1 The influence of vegetation recovery on soil hydrology and ...

1 downloads 0 Views 2MB Size Report
Singleton Park, Swansea SA2 8PP, UK ([email protected]ac.uk) ... eastern Mediterranean was examined over an 11-year period. ...... AJD, Walsh RPD, Shakesby RA, Leighton-Boyce G, Coelho C.O.A. (2003) Soil water repellency as a potential.

International Journal of Wildland Fire (14[4], 2005), in press

The influence of vegetation recovery on soil hydrology and erodibility following fire: an elevenyear investigation Artemi CerdàA,B* and Stefan H. DoerrC A

Department of Geography, Universitat de València. Blasco Ibáñez, 28, 46010-Valencia, Spain ([email protected]) Instituto Pirenaico de Ecología, Consejo Superior de Investigaciones Científicas. Aula Dei, 202, 50080-Zaragoza, Spain C University of Wales Swansea, Department of Geography. Singleton Park, Swansea SA2 8PP, UK ([email protected]) B

*Corresponding Author. Telephone +34 6 3864237; Fax +34 6 3864249

Suggested running heading title: Vegetation recovery effects on post-fire soil hydrology

Additional keywords: Soil Erosion, Overland flow, Wildfire, Vegetation Recovery, Mediterranean Environments, Rainfall Simulator, Soil Hydrophobicity, Water Repellency. Short summary: The effectiveness of different types of vegetation in reducing runoff and soil losses following a severe wildfire in the eastern Mediterranean was examined over an 11-year period. In addition to vegetation density, vegetation type was also important, with herbs and shrubs being more effective compared to dwarf shrubs and particularly trees. Abstract This study investigates long-term changes in soil hydrological properties and erodibility during the regrowth of different types and densities of vegetation following a severe wildfire in the eastern Mediterranean (Serra Grossa Range, eastern Spain). Twelve plots of similar slope and soil characteristics, naturally re-colonized by four different vegetation types (trees, herbs, shrubs and dwarf shrubs) were examined using rainfall simulations during an 11-year period. The mean erosion rate was 80 g m-2 h-1 six months after the fire under wet-winter conditions, declining to 30 g m-2 h-1 in the following summer and reaching < 10 g m-2 h-1 after 2 years. Considerable variation under the different vegetation types was observed. Herbs and shrubs reduced erosion and overland flow coefficients to negligible values 2 years after fire, whereas under trees and dwarf shrubs appreciable overland flow and soil loss still occurred after 5 years. On tree covered plots (Pinus halepensis), overland flow actually increased over time in association with the development of topsoil hydrophobicity, reaching a coefficient of 27 % ten years after burning. Rates of post-fire overland flow and erosion reduction were not only strongly influenced by vegetation coverage, but also by the type of cover and its effects on soil hydrophobicity.

International Journal of Wildland Fire (14[4], 2005), in press

1

1. Introduction Runoff and soil erosion tend to increase following wildfire due to the removal of vegetation and litter and fire-induced changes in soil properties such as hydrophobicity (water repellency) (DeBano et al. 1970; Doerr et al. 2000), porosity (Imeson et al. 1992), fertility (Anderson 1949; Brown 1972; Úbeda and Sala 1998; Andreu et al. 2001) and aggregate stability and distribution (Llovet López 2005), which in turn affect soil erodibility (i.e. the soil’s susceptibility to detachment and transport by erosion; Bryan 1968-1969). Vegetation cover is viewed as the key factor on the ground in controlling soil erosion (Morgan 1986; Shakesby et al. 2000) and its recovery normally leads to a decline in post-fire runoff and soil erosion rates as demonstrated by many authors at different spatial scales and under different ecological conditions (Cerdà 1998a,b; Benavides-Solorio and MacDonald 2001; Moody and Martin 2001a,b; Shakesby et al. 1993). Data on post-fire runoff and erosion, however, have often been collected only for the first months or a few years following burning, although effects on overland flow and erosion can be more persistent associated with longer-term fire impacts on flora, fauna, geomorphology, and soil properties (crusting, aggregate stability, porosity, hydrophobicity) (Neary et al. 1999; Doerr et al. accepted subject to revision). Long-term studies on the effect of fire and other disturbance events are thus comparatively rare, despite their importance in the understanding of ecosystem functioning (Wolman and Miller 1960). Furthermore, much of the research carried out on the relationship of vegetation to soil erosion has focussed on the overall vegetation cover and its biomass, but comparatively little is known about the specific effects of different types of vegetation or specific plant species on soil erosion (Hester et al. 1997). Recent research on the spatial distribution of vegetation unrelated to fire has demonstrated important differences in the effects of different plant species on soil erosion processes (Cerdà 1997; Reid et al. 1999). Examining vegetation recovery effects under Mediterranean environmental conditions Compared to many other environmental conditions, vegetation recovery after forest fire in the Mediterranean can be rather rapid due to the adaptation of vegetation to disturbance by fire, and, following burning, the low competition for sunlight, increased nutrient availability and reduced water losses by transpiration (Naveh 1974; Trabaud 1981). The reliable determination of associated temporal changes in soil hydrological and erosional processes, however, is notoriously difficult to achieve under Mediterranean climatic conditions due to the high temporal variability of the rainfall, which results in large differences in inter-annual responses. For example, rainfall events of 600 mm during two days were recorded in the western Mediterranean basin (Olcina 1994), and daily rainfall events exceeding 300 mm are not unusual (López Bermúdez 1990; Pérez Cueva 1994). On the other hand, prolonged drought periods with rainfall lower than 200 mm y-1 are also recurrent and their frequency is expected to increase with predicted future climate change (De Luis et al. 2000, 2001; Ceballos et al. 2004). The importance of the temporal variability of climatic conditions in soil hydrology and erosion has been highlighted in a number of studies in the Mediterranean. For example, a ten-year study on 15 x 4 m plots on a southfacing slope in the Ebro Valley, Spain, demonstrated that during wet years the erosion rate can be three orders of magnitude higher that during dry years with, annual sediment yield during 1991 being 24.8 Mg ha-1 compared to 0.06 Mg ha-1 in 1994 (Desir 2000). Similar findings have been reported from fire-affected land. For example, during a study using 4 x 20 m plots under a Matorral cover near Alicante burnt in September 1989 (see Figure 1), a very high variability in rainfall resulted in runoff coefficients of between 20 % and 80 % during the three years prior to, but only 3.4 % in the year after fire (Sánchez et al. 1994). In a study in burnt Pinus halepensis woodland using 2 x 8 m plots, Bautista (1999) found that soil loss on one day (7.2 Mg ha-1; September 30th 1997) was higher than the total soil eroded during the 3 previous years (5.7 Mg ha-1; 1993-1996). Thus, in addition to vegetation and soil related parameters being key controls associated soil hydrological and erosional responses can remain highly variable when driven by a few extreme events each decade (Wolman and Miller 1960). Data collected during a period dominated by high frequency-low magnitude rainfall events can result in below-average overland flow and erosion, whereas the occurrence of one or several high magnitude-low frequency events will have the opposite result. The problems in determining the effects of vegetation recovery under the highly variable Mediterranean climatic conditions can be circumvented using simulated rainfall. While this approach provides data only on soil hydrological and erosional responses collected for the selected rainfall characteristics, with measured rates that may not necessarily represent those actually occurring under natural rainfall, the standardisation of rainfall characteristics allows the detailed study of the effects of vegetation and soil characteristics independent of the variations in natural rainfall (Meyer 1988). In the present study, we report on rainfall simulations and associated investigations carried out over a 11-year period following a wildfire in the Serra Grossa Range in eastern Spain with the aim to examine the effects of (i) different types of vegetation and their coverage, and (ii) temporal changes in vegetation and soil hydrophobicity characteristics, on soil hydrological and erosional responses following fire, covering a period that extends well beyond those typically examined in previous studies carried out on the effects of fire on soil hydrology and erosion in the Mediterranean and elsewhere. 2. Study sites A wildfire in August 1989 burnt an area of 87 ha in the Serra Grossa Range in La Costera district southwest of Valencia in eastern Spain (Figure 1). The north-facing slope, covered by an Aleppo pine open forest (Pinus halepensis) with a dense understorey matorral (Quercus coccifera, Pistacia lentiscus, Erica multiflora, Rosmarinus officinalis) was completely burnt. Fire severity was high associated with the hot daytime temperature (38 ºC), low relative humidity (35 %), low soil moisture (< 5 % at 0-2 cm depth) and the dense biomass of the P. halepensis and scrubland. The presence of white ash International Journal of Wildland Fire (14[4], 2005), in press

2

after the fire confirmed the high fire severity (Bentley and Fenner 1958). The dense vegetation cover of the north-facing slopes generally results in comparatively high burn temperatures as recently demonstrated by Ruiz Gallardo (2004) for a nearby area farther inland. This vegetation cover is typical of non-cultivated calcareous soils in eastern Spain where the felling of trees for charcoal production and of shrubs for fuel, as well as grazing over millennia has resulted in dramatic changes in floristic composition. As a consequence, the climax vegetation, dominated by Quercus ilex, is now uncommon. The studied slope is north facing and has an elevation range of 150-299 m. The experimental plots were located on a slope of 14º at 200 m elevation. The bedrock comprises homogeneous Cretaceous limestone with a 5-50 cm deep soil cover and occasional rock outcrops. The soils are Leptosols and Luvisols with Lithic Leptosols occurring at exposed locations with depths of < 5 cm. Where the experiments took place, the soil depth is 15-50 cm. Soils are of loamy sand texture with a typical composition of 69 % sand, 16 % silt and 15 % clay at 0-2 cm depth. A discontinuous clay horizon (Bt) is found in some locations at ca 20 cm depth. Organic matter content averages about 4.8 % at 0-2 cm depth and 2.7 % at 4-6 cm depth and calcium carbonate content about 4.4 % at 0-2 cm depth. Bulk density at 0-2 cm depth was about 1.1 g cm-3. Surface cracking, which could enhance preferential flow, was not observed. The climate is typically Mediterranean, with dry and hot summers and warm winters. Mean annual rainfall was 688 mm y-1 at the nearest meteorological station 15 km to the east, with our records at the study area further inland showing a mean of 505 mm y-1. October is the wettest month, with daily rainfall in excess of 100 mm day-1 occurring on average once a decade (Elias and Ruiz 1979) although in the study area, daily rainfall amounts of > 300 mm occurred at least twice during last 20 years. Drought periods with precipitation < 300 mm y-1 also occur, the most recent being 19801982 and 1993-1994. Rainfall is often also spatially highly variable as for example demonstrated by a thunderstorm in summer 1998, when 19 mm fell on the north-facing study slope while only 3.9 mm were measured on the south-facing slope of the same hill. Monthly rainfall shows the highest amounts during September. In the year 1989, shortly after the fire, 50 mm fell at the end of August and 392.7 mm during September (Table 1). However, no rills or other visible erosion features were found after these events. In the post-fire period, no other external disturbance events, such as grazing or human interference, occurred on any of the plots.

3. Methods Twelve plots with the same aspect, comparable slope angle (9-14 º) and with similar soil characteristics were selected at mid- to lower slope positions six months after the fire in early 1990. To determine temporal changes in post-fire soil hydrological and erosional characteristics, rainfall simulation experiments were carried out on these plots (Figure 2) following dry summer conditions (> 15 days with no rainfall; volumetric soil moisture at 0-2 cm: 2-6 %) and wet autumn/winter conditions (following one week without rainfall to avoid the effect of saturated soil conditions affecting runoff generation; volumetric soil moisture at 0-2 cm: 12-21 % in 1990, 1991, 1992, 1995, 1997 and 2000. Thus, twelve rainfall simulations were performed twice a year during six selected years between 1990 and 2000, resulting in 144 individual simulations. During initial plot selection, vegetation had not been considered, but it soon became clear that different plots became dominated by different plant communities, which provided the opportunity to examine any differences the effects of contrasting vegetation types on hydrological and erosional response. Some vegetation sprouted after the fire (e.g. Quercus coccifera, Pistacia lentiscus, Erica multiflora), while other vegetation grew from seedlings after successful germination (Brachypodium retusum, Cistus albidus, Pinus halepensis, Ulex parviflorus). The emerging differences in vegetation type and coverage were recorded using repeat plot photographs, and plots were classed into four main types: trees (2 plots), herbs (2 plots), shrubs (3 plots) and dwarf shrubs (5 plots). Simulated rainfall of 55 mm over one hour duration was produced with deionized by means of a sprinkler rainfall simulator (Cerdà et al. 1997) (Figure 2) over a 1 m2 plot surface (0.25 m2 were used as a rainfall simulation plot surface and 0.75 m2 as a border surface). The plots were initially left permanently in place, but after the experiments of 1995 they had to be removed and re-inserted in directly adjacent comparable locations for the experiments of 1997 and 2000 (Fig. 3) to avoid sediment exhaustion, which could otherwise have been responsible for a reduction in measured erosion rates. Overland flow (i.e. surface runoff) was collected at 1 to 2 minute intervals. At least 3 samples were collected in bottles and the sediment content was measured in the laboratory following complete evaporation of the water. These measurements allow determination of overland flow discharge and coefficient, sediment concentration, sediment yield and erosion rates. As an additional indicator of soil hydrological behaviour, we also measured ‘time to ponding’ (tp), where tp is the time from the onset of the rainfall initiation to the development of ponding on ca 40 % of the total plot surface (Imeson et al. 1992). Soil surface hydrophobicity was also measured in situ prior to every simulation using the Water Drop Penetration Time (WDPT) test (Letey 1969). The test comprised placing ten drops (0.05 ml) on a representative soil surface close to the rainfall simulation plot following careful removal of any litter, and measuring the time until complete infiltration for each drop. The average of these ten values was taken as the respective WDPT. Following the widely-adopted classification of Bisdom et al. (1993), WDPTs ≤5 s were taken as indicative of wettable soil conditions, 5-60 s as slight, 60-600 s as strong, 600-3600 s as severe and >3600 s as extreme hydrophobicity. Plot removal and reinstallation in 1995 and 2000 allowed excavation of parts of the plots for visual examination of the wetting front.

International Journal of Wildland Fire (14[4], 2005), in press

3

4. Results and Discussion 4.1 Post-fire vegetation development Vegetation recovery was relatively rapid as is typical of many Mediterranean environments (Naveh 1974; Trabaud 1981) with some plots under herbs or dwarf shrubs plots reaching > 100 % coverage (i.e. consisting of several layers provided by moss, litter, herbs and shrubs) within 2 years after burning. At this stage, recovery was so distinct that plots could be grouped by vegetation type and classified by their dominant plant species: trees (two plots with Pinus halepensis), herbs (two plots with Bachypodium retusum), shrubs (three plots with, Quercus coccifera, Juniperus oxycedrus and Pistacia lentiscus) and dwarf shrubs (five plots with, Ulex parviflorus, Erica multiflora, Rosmarinus officinnalis, Cistus albidus and Thymus vulgaris). The post-fire vegetation cover present at the time of each rainfall simulation for each plot is given in Table 2 (total vegetation cover). The herb-dominated plots showed the fastest vegetation growth, reaching a dense cover of > 50 % within only six months of the fire (1990). Amongst shrubs and dwarf shrubs, the increase in vegetation cover varied widely. The lowest coverage was by Ulex parviflorus and Erica multiflora with values of < 50 % after 3 years (1992), whereas Quercus coccifera, Cistus Albidus and Thymus vulgaris exceeded 100 % cover at this stage. Vegetation coverage by trees was intermediate between these, reaching 60-80 % in 1992. It is notable that the increase in cover during 1990 and 1991 exceeds the variation caused by the seasonal reduction in cover during the dry season (Table 2). In later years, however, a reduction in cover during the dry season becomes evident, particularly for the herb and dwarf shrub dominated plots (Table 1). Eight years after burning (1997), vegetation cover was well above 100 % at all plots (109-269 %). In comparison, at long unburned (>10 years since a fire) sites nearby, vegetation cover, comprising mainly shrubs with some herbs patches is typically above 100 % 4.2 Post-fire soil hydrophobicity Soil surface hydrophobicity data, obtained using the WDPT test, is summarised in Table 3. None of the plots exhibited hydrophobic soil surface conditions (WDPT > 5 s) six and twelve months after fire. Exploratory measurements carried out in similar unburnt vegetation (unpublished data collected in 1998) suggest that before the fire low levels of hydrophobicity may have existed here under pine. The wettable soil condition after the fire suggests that any pre-fire surface soil hydrophobicity has been eliminated during burning. The high severity of the burn, as indicated by the presence of white ash, suggests that surface soil temperatures may have been sufficiently high to eliminate any pre-existing surface hydrophobicity. If sufficient oxygen for combustion of hydrophobic compounds would have been available, this threshold for soil hydrophobicity elimination would have been around 300 ºC (Doerr et al. 2004), rising to 500 ºC under oxygendepleted conditions (Bryant et al. this issue). All except the tree-covered plots retain wettable surface soil conditions (WDPT ≤ 5 s) during the entire 11-year period of investigation, which supports the notion that surface soil hydrophobicity is not a feature of calcareous soils under these vegetation types in the region (Mataix-Solera and Doerr 2004). Hydrophobic soil conditions did, however, develop from year two onwards under tree cover (Pinus halepensis plots), supporting the view that any pre-fire surface hydrophobicity at the plots was eliminated during the burn. During the following years, hydrophobicity under pine increased until the eighth year after fire, but did not exceed slight hydrophobicity (WDPT 5-60 s). The slight hydrophobicity measured here matches hydrophobicity levels reported from calcareous soils under long unburnt (>30 years) P. halepensis in south-east Spain reported by Mataix-Solera and Doerr (2004), but contrasts with the often much higher hydrophobicity levels found under shrub or pine vegetation in some other regions with a Mediterranean type climate (e.g. Giovannini and Lucchesi 1983; Soto et al. 1994; Doerr et al. 1998) and it is suggested that the calcareous soil conditions present here do not promote the natural development of hydrophobicity compared with the acidic conditions typically associated with more severe levels of hydrophobicity. A potential reason may be that fungal activity, which has been implied in the development of hydrophobicity (Jex et al. 1985; Hallett et al. 2001), is more prominent under alkaline conditions (Paul and Clark 1996). Although hydrophobicity was only measured at the soil surface, the fact that it increased naturally under pine in the post-fire period supports the notion that it may well have been present also to some depth in the soils before and after the fire as observed in studies conducted under the same and other pine species elsewhere (e.g. Doerr et al. 1996; Scott 2000; Mataix-Solera and Doerr 2004). Throughout the experimental period, hydrophobicity was always higher under dry summer compared to wet autumn/winter conditions (Table 3). This seasonal pattern is typical of hydrophobic soils and is thought to be related to soil moisture. Hydrophobicity tends to be absent above, and present below a critical moisture threshold zone. Within this zone, hydrophobicity has the potential to be present, but may not be fully expressed. The limits of this zone differ between soils and are thought to depend on soil texture and organic matter characteristics (Dekker et al. 2001). In the present study, volumetric surface soil moisture (0-2 cm depth) was consistently 4-5 % for dry season measurements and 15-17 % in the wet season (average of 3 samples per plot). Hydrophobicity was reduced, but always present for wet season measurements, which suggests that the 15-17 % moisture content measured here is within the soil moisture threshold zone for hydrophobicity expression. The data obtained here do not allow determination of the upper soil moisture threshold, but studies under pine (Doerr and Thomas 2000) and other vegetation (Soto et al. 1994) conducted elsewhere in the Mediterranean suggest an upper limit in the region of 20-28 %. 4.3 Post-fire soil hydrology and erodibility International Journal of Wildland Fire (14[4], 2005), in press

4

Overland flow at all burnt plots (Table 4) was initially much higher than that recorded for unburnt scrub or grass covered terrain of otherwise similar characteristics near the study area (Cerdà, 1998b). From the initially highest levels measured six months after the fire, all twelve plots show a strong decline in overland flow and soil loss under simulated rainfall during the first 3 years with superimposed variations in overland flow (and sediment concentration) between wet autumn/winter and dry summer conditions (Figure 4). Thus average wet and dry season overland flow coefficient decline from 44.6 % and 11.5 % six and twelve months after the fire to 5.0-6.6 % and 7.6-4.6 % respectively from year three onwards (Table 4). Average wet and dry season soil loss decline sharply from 79.7 and 30.0 g m2 h-1 in months six and twelve to 4.8 and 2.8 g m2 h-1 in months 30 and 36 respectively. Thereafter, soil loss declines further, reaching 0.33 and 0.24 g m2 h-1 ten years after the first respective measurement (Table 4). Although greater overland flow was generally associated with higher erosion rates, the latter declined much more sharply than the former (Figures 5 and 6). This is a pattern typically observed under recovering vegetation and it is thought that the increase in vegetation coverage, in addition to reducing eroding overland flow, also resulted in a lower rate of entrainment of soil particles for a given rate of overland flow as evident from the sediment concentration data presented in Table 6 and Figure 7. The effects of the various vegetation types are discussed in more detail in the following Section 4.4. The considerable variations in overland flow and soil loss between wet and dry seasons are thought to be caused primarily by the higher antecedent soil moisture content in the wet season, leading to a more rapid development of saturation overland flow. The average overland flow coefficient over the whole period of investigation was 24 % during wet, but only 11 % during dry periods with the differences between wet and dry seasons declining over the years. Sediment concentration in the overland flow is somewhat higher during the dry compared to wet periods of the first three post-fire years (Figure 7). This may be due to a number of reasons including sediment exhaustion, the dilution effect associated with the greater overland flow volume generated during the wet season rainfall simulations and aggregate behaviour under wet conditions. During the wet periods, any overland flow generated from natural rainfall (and during simulated rainfall) would have caused soil particle and aggregate wash, resulting in the partial exhaustion of the available sediment. During the dry and hot conditions dominant in the Mediterranean summer, surface wash does not take place, allowing uninterrupted particle accumulation on the soil surface and air trapped in dry aggregates makes them more vulnerable to breakdown (i.e. slaking) during rapid wetting. As a consequence, the sediment available during a rainfall simulation following prolonged dry conditions is greater. Soil erosion rates in similar terrain, though not affected by recent fires, showed comparable seasonal trends with a similar experimental layout 1995 (Cerdà 1998b). Similar results were also found by Simanton and Emmerich (1994) under natural vegetation and simulated rainfall in USA. The above data demonstrate (i) the increase of the overland flow and erosion rates after the fire in comparison to soils not recently affected by fire, (ii) the rapid reduction in overland flow generation and soil erodibility within three years of burning, and (iii) the high seasonal variability of the hydrological and erosional response typical of the Mediterranean ecosystems. 4.4 The influence of vegetation type and soil hydrophobicity on soil hydrology and erodibility Average overland flow generation at the plots following wet antecedent conditions shows a steep decline with increasing vegetation until ca 50 % cover was reached in 1995 (Table 4 and 5). As overland flow generation levels off thereafter, the further increase in vegetation cover appears not to have an effect on wet season overland flow generation. Following dry antecedent conditions, the decline in overland flow with increasing vegetation recovery continues throughout the whole study period. Overland flow generation during the dry season, however, is much lower than during the wet season, resulting in a lower rate of decline (Table 4). Erosion rates decline much more sharply over the study period than overland flow as already pointed out. This is an expected outcome as increasing vegetation cover tends not only to reduce overland flow totals, but also the potential of a given amount of overland flow to entrain soil particles (Morgan 1986). As regards vegetation type affecting overland flow and erosion some important differences emerge between vegetation types (Tables 2-6 and Figures 5, 6). The reduction in overland flow coefficient measured for both wet and dry antecedent conditions over the whole study period was greatest for plots recovering with herbs, followed by shrubs and dwarf shrubs (Figure 5). The plots recovering with pine showed the weakest reduction in overland flow under wet conditions, but, in contrast to all other vegetation types, an overall increase during the dry conditions compared to year 1. The same ranking of different vegetation types applies to their effectiveness in reducing erosion, except that in contrast to overland flow, erosion also continued to decline under trees with increasing cover. More specifically, the vegetation in herb-dominated plots recovered most rapidly (96-142 % cover after 1 year), reducing dry season overland flow to around 5-6 % after one year, 0.5-1 % after two years and no overland flow occurring three years later. Shrub-dominated plots recovered less rapidly (38-69 % after 1 year) and overland flow was accordingly reduced at that time to only 9-10 %. However, when shrubs exceeded coverage of 100 % in year six, dry-season overland flow was also reduced to zero. Plots dominated by dwarf-shrubs had a similarly rapid vegetation recovery, exceeding on average 100 % in year 6, however, here overland flow never reached zero. Vegetation recovery rate under tree covered plots was similar to shrubs and dwarf-shrubs, also exceeding 100 % in year 6. However, dry-season overland flow showed the aforementioned overall increase from 15-18 % in year 1 to a maximum of 36-45 % in year 6, followed by a decline to 24-29 % in year 11.

International Journal of Wildland Fire (14[4], 2005), in press

5

As regards erosion, within two years erosion rates at the herb plots fell below what has been classified as negligible for forested terrain under natural rainfall under western Mediterranean conditions (0.02 g m2 per mm rainfall, equivalent to 1.1 g m2 h-1 here; Shakesby et al. 2002). The reduction to such negligible erosion rates was reached during the next measuring campaign six months later under shrubs and by year 6 under dwarf-shrubs. Under trees, despite a progressive decline, erosion rates reached levels close to this threshold (1.1-1.4 g m2 h-1) only during year 11 (Table 5, Figure 6). The overland flow exceeding 24 % during each campaign allowed significant erosion to occur until year 10 despite the vegetation cover exceeding 100 % from year 6 onwards. From the results presented above, it is evident that the changes in post-fire erosion and, to a greater extent, overland flow rates, are not related to vegetation recovery alone. For example, despite showing very similarly trends in vegetation recovery, both overland flow and erosion remained generally higher under dwarf shrubs compared to shrubs (Table 2 and Figures 5, 6) despite the overall similar morphology of these plants. Also, dry season overland flow under herbs showed a significant reduction between 1991 and 1995 (Table 4) despite negligible differences in vegetation cover during the same period (Table 1). Soil surface hydrophobicity for all these plots shows consistently low values (WDPT < 5 s), and the same applies to dry season soil moisture (≥ 5 %), effectively ruling out their influence. Determining the reasons for the differences in the effects of similar coverage within and between vegetation types clearly warrants further investigation. For the pine-dominated plots, the relationship between vegetation recovery and post-fire overland flow and erosion is more complex than under other vegetation types. Here, however, soil hydrophobicity increases to significant levels with vegetation recovery, as already discussed in more detail in Section 4.2, which would be expected to counter some of the effects of vegetation cover increase. The data in Table 3 demonstrate that soil hydrophobicity was present at the soil surface for both wet and dry season rainfall simulations and could thus have contributed to the higher overland flow and associated erosion rates on the pine compared to all other plots, where significant hydrophobicity (WDPT > 5 s) was absent throughout. Evidence for the potential contribution of soil hydrophobicity here is confirmed by the fact that higher overland flow rates during summer compared to winter conditions from 1995 onwards coincide with higher hydrophobicity levels. If soil had been wettable throughout, greater overland flow would be expected during winter due to more rapid saturation of the already moist or wet soil, as observed for nearly all rainfall simulations except under pine plots. For the latter, the higher hydrophobicity levels, that often accompany drier soil conditions (Doerr et al. 2000), appeared to be more influential on overland flow generation and associated erosion than the higher soil moisture contents during winter. Thus, from 1995 onwards, the further increase in vegetation cover between wet and subsequent dry seasons appears to have been insufficient to counter the effects of increased dry-season hydrophobicity. Between the fire and 1995, soil hydrophobicity is also likely to have affected overland flow generation, however, during that period, the higher vegetation recovery rate is considered to have been more influential here than the slowly increasing hydrophobicity levels. Overland flow also showed the effect of soil hydrophobicity development associated with the recovery of Pinus halepensis cover. Figure 8 shows the hydrographs of the 1990 and 2000 experiments under dry conditions on Pinus halepensis covered soils. The shape of the overland flow curve for the measurements in 2000 shows a sharp increase during the first 5 minutes of rainfall before reaching a steady-state infiltration rate thereafter, whereas the 1990 post-fire rainfall simulation did not show this response. The response in 2000 is similar to that typically observed for hydrophobic soil conditions following fire (Robichaud 2000). Given the otherwise comparable soil conditions and the increase in vegetation cover observed at all plots, it is reasonable to conclude that increasing soil surface hydrophobicity is the key factor in increasing, or maintaining the occurrence of, dry-season overland flow under pine. In-newly burnt eucalyptus stands in Portugal (Leighton-Boyce et al. 2005), the influence of hydrophobicity has been isolated under simulated 30 minute rainstorms (~100 mm h-1) by using wetting agents in some simulations to impart wettable soil conditions. Despite the high rainfall intensity, no overland flow occurred when wetting agents were used, whereas overland flow ranged between 48.4 and 92.5 % under hydrophobic soil conditions. At their study site, however, soil hydrophobicity levels were predominantly extreme (Leighton-Boyce et al. 2005), as classified by the Critical Surface Tension test, which equates to WDPTs > 1 h (Doerr et al. 1998). Given the comparatively low hydrophobicity levels measured in the current study, it is perhaps remarkable that hydrophobicity appears nevertheless to have such a marked effect on overland flow generation. Additional information on the potential importance of various factors on overland flow generation and associated erosion can be gained from the time to ponding (tp) data (Figure 9); i.e. the time taken from the onset of the rainfall initiation to the development of ponds on ca 40 % of the total plot surface. In addition to WDPT data, which is only indicates the degree of hydrophobicity at the very soil surface within the contact area of the respective water droplet, tp will be affected by soil surface micro-topography and the infiltration capacity of the topsoil layer, which in turn will be influenced by the spatial distribution of any existing hydrophobicity and the effectiveness of more wettable soil patches and of macropores in allowing infiltration to bypass less wettable soil areas (Doerr et al. 2003). As one would expect, tp, averaged for each of the vegetation types, showed inverse trends to overland flow, with herbs having the longest and trees shortest times. At the pine plots, where significant hydrophobicity was present, tp lengthened with increasing WDPTs, suggesting that hydrophobicity is considerably more influential here than the interception capacity of the vegetation. Finally, useful insight into the effects of vegetation and soil hydrophobicity can be gained from the shape of the wetting front present after simulated rainfall during the dry season (Figure 10). Perhaps most striking is the observation that the wetting fronts were homogeneous following the first simulation after fire (1990), indicating that matrix flow dominated the infiltration process. Six years after burning (1995) the wetting fronts are heterogeneous and patchy due to International Journal of Wildland Fire (14[4], 2005), in press

6

the influence of macropore flow associated with cracks, rootholes, and faunal burrows. The soil horizon boundaries were not unusually discontinuous and are thus though to have had only a minor, if any, influence on the shapes of the wetting fronts. Except for pine, there was an increase in the depth of the wetting front over the years as more rainfall infiltrated and less was lost as overland flow. Under Pinus halepensis, however, the wetting front reached only shallower depths as less infiltration and more overland flow occurred except that here some preferential flow to a depth of 20-30 cm was observed. Enhanced preferential flow is a typical feature of hydrophobic soils (see review by Doerr et al. 2000) and hydrophobicity is also viewed as promoting its occurrence here. Some preferential flow was also present within the root zone of other plots in 2000 (Figure 10) and, given the absence of surface hydrophobicity at these plots and the fact that natural (i.e. not fire-induced) hydrophobicity tends to decrease with depth (see review by Doerr et al. 2000), it is speculated here that the root channel developed during re-vegetation and cracks have promoted preferential flow here. 4.5 Implications for post-fire terrain recovery and land management under Mediterranean environmental conditions A number of wider implications for post-fire terrain recovery under Mediterranean environmental conditions arise from the results of this study. (i) Overland flow and erosion rates are enhanced following fire associated with the removal of vegetation and changes to the soil as a result of heating. The recovery of the terrain to pre-fire erosion rates requires 2-4 years under the shrub and herb vegetation examined. Under Pinus halepensis, however, soil heating during the fire appears to have been sufficient to reduce soil hydrophobicity and increase the dry-season infiltration rates. Hydrophobicity re-establishment associated with pine regrowth increases dry-season overland flow during the first few years of vegetation recovery, which appears to be the cause of the considerable delay in the reduction of erosion rates to pre-fire conditions under pine. Had soil temperatures remained lower during the burn, maintaining or enhancing pre-fire soil hydrophobicity, overland flow and erosion would be expected to have been even higher during the first few years following the fire. These findings demonstrate that where vegetation growth is coupled with soil hydrophobicity development, an increase in vegetation cover does not necessarily reduce overland flow as might otherwise be expected. (ii) In Spain, most of the past and current afforestation is carried out with Pinus halepensis due to his fast growth and the comparatively high value of the timber. When mature, these forests are undoubtedly just as, if not more, effective in preventing soil erosion as a dense grass or shrub cover. The results presented here, however, suggest that, following disturbance events such as fire, shrubs and herbs are more efficient for soil erosion control. Thus, where disturbance events are frequent, as is increasingly the case for many parts of the Mediterranean, shrubs and herb vegetation appears also to be more effective in preventing erosion in the long-term compared to P. halepensis. (iii) The results suggest that where soil erosion control measures after fire are envisaged, they would be most effective in terms of encouraging recovery of herbs as a result of their fast growth followed by sprouting shrubs such as Quercus coccifera, Pistacia lentiscus and Juniperus oxycedrus. For herbs, aerial sowing can be an efficient technique as it reduces trampling and related soil disturbance. The resulting patchy distribution of shrubs and presence of herbs between them is a typical feature of Mediterranean rangelands, which also helps to reduce the connectivity of future fires. 5. Conclusions The rainfall simulations carried out over a period of 11 years following a severe wildfire in Mediterranean eastern Spain demonstrate different rates of plant re-establishment and in their effectiveness in reducing overland flow and erosion, resulting in different responses throughout the 11 years experimental period. Overland flow and soil losses reached a peak in the winter following the fire in summer 1989. Overland flow was then reduced most rapidly under herbs, followed by shrubs and dwarf shrubs, whereas under a tree cover (Pinus halepensis), the re-development of a hydrophobic surface layer during the post-fire regeneration period resulted in an increase in overland flow rates, especially during summer. Post-fire erosion rates showed a more rapid decline than overland flow responses, with less distinctive differences between herb, shrub and dwarf-shrub covered plots. Only under trees did re-development of surface soil hydrophobicity appear to have delayed erosion rate reduction. The differences in the effectiveness in reducing overland flow and soil erodibility are clearly not only related to the degree of vegetation cover, but also to the type of cover, with herbs and shrubs being more efficient in reducing overland flow and erosion than dwarf shrubs and particularly trees. The influence of vegetation type on the hydrophobic behaviour of the soil is thought to be a key factor here. The findings demonstrate that where vegetation growth is coupled with soil hydrophobicity development, an increase in vegetation cover does not necessarily reduce overland flow as might otherwise be expected. The results of this study also demonstrate that, while significant terrain recovery towards pre-fire conditions has taken place during the first two years of investigation, major changes to soil hydrology and erodibility have occurred much later in this investigation. This highlights the importance of long-term monitoring approaches in study of fire effects. Shorter-term studies may not always be sufficient in allowing a thorough determination of (i) the changes to soil hydrology and erodibility caused by a fire and (ii) the effectiveness of various vegetation types or management measures in reducing accelerated post-fire overland flow and soil erosion responses.

International Journal of Wildland Fire (14[4], 2005), in press

7

Acknowledgements This work was supported by the REN2002-00133/GLO (Spain) project and a NERC Advanced Fellowship NER/J/S/2002/00662 (UK). We thank Anna Ratcliffe for drawing Figure 1, and Richard Shakesby and the anonymous referees constructive comments on a previous version of the manuscript. References Anderson HW (1949) Does burning increase surface runoff? Journal of Forestry 47, 54-57. Andreu V, Imeson AC, Rubio JL (2001) Temporal changes in soil aggregates and water erosion after a wildfire in a Mediterranean pine forest. Catena 44, 69-79. Bautista S (1999) ‘Regeneración post-incendio de un pinar (Pinus halepensis, Miller): en ambiente semiárido. Erosión del suelo y medidas de conservación a corto plazo.’ PhD Thesis, Universidad de Alicante, 238 pp. Benavides-Solorio J, MacDonald LH (2001) Post-fire runoff and erosion from simulated rainfall on small plots, Colorado Front Range. Hydrological Processes 15 2931-2952. Bentley JR, Fenner RL (1958) Soil temperatures during burning related to postfire seedbeds on woodland range. Journal of Forestry 56, 737-740. Bisdom EBA, Dekker LW, Schoute JF Th (1993) Water repellency of sieve fractions from sandy soils and relationships with organic material and soil structure. Geoderma 56, 105-118. Brown JHA (1972) Hydrological Effects of a Bushfire in a Catchment in South-Eastern New South Wales. Journal of Hydrology 15, 77-96. Bryan RB 1968-1969. The development, use and efficiency of indices of soil erodibility. Geoderma 2, 5-26. Bryant R, Doerr SH, Helbig M (2005) The effect of oxygen depravation on soil hydrophobicity during heating. International Journal of Wildland Fire (this issue). Ceballos A, Martínez-Fernández J, Luengo-Ugidos MA (2004). Analysis of rainfall trends and dry periods on a pluviometric gradient representative of Mediterranean climate in the Duero Basin, Spain. Journal of Arid Environments 54, 214-232. Cerdà A (1997) The seasonal changes of the infiltration rates in a typical Mediterranean scrubland on limestone in southeast Spain. Journal of Hydrology 198, 209-225. Cerdà A (1998a) Changes in overland flow and infiltration after a rangeland fire in a Mediterranean scrubland. Hydrological Processes 12, 1031-1042. Cerdà A (1998b) Postfire dynamics of erosional processes under mediterranean climatic conditions. Zeitschrift für Geomorphologie 42, 373-398. Cerdà A, Ibáñez S, Calvo A (1997) Design and operation of a small and portable rainfall simulator for rugged terrain. Soil Technology 11, 163-170. DeBano LF, Mann LD, Hamilton DA (1970) Translocation of hydrophobic substances into soil by burning organic litter. Soil Science Society of America Proceedings 34, 130-133. Dekker LW, Doerr SH, Oostindie K, Ziogas AK, Ritsema CJ (2001) Actual water repellency and critical soil water content in a dune sand. Soil Science Society of America Journal 65, 1667-1675. De Luis M, García-Cano MF, Cortina J, Raventós J, González-Hidalgo JC, Sánchez JR (2000) Climatic trends, disturbances and shortterm vegetation dynamics in a Mediterranean shrubland. Forest Ecology and Management 147, 25-37. De Luis M, Raventós J, González-Hidalgo JC, Sánchez JR, Cortina J (2001) Spatial analysis of rainfall trends in the region of Valencia (Eastern Spain). International Journal of Climatology 20, 1451-1469. Desir G (2000) ‘Erosion h´ıdrica de terrenos yes´ıferos en el sector central de la Depresi´on del Ebro’. PhD thesis, University OF Zaragoza, 318pp. Doerr SH, Thomas AD (2000) The role of soil moisture in controlling water repellency: new evidence from forest soils in Portugal. Journal of Hydrology 231-232, 134-147. Doerr SH, Shakesby RA, Walsh RPD (1996) Soil hydrophobicity variations with depth and particle size fraction in burned and unburned Eucalyptus globulus and Pinus pinaster forest terrain in Agueda Basin, Portugal. Catena 27, 25-47. Doerr SH, Shakesby RA, Walsh RPD (1998) Spatial variability of soil hydrophobicity in fire-prone eucalyptus and pine forests, Portugal. Soil Science 163, 313-324. Doerr SH, Shakesby RA, Walsh RPD (2000) Soil water repellency: its causes, characteristics and hydro-geomorphological significance. Earth Science Review 51, 33-65. Doerr SH, Ferreira AJD, Walsh RPD, Shakesby RA, Leighton-Boyce G, Coelho C.O.A. (2003) Soil water repellency as a potential parameter in rainfall-runoff modelling: experimental evidence at point to catchment scales from Portugal. Hydrological Processes 17, 363-377. Doerr SH, Blake WH, Shakesby RA, Stagnitti F, Vuurens SH, Humphreys GS, Wallbrink P (2004) Heating effects on water repellency in Australian eucalypt forest soils and their value in estimating wildfire soil temperatures. International Journal of Wildland Fire 13, 157-163. Doerr SH, Blake WH, Shakesby RA, Humphreys GS, Wallbrink PJ. Effects of contrasting wildfire severity on soil wettability in Australian eucalypt catchments. Submitted to Journal of Hydrology, (accepted subject to revision, May 2005). Elias F, Ruiz L (1979) ‘Precipitaciones máximas en España’. Monografía 21, ICONA, Ministerio de Agricultura, 545 pp. Giovannini G, Lucchesi S (1983) Effect of fire on hydrophobic and cementing substances of soil aggregates. Soil Science 136, 231-236. Hallett PD, Ritz K, Wheatley RE (2001). Microbial derived water repellency in soil. International Turfgrass Society Research Journal 9, 518-524. Hester JW, Thurow TL, Taylor CA (1997) Hydrologic characteristics of vegetation types as affected by prescribed burning. Journal of Range Management 50, 199-204. Imeson AC, Verstraten JM, van Mulligen EJ, Sevink J (1992) The effect of fire and water repellency on infiltration and runoff under Mediterranean type forest. Catena 19, 345-361. Jex GW, Bleakley BH, Hubbell DH, Munro LL (1985) High humidity-induced increase in water repellency in some sandy soils. Soil Science Society of America Journal 49, 1177-1182. Leighton-Boyce G, Doerr SH, Shakesby RA, Walsh, RPD, Ferreira AJD, Boulet AK, Coelho COA (2005) Temporal dynamics of water repellency and soil moisture in eucalypt plantations, Portugal. Australian Journal of Soil Research 43, 269-280. International Journal of Wildland Fire (14[4], 2005), in press

8

Letey J (1969) Measurement of contact angle, water drop penetration time, and critical surface tension. In ‘Proceedings of a Symposium on Water Repellent Soils’. (Eds DeBano LF, Letey J) pp. 43-47, (University of California: Riverside CA) López Bérmudez F (1990) El clima mediterráneo semiárido como factor de erosión. Estudios Geográficos 199-200, 489-506. Llovet López J (2005) ‘Degradación del suelos posterior al fuego en condiciones mediterráneas. Identificación de factores de riesgo.’ Unpublished Doctoral Thesis, Departament d'Ecologia, Universitat d'Alacant, 188 pp. Mataix-Solera J, Doerr SH (2004) Hydrophobicity and aggregate stability in calcareous topsoils from fire-affected pine forests in southeastern Spain. Geoderma 118, 77-88. Meyer LD (1988) Rainfall simulators for soil conservation research. In ‘Soil Erosion Research Methods’ (Ed. Lal R), Soil and Water Conservation Society, pp. 74-95, Ankeny, Iowa. Moody JA, Martin DA (2001a) Initial hydrologic and geomorphic response following a wildfire in the Colorado Front Range. Earth Surface Processes and Landforms 26, 1049-1070. Moody JA, Martin DA (2001b) Post-fire, rainfall intensity-peak discharge relations for three mountainous watersheds in the western USA. Hydrological Processes 15, 2981-2993. Morgan RPC (1986) ‘Soil Erosion and Conservation’. Longman, New York, 298 pp. Naveh Z (1974) Effects of fire in the Mediterranean region. In ‘Fire and Ecosystems’ (Eds Koxlowski T, Ahlgren CE), pp. 372-390, Academic Press, New York. Neary DG, Klopatek CC, DeBano LF, Ffolliott PF (1999) Fire effects on belowground sustainability: a review and synthesis. Forest Ecology and Management 122, 51-71. Olcina J (1994) ‘Riesgos climáticos en la Península Ibérica’. Libros Penthalon, Madrid, 439 pp. Paul EA, Clark FE (1996) ‘Soil Microbiology and Biochemistry’. Academic Press, New York, 340pp. Pérez Cueva AJ (1994) ‘Atlas climático de la Comunidad Valenciana’. Generalitat Valenciana, València, 243 pp. Reid KD, Wilcox BP, Breshears DD, MacDonald LH (1999) Runoff and erosion in a pinyon-juniper woodland: influence of vegetation patches. Soil Science Society of America Journal 63, 1869-1879. Ruiz Gallardo JR (2004) ‘Teledetección y SIG en la Asistencia a la actuación forestal postincendio. Método de estimación de la prioridad de intervención forestal. Análisiad de tres casos de estudio’. Universidad de Castilla-La Mancha, Albacete, 251 pp. Robichaud PR (2000) Fire effects on infiltration rates after prescribed fire in northern rocky mountain forest, USA. Journal of Hydrology 231-232, 220-229. Sánchez JR, Mangas VJ, Ortiz C, Bellot J (1994) Forest fire effects on soil chemical properties and runoff. In 'Soil erosion and degradation as a consequence of forest fires’. (Eds Sala M, Rubio JL) pp. 53-65, Geoforma Ediciones, Logroño, Spain. Scott DF (2000) Soil wettability in forested catchments in South Africa: as measured by different methods and as affected by vegetation cover and soil characteristics, Journal of Hydrology 231-232, 87-104. Shakesby RA, Coelho C de OA, Ferreira AJD, Terry JP, Walsh RPD (1993) Wildfire impacts on soil erosion and hydrology in wet Mediterranean forest, Portugal. International Journal of Wildland Fire 3, 95-110. Shakesby RA, Doerr SH, Walsh RPD (2000) The erosional impact of soil hydrophobicity: current problems and future research directions. Journal of Hydrology 231-232, 178-191. Shakesby RA, Coelho COA, Schnabel S, Keizer JJ, Clarke MA, Lavado Contador JF, Walsh RPD, Ferreira AJD, Doerr SH (2002) Dehesas and montados: do they represent low erosion-risk Mediterranean land uses? Land Degradation and Development 13, 129140. Simanton JR, Emmerich WE (1994) Temporal variability in rangeland erosion processes. In ‘Variability of Rangeland Water Erosion Processes’ (Eds Blackburn WH, Pierson FB Jr, Schuman GE, Zartman R), pp. 51-65. Soil Science Society of America Special Publication 38. Soto B, Basanta R, Benito E, Perez R, Diaz-Fierros F (1994) Runoff and erosion from burnt soils in northwest Spain. In 'Soil erosion and degradation as a consequence of forest fires’. (Eds Sala M, Rubio JL) pp. 91-98. Geoforma Ediciones, Logroño, Spain. Trabaud L (1981) Man and fire: impacts on Mediterranean vegetation. In ‘Mediterranean-Type Shrublands’. (Eds di Castri F, Goodall DW, Specht RL), pp. 523-537. Elsevier, Amsterdam. Úbeda X, Sala M (1998) Variation in runoff and erosion in three areas with different fire intensities. Geoökodynamik 19, 179-188. Wolman MG, Miller JP (1960) Magnitude and frequency of forces in geomorphic processes. Journal of Geology 68, 54-74.

International Journal of Wildland Fire (14[4], 2005), in press

9

International Journal of Wildland Fire (14[4], 2005), in press

10

Table 1. Monthly rainfall at the nearest meteorological station at Xàtiva (see Figure 1). The wildfire took place prior to the 1.5 mm rainfall event in August 1989. Values are rounded to the nearest mm.

A

Year

Jan.

Feb.

March

April

May

June

July

Aug.

Sept.

Oct.

Nov.

Dec.

Total

1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 Average

102 372 347 20 2 14 0 56 104 77 89 47

38 1 111 213 304 1.5 48 41 0 8 15 0 33

158 77 131 37 39 5 95 53 6 0 97 15 54

36 102 43 6 25 89 24 13 170 7 24 20 47

52 95 13 209 36 10 2 55 15 40 9 13 61

15 0 17 123 42 0 6 3 45 7 1 28 14

0 6 4B 6 65 7 0B 2 8 0 12 0 10

5A 1B 38 0B 0 7 163 0 0B 22 0 0B 4

398 58 12 0 13 129 8 289 438 31 68 4 107

27 177A 377A 71 205 115 105A 17 31 1 65 451 69

164 26 6 0 124 39 12 56 11 33 95 9A 64

310 33 70.2 288B 10 2 124 89 149A 23 17 58

1301 949 1164 975 866 420 588 676 979 646 548

Sampling carried our under wet conditions; B sampling carried out under dry conditions.

International Journal of Wildland Fire (14[4], 2005), in press

11

Table 2. Post-fire evolution of total vegetation and litter cover (plant, trees, moss and herbs) in percent at each individual plot over the whole period of investigation measured for predominantly wet (autumn, winter) and dry (summer) conditions. Primary species on plot Season Trees Pinus halepensis Pinus halepensis Herbs Brachypodium retusum Brachypodium retusum Shrubs Pistacia lentiscus Juniperus oxycedrus Quercus coccifera Dwarf shrubs Ulex parviflorus Erica multiflora Rosmarinus officinalis Cistus albidus Thymus vulgaris Average StDev

1990 wet dry

1991 wet dry

1992 wet dry

1995 wet dry

1997 wet dry

2000 wet dry

15 16

30 19

40 25

42 38

80 60

78 56

120 120

110 121

135 178

125 168

149 195

156 168

60 57

142 96

163 143

175 162

185 150

165 132

250 241

168 169

269 265

175 158

287 286

268 245

24 11 53

38 35 69

69 45 95

75 58 110

97 78 130

79 86 125

124 130 165

112 130 145

135 145 178

125 140 165

169 168 210

174 135 198

16 3 37 41 39 31.0 18.6

30 25 45 54 84 55.6 34.8

35 31 65 70 110 74.3 43.1

45 56 72 89 115 86.4 43.8

35 42 74 95 160 98.8 45.6

42 48 75 110 145 95.1 38.1

78 98 115 115 158 142.8 51.0

85 89 114 110 158 125.9 27.3

109 124 128 165 158 165.8 49.7

102 136 135 145 158 144.3 20.4

165 125 135 199 165 187.8 50.1

156 154 125 189 154 176.8 40.8

Table 3. Water Drop Penetration Times (WDPT in seconds; average values of ten drops) at each plot measured over the whole period of investigation for predominantly wet (autumn, winter) and dry (summer) conditions. Cells with WDPTs > 5 s (i.e. hydrophobic conditions) are highlighted with dark shading. Primary species on plot Season Trees Pinus halepensis Pinus halepensis Herbs Brachypodium retusum Brachypodium retusum Shrubs Pistacia lentiscus Juniperus oxycedrus Quercus coccifera Dwarf shrubs Ulex parviflorus Erica multiflora Rosmarinus officinalis Cistus albidus Thymus vulgaris

1990 wet dry

1991 wet dry

1992 wet dry

1995 wet dry

1997 wet dry

1.36 1.25

2.36 2.05

5.25 4.23

5.36 6.35

6.35 7.32

10.25 9.65

7.52 7.36

15.65 18.25

8.65 8.25

24.65 25.32

10.21 9.36

23.51 24.65

0.50 0.62

2.32 2.30

1.25 1.14

3.25 3.15

1.36 2.25

2.32 3.01

2.25 1.25

2.65 2.01

1.25 1.25

2.32 1.85

2.32 1.25

2.03 1.96

1.02 0.59 0.58

3.25 2.15 2.58

1.26 1.25 1.54

4.25 2.15 3.35

1.24 1.85 1.96

2.65 2.36 2.45

1.36 2.25 1.45

2.36 2.14 2.65

2.32 1.52 1.96

1.58 1.95 1.65

0.99 2.25 1.23

1.75 1.45 1.78

0.69 0.85 0.75 1.25 1.36

2.41 2.32 2.14 2.35 2.74

0.95 1.26 1.38 1.56 1.25

4.25 4.32 3.25 2.65 3.25

2.35 2.48 2.65 2.58 3.25

2.48 2.35 2.14 3.10 2.65

1.69 2.85 1.96 2.35 2.98

2.48 2.35 2.31 2.45 2.15

1.85 1.75 2.35 1.25 1.25

1.48 2.35 2.14 1.24 1.65

1.08 1.47 2.32 0.98 0.58

2.02 1.36 2.45 1.98 1.65

International Journal of Wildland Fire (14[4], 2005), in press 12

2000 wet dry

Table 4. Overland flow coefficient (% of rainfall) after the forest fire (August 1989) for each plot over the whole period of investigation measured for predominantly wet (autumn, winter) and dry (summer) conditions. Primary species on plot Season Trees Pinus halepensis Pinus halepensis Herbs Brachypodium retusum Brachypodium retusum Shrubs Pistacia lentiscus Juniperus oxycedrus Quercus coccifera Swarf shrubs Ulex parviflorus Erica multiflora Rosmarinus officinalis Cistus albidus Thymus vulgaris Average StDev

1990 wet dry

1991 wet dry

wet

1992 dry

70.00 17.65 54.60 32.00 42.56 47.80 15.32 32.70 36.24 65.32

1997

2000

dry

wet

dry

wet

dry

30.25 24.35

35.26 34.26

45.00 36.50

25.60 35.65

33.15 36.90

25.35 28.15

29.35 24.30

1.90 11.30

1.02 0.35

2.32 3.56

0.00 0.00

0.00 0.00

0.00 0.00

0.00 0.00

0.00 0.00

0.00 0.00

0.00 0.00

35.60 10.25 21.10 49.10 16.35 32.50 29.00 9.00 10.40

2.36 5.69 1.03

6.35 6.89 4.65

0.36 1.05 0.65

0.00 0.00 0.00

0.00 0.00 0.00

0.00 0.00 0.00

0.00 0.00 0.00

0.00 0.00 0.00

0.00 0.00 0.00

12.36 15.65 10.25 11.32 8.36 11.52 4.02

10.25 10.36 12.54 11.36 9.65 11.07 11.67

15.36 16.35 18.56 17.98 15.36 17.94 18.47

3.50 3.45 3.26 4.65 2.63 6.18 10.07

2.23 1.35 2.65 3.65 0.25 6.64 13.20

2.35 1.26 2.35 2.65 1.05 7.60 15.62

1.25 1.39 2.14 2.25 0.56 5.74 11.85

0.65 1.05 0.98 0.87 0.48 6.17 13.51

2.32 1.24 0.98 0.97 1.25 5.02 10.19

0.87 0.05 0.35 0.59 0.25 4.65 10.42

12.00 30.00

49.20 60.50 53.10 55.70 43.70 44.64 15.80

5.36 6.35

1995 wet

43.90 30.00 44.30 34.70 23.20 28.38 15.54

Table 5. Post-fire soil erosion rates (g m-2 h-1) during 55 mm h-1 rainfall simulations for each plot over the whole period of investigation measured for predominantly wet (autumn, winter) and dry (summer) conditions. Primary species On plot Season Trees Pinus halepensis Pinus halepensis Herbs Brachypodium retusum Brachypodium retusum Shrubs Pistacia lentiscus Juniperus oxycedrus Quercus coccifera Dwarf shrubs Ulex parviflorus Erica multiflora Rosmarinus officinalis Cistus albidus Thymus vulgaris Average StDev

1990 wet

dry

1991 wet

dry

1992 wet

1995

1997

2000

dry

wet

dry

wet

dry

wet

dry

164.0 52.23 37.54 37.66 14.75 65.73 42.13 18.34 25.11 18.68

16.97 9.24

6.79 7.91

5.69 3.01

4.51 2.75

1.64 1.62

2.09 1.55

1.45 1.07

16.37 12.85 58.08 16.90

0.26 2.24

0.13 0.13

0.03 0.04

0.00 0.00

0.00 0.00

0.00 0.00

0.00 0.00

0.00 0.00

0.00 0.00

0.00 0.00

47.97 22.44 11.84 98.57 48.74 18.41 41.15 18.27 3.89

3.01 7.67 1.12

0.91 0.57 0.23

0.07 0.32 0.07

0.00 0.00 0.00

0.00 0.00 0.00

0.00 0.00 0.00

0.00 0.00 0.00

0.00 0.00 0.00

0.00 0.00 0.00

31.07 36.58 21.37 35.18 22.90 30.05 12.96

13.30 11.45 16.62 16.56 16.35 12.43 11.25

2.96 5.85 8.68 3.46 1.61 4.81 6.20

1.25 1.69 1.70 2.15 0.78 2.85 5.12

0.39 0.19 0.99 0.30 0.06 1.39 2.81

0.16 0.02 0.41 0.16 0.01 0.79 1.76

0.18 0.11 0.56 0.11 0.11 0.69 1.43

0.01 0.01 0.01 0.01 0.00 0.28 0.63

0.06 0.08 0.08 0.04 0.10 0.33 0.70

0.17 0.00 0.03 0.08 0.03 0.24 0.49

63.86 114.8 68.63 139.7 78.11 79.75 42.47

30.42 22.28 32.41 24.81 15.82 18.19 12.11

Table 6. Sediment concentration in the overland flow (g L-1) for each plot over the whole period of investigation measured for predominantly wet (autumn, winter) and dry (summer) conditions. International Journal of Wildland Fire (14[4], 2005), in press 13

Primary species on plot Season Trees Pinus halepensis Pinus halepensis Herbs Brachypodium retusum Brachypodium retusum Shrubs Pistacia lentiscus Juniperus oxycedrus Quercus coccifera Swarf shrubs Ulex parviflorus Erica multiflora Rosmarinus officinalis Cistus albidus Thymus vulgaris Average StDev

wet

1990 dry

wet

1991 dry

wet

1992 dry

1995

1997

2000

wet

dry

wet

dry

wet

dry

4.26 2.50

5.38 5.00

1.25 1.02

2.14 1.26

0.63 0.52

1.02 0.69

0.35 0.42

0.23 0.15

0.32 0.14

0.09 0.08

0.15 0.10

0.09 0.08

2.48 3.52

4.36 4.84

0.25 0.36

0.23 0.65

0.02 0.02

0.00 0.00

0.00 0.00

0.00 0.00

0.00 0.00

0.00 0.00

0.00 0.00

0.00 0.00

2.45 3.65 2.58

3.98 5.42 3.69

1.02 1.03 0.68

2.32 2.45 1.98

0.26 0.15 0.09

0.36 0.56 0.20

0.00 0.00 0.00

0.00 0.00 0.00

0.00 0.00 0.00

0.00 0.00 0.00

0.00 0.00 0.00

0.00 0.00 0.00

2.36 3.45 2.35 4.56 3.25 3.12 0.78

4.57 4.25 3.79 5.65 4.98 4.66 0.66

1.26 1.35 1.33 1.30 1.24 1.01 0.38

2.36 2.01 2.41 2.65 3.08 1.96 0.84

0.35 0.65 0.85 0.35 0.19 0.34 0.27

0.65 0.89 0.95 0.84 0.54 0.56 0.35

0.32 0.25 0.68 0.15 0.45 0.22 0.23

0.13 0.03 0.32 0.11 0.01 0.08 0.11

0.26 0.15 0.48 0.09 0.36 0.15 0.17

0.02 0.02 0.01 0.02 0.02 0.02 0.03

0.05 0.12 0.14 0.08 0.14 0.07 0.06

0.35 0.02 0.15 0.26 0.21 0.10 0.12

International Journal of Wildland Fire (14[4], 2005), in press 14

Figure 1. Location map of the study area in eastern Spain.

International Journal of Wildland Fire (14[4], 2005), in press 15

Figure 2. View of the rainfall simulation equipment. The rainfall is provided by a nozzle 2 m above the ground. The tent prevents wind from distorting the rainfall distribution.

International Journal of Wildland Fire (14[4], 2005), in press 16

Figure 3. View of the four types of vegetation: (a) trees (understorey of Pinus halepensis) (b) herbs (Brachypodium retusum), (c, d) shrubs (Juniperus oxycedrus and Pistacia lentiscus), and (e, f) dwarf shrubs (Erica multiflora and Cistus albidus).

a

b

c

d

e

f

Figure 4. Average overland flow coefficient, sediment concentration and erosion rates expressed as a percentage of the maximum value measured, following the fire in 1989 (first measurement in 1990) up to 2000 (11 years later) for dry (summer) and wet (winter) conditions.

International Journal of Wildland Fire (14[4], 2005), in press 17

Percentage the maximum measured Percentage ofto maximum valuevalue measured

120

100

Sediment concentration

80 Overland flow coefficient

60

40

20

0 Erosion rate

0

2

4

6

8

Years since burning

International Journal of Wildland Fire (14[4], 2005), in press 18

10

12

Figure 5. Post-fire overland flow coefficient (% of rainfall) evolution for dry and wet antecedent conditions over the study period distinguishing between plots covered by trees, herbs, shrubs and dwarf shrubs.

70

Overland flow coefficient (%)

60

Wet conditions

50

40

30

Trees

Dwarf shrubs

20

Shrubs 10

Herbs 0

Dry conditions

Overland flow coefficient (%)

60

40

20

0

1990

1992

1994

International Journal of Wildland Fire (14[4], 2005), in press 19

1996

1998

2000

Figure 6. Post-fire erosion rate evolution for dry and wet antecedent conditions over the study period distinguishing between plots covered by trees, herbs, shrubs and dwarf shrubs.

120

Trees Wet conditions Dwarf shrubs

2

-1

Erosion m 2h Erosionrate rate(g(g/m /h))

100

80

Shrubs

60

40

20

Herbs 0

Dry conditions

2

-1

Erosion rate (g m 2h ) Erosion rate (g/m /h)

60

40

20

0

1990

1992

1994

International Journal of Wildland Fire (14[4], 2005), in press 20

1996

1998

2000

Figure 7. Sediment concentration in overland flow for dry and wet antecedent conditions over the study period distinguishing between plots covered by trees, herbs, shrubs and dwarf shrubs.

Sediment concentration (g l-1)

4

Wet conditions 3

2

1

0

6

Dry conditions

Sediment concentration (g l-1)

Trees 5

4

3

2

Dwarf shrubs 1

Shrubs Herbs

0

1990

1992

International Journal of Wildland Fire (14[4], 2005), in press 21

1994

1996

1998

2000

Figure 8. Overland flow curves for rainfall simulations (55 mm h-1) on Pinus halepensis covered plots following dry antecedent conditions in 1990 and 2000. Overland flow coefficients were 17.65 % and 29.35 % 1 and 11 years after the fire respectively.

30

Runoff curves on Pinus halepensis plot

Overland flow (mm h-1)

25

2000 (11 years after the fire) 20

15

10

5

1989 (1 year after fire) 0 0

10

20

30

40

Time (minutes)

International Journal of Wildland Fire (14[4], 2005), in press 22

50

60

70

Figure 9. Time to ponding (TP) for dry and wet antecedent conditions over the study period distinguishing between plots covered by trees, herbs, shrubs and dwarf shrubs.

4000

Time to ponding (seconds)

Wet conditions 3000

Herbs Shrubs 2000

Dwarf shrubs 1000

Trees 0 4000

Time to ponding (seconds)

Dry conditions 3000

2000

1000

0

1990

1992

1994

International Journal of Wildland Fire (14[4], 2005), in press 23

1996

1998

2000

Figure 10: Shape and position of the wetting front (summer experiments) as observed after rainfall simulations in 1990, 1995 and 2000 for one plot per vegetation type.

.

.

.. .. .

.... . .

.

.

.. .. .

..

International Journal of Wildland Fire (14[4], 2005), in press 24

. .. . . . . . ... . .... . . ... ... . .... ....... . . . . . . .. .. .. ... ... ..... ....... ... .. . .. ....... 2000

.

. . . .... . . ... . . ... .. .... .... .. . ... .. . ...... ... .. ... .. ........... .. .. .. ........ ..... . .. .... . ..... . ..... .. . .. .. ..... .... .. .. .. . .. . . . . ... . ..... .. . . ..... . .... ...

.

..

10 cm

.... . .

2000

1995

10 cm

... ... .. ...... . .. ... ... . .... .. ...... ......... ...... .. ............ .... ............... . .. . . . . . . . . . . .. .... ... . .... ....... ....... ... ......... ... .... ................. .. .. . . . .. .. .. . .. . . . . ... ... . . . .. .. .. . .. ..... . . . . . .. .. .

. .. . . . . . ... . .... . . ... ... . .... ....... . . . . .... .. .. .. ... ... .... ....... ... .. . .. .

........

.. . .... . . ... . . ... .. .... .... .... ... ... . . .... .. .. ... . . ......... .. .. .. ....... ....... . .... ...... . ..... ... .. .. ... .... ... .

... .... .. ... ... . .. ... ... . .... .. ....... ......... ...... ................ .... ... ............ . .. . . . . . . . . . .. .... ... . .... .............. .. ......... .... .... ........ ......... . . . . ... ... . ... . . .

. .. .. .... . . ... .. .. . . . . ..

1990

10 cm

1995

. . .... . . .. . . ... .... . .... . . ... ... . ...... .. .. ... .. . .. ....... .. .. ... ... .....

.

..

1990

.................. .. .. .... .... ... .. ... .... .... .

........

. .. .. .

wet soil

.

wetting front

.. . .... . . ... . . ... . .. . .... . . ... .. . ..... .. .. ... .. ............... .. .. .... ... ..... . . . .. . . . .. . . .

... .... .. ...... .... ... ... . .... ... ....... ......... ..... ....... ....... .... ................ . .. . . . . . . . . .. .. .. ...

Dwarf shrubs (various species)

. .. .. .

.................. .. ... .... ... ... .. ... .... ...... . . . . . . ... .. ... .. .... ..

Shrubs (various species)

.

Herbs (Brachypodium retusum)

.

Trees (Pinus halepensis)

Suggest Documents