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Canopy characteristics and growth rates of ponderosa pine and Douglas-fir at longestablished forest edges Kelsey Sherich, Amy Pocewicz, and Penelope Morgan
Abstract: Trees respond to edge-to-interior microclimate differences in fragmented forests. To better understand tree physiological responses to fragmentation, we measured ponderosa pine (Pinus ponderosa Dougl. ex P. & C. Laws) and Douglasfir (Pseudotsuga menziesii (Mirbel) Franco) leaf area, crown ratios, sapwood area, basal area (BA) growth rates, and BA growth efficiency at 23 long-established (>50 year) forest edges in northern Idaho. Trees located at forest edges had more leaf area, deeper crowns, higher BA growth rates, and more sapwood area at breast height than interior trees. Ponderosa pine had significantly higher BA growth efficiency at forest edges than interiors, but Douglas-fir BA growth efficiency did not differ, which may relate to differences in photosynthetic capacity and drought and shade tolerance. Edge orientation affected BA growth efficiency, with higher values at northeast-facing edges for both species. Edge effects were significant even after accounting for variation in stand density, which did not differ between the forest edge and interior. Although edge trees had significantly greater canopy depth on their edge-facing than forest-facing side, sapwood area was evenly distributed. We found no evidence that growing conditions at the forest edge were currently subjecting trees to stress, but higher leaf area and deeper crowns could result in lower tolerance to future drought conditions. Re´sume´ : Les arbres re´agissent aux diffe´rences de microclimat qui existent entre la bordure et l’inte´rieur des foreˆts fragmente´es. Pour mieux comprendre les re´actions physiologiques des arbres a` la fragmentation, nous avons mesure´ la surface foliaire, le rapport de cime, la surface d’aubier, le taux de croissance en surface terrie`re (ST) et l’efficacite´ de croissance en ST du pin ponderosa (Pinus ponderosa Dougl. ex P. & C. Laws) et du douglas vert (Pseudotsuga menziesii (Mirbel) Franco) dans 23 bordures forestie`res e´tablies depuis longtemps (plus de 50 ans) au nord de l’Idaho, aux E´tats-Unis. Les arbres situe´s le long d’une bordure forestie`re avaient une plus grande surface foliaire, une cime plus longue, un plus fort taux de croissance en ST et plus de superficie d’aubier a` hauteur de poitrine que les arbres situe´s a` l’inte´rieur de la foreˆt. L’efficacite´ de croissance en ST des pins ponderosa situe´s le long d’une bordure forestie`re e´tait significativement plus grande que celle des pins situe´s a` l’inte´rieur de la foreˆt, mais pas dans le cas du douglas vert, ce qui peut eˆtre associe´ a` des diffe´rences de capacite´ photosynthe´tique et de tole´rance a` la se´cheresse et a` l’ombre. L’orientation de la bordure affectait l’efficacite´ de croissance en ST des deux espe`ces; elle e´tait plus e´leve´e dans les bordures faisant face au nord-est. Les effets de bordure e´taient significatifs meˆme en tenant compte de la variation de la densite´ des peuplements qui, toutefois, n’e´tait pas diffe´rente entre la bordure et l’inte´rieur de la foreˆt. Meˆme si la cime des arbres du coˆte´ faisant face a` la bordure e´tait significativement plus longue que celle du coˆte´ faisant face a` la foreˆt, la superficie d’aubier e´tait uniforme´ment distribue´e. Nous n’avons trouve´ aucun indice indiquant que les conditions de croissance en bordure de la foreˆt soumettaient les arbres a` des stress imme´diats, mais une plus grande surface foliaire et une cime plus longue pourraient diminuer la tole´rance des arbres a` d’e´ventuelles conditions de se´cheresse. [Traduit par la Re´daction]
Introduction Forest fragmentation resulting from agricultural conversion, timber harvesting, and other land uses creates distinct edges between remaining forests and the adjacent cleared land. The structural contrast in vegetation at such edges often results in a gradient in forest microclimate from the forest edge to interior. Temperature, light availability, and Received 2 October 2006. Accepted 29 May 2007. Published on the NRC Research Press Web site at cjfr.nrc.ca on 9 November 2007. K. Sherich, A. Pocewicz,1 and P. Morgan. Department of Forest Resources, University of Idaho, P.O. Box 441133, Moscow, ID 83844-1133, USA. 1Corresponding
author (e-mail:
[email protected]).
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wind speed decrease from the forest edge to interior, whereas relative humidity increases (Matlack 1993; Chen et al. 1995). These gradients may affect physiological processes in vegetation and thus influence productivity at forest edges. Resulting changes in forest productivity could be widespread, given that 62% of forested lands in the continental United States are located within 150 m of a forest edge (Riitters et al. 2002). Although numerous researchers have contrasted forest edges and interiors (Matlack 1993, 1994; Fraver 1994; Chen et al. 1995; Harper and Macdonald 2002; Harper et al. 2004), few studies have considered how important canopy structural attributes (e.g., leaf area) and measures of productivity (e.g., tree growth rates) are influenced by these patterns (Chen et al. 1992; Cienciala et al. 2002; McDonald and Urban 2004). Tree leaf area may be affected as light, water, and nu-
doi:10.1139/X07-105
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trient availability change with varying microclimate within the forest edge zone. Leaf area, the total surface area of a tree’s foliage, influences the amount of photosynthetically active radiation absorbed (Gholz 1982; Bonan 1993). However, variation in leaf area between forest edges and interiors has only been studied in tropical ecosystems (Harper et al. 2005). Furthermore, the orientation of an edge can also influence the forest microclimate and the strength of edge gradients (Matlack 1993; Chen et al. 1995; Murcia 1995; Gehlhausen et al. 2000). In northern temperate forests, south-facing forest edges receive more solar radiation than north-facing edges (Matlack 1993), which may result in more pronounced increases in leaf area at south-facing edges. Increases in the leaf area of trees located in forest edge environments may lead to increases in photosynthetic capacity and tree growth rates. Cienciala et al. (2002) found increased stem growth increments in Scots pines (Pinus sylvesteris L.) located at a clear-cut forest edge, relative to trees located in the interior forest. Growth rates of canopydominant Douglas-fir (Pseudotsuga menziesii (Mirbel) Franco) and western hemlock (Tsuga heterophylla (Raf.) Sarg.) have also increased near clear-cut forest edges, with growth rates decreasing with distance from the forest edge (Chen et al. 1992). Wider growth rings were also found at the forest edge relative to interior in two tree species in the southeastern US (McDonald and Urban 2004). Tree basal area (BA) growth efficiency, the amount of production per unit leaf area, may also be greater at forest edges than interiors. Waring (1983) found that Douglas-fir trees exposed to greater light levels had increased growth efficiency, but only until the optimal leaf area was attained. The shade tolerance of a tree species may also affect its growth efficiency. Ponderosa pine (Pinus ponderosa Dougl. ex P. & C. Laws) is shade-intolerant, whereas Douglas-fir has intermediate shade tolerance (Burns and Honkala 1990), and ponderosa pine has shown higher growth efficiency than Douglas-fir (Gersonde and O’Hara 2005). Changes in the spatial allocation of the tree canopy occur at forest edges. Trees grow towards areas of decreased competition and increased resource availability (Rouvinen and Kuuluvainen 1997; Muth and Bazzaz 2002, 2003). Trees located at forest edges have greater exposure to light on the side adjacent to the opening and are more likely to develop advantageous limbs in that direction (Matlack 1994; Muth and Bazzaz 2002); this would result in increased leaf area of edge trees. Muth and Bazzaz (2003) found that trees responded to spatial variation in light availability primarily by differential growth and survival of branches rather than stem leaning. Trees located along a forest edge would be expected to exhibit canopy unevenness, with higher crown ratios (canopy depth divided by tree height) on the edgefacing side that has no competing neighbors. Consistent with these expectations, Douglas-fir tree crowns located at forest gap edges were 62% wider and 36% deeper than those located within the closed canopy (Wardman and Schmidt 1998). Unbalanced canopies (deeper crowns on one side of a tree) may lead to unbalanced sapwood distribution. Sapwood, the conductive xylem, transports water and nutrients to support the crown of a tree. Our research objectives were to compare leaf area, BA growth rate, and BA growth efficiency between forest edges
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and interiors and to determine if trees located at forest edges have significantly greater canopy depth and more sapwood area on the edge-facing side of the tree. Our sampling sites were located at long-established (>50 year) maintained and natural forest edges bordering agricultural lands, previously cultivated grasslands, or relatively undisturbed prairies and meadows. This variety in land use types provided us with a gradient related to nitrogen fertilization influences (Pocewicz et al. 2007), and we expected that trees at forest edges receiving greater nitrogen inputs would be more productive. We addressed the following research questions related to tree canopy and growth variables (leaf area, crown ratio, sapwood area, BA growth rate, and BA growth efficiency). (i) Are tree canopy and growth variables greater at forest edges than interiors (>50 m) and what is the magnitude of edge influence (Harper et al. 2005)? (ii) Are tree canopy and growth variables greater at forest edges facing southwest versus northeast? (iii) Are tree canopy and growth variables greater at edges bordering agricultural land uses? (iv) Do trees located at the forest edge have significantly greater crown ratios and more sapwood area on the side of the tree facing the edge (open side) than the side facing the forest interior (closed side)?
Methods Study sites Our study sites were located in western Latah and Benewah counties (4763 km2) in northern Idaho. The town of Potlatch, Idaho, is centrally located within the study area (478N, 116.98W). This region is located at the interface of the Palouse Prairie and Bitterroot Mountains ecoregions and is dominated by forest (60%), agriculture (21%), and shrub and grasslands (17%) (Scott et al. 2002). Our focus was on low-elevation forests dominated by ponderosa pine or rocky mountain Douglas-fir (Pseudotsuga menziesii var. glauca (Beissn.) Franco). Other tree species occurring at sampling sites were grand fir (Abies grandis (Dougl. ex D. Don) Lindl.), western larch (Larix occidentalis Nutt.), lodgepole pine (Pinus contorta var. latifolia Engelm. ex S. Wats.), and western redcedar (Thuja plicata Donn ex. D. Don). The soils are deep, well-drained silt loams of the orders Mollisols, Inceptisols, and Alfisols that were formed primarily in loess, with volcanic ash, granite, and metasedimentary rock contributing to soil development at some study sites (Weisel 1980; Barker 1981). The forest sites ranged in elevation from 820 to 1050 m above sea level. Annual precipitation averages 63 cm (1915–2005) in Potlatch, Idaho (Western Regional Climate Center, www.wrcc.dri.edu). Most precipitation is received from November to May, whereas only 8 cm is received on average during the warmest portion of the growing season from July to September. Mean minimum and maximum temperatures are –6.2 and 2.1 8C in January and 7.6 and 28.2 8C in July, respectively. We collected data at 23 forest edge sites adjacent to three different land use types that represented gradients in nitrogen fertilization history and edge sharpness. These included agricultural lands that had well-defined edges maintained by field cultivation (nine sites), formerly cultivated grasslands that had well-defined edges once maintained by field cultivation at most sites and some edge expansion with small #
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trees up to 50 m from the primary canopy drip line at others (nine sites), and five natural or inherent forest edges adjacent to unplowed Palouse prairie or forest meadows. The inherent edges were diffuse, with small trees extending beyond the drip line of the canopy-dominant edge in some locations. The formerly cultivated grasslands are dominated by perennial mat-forming grasses, and most had been out of agricultural production for approximately 20 years. We located 10 of the 23 sites by randomly generating points throughout forests of the two counties, using the Idaho GAP Analysis land cover data set (Scott et al. 2002). We identified forest edges located within a 1 km radius of each random point using recent digital imagery. The remaining 13 sites were chosen purposively to achieve a balance of edge types for comparison. All sampling sites were located at least 1 km apart, and only forests dominated by either ponderosa pine or Douglas-fir were included. Each sampling site was characterized by a 150 to 400 m length of forest edge that was at least 50 years old. At each site, we randomly located two points along the forest edge, which was located at the canopy drip line. These points were located at least 40 m apart and at least 50 m from any other edge. Transects were laid out from these points, running perpendicular to the edge and 50 m into the forest. Two sampling points were established along each transect: at the edge and 50 m into the forest. The forest edges faced a number of azimuth directions and were grouped into two contrasting aspect classes: north to east (nine sites) and south to west (14 sites), because here the prevailing winds are from the south and west. Ranney et al. (1981) found that south and west edges supported tree species typical of more xeric habitats relative to north and east edges. We were unable to consider each cardinal direction independently because we lacked adequate representation of sites across these four groups. Tree measurements Two trees each of ponderosa pine and Douglas-fir were randomly selected, when present, at each forest edge and interior sampling point from among all trees fulfilling desired criteria within predetermined search areas. Search areas were centered on the sampling point and extended 10 m in either direction, parallel with the forest edge. Because the edges were diffuse with relatively open forest canopies, the environment for edge trees was similar for trees within 10 m of the edge. We accepted trees with boles located within 10 m of the main forest drip line as edge trees. Interior tree boles were located between 50 and 75 m from the forest edge. Only ponderosa pine was sampled at 11 of the sites, only Douglas-fir at six, and both species were sampled at six sites. The 192 sampled trees were canopy-dominant or codominant trees, had live crowns that were not forked low on the tree, and had no excessive sap on the bark surface or large fire or lightning scars. We restricted the diameter at breast height (DBH) of the 76 sampled Douglas-fir trees to between 30 and 50 cm and the DBH of the 116 sampled ponderosa pine trees to between 35 and 65 cm. The DBH was recorded, and the trees were cored three times at breast height (1.3 m). One core was oriented towards the forest edge and another directly opposite. The third was between the other two, favoring the uphill side of
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the tree. This method was used to provide a more thorough representation of the sapwood area than would a single core and to examine if there were edge-related trends in sapwood allocation within trees located at the forest edge. The sapwood was identified based on translucency and marked in the field. Bark depths were recorded with a bark gauge at each core location. Dried cores were glued into a routed wooden board, and the sapwood mark was transferred to the board. The cores were sanded with a belt sander and then by hand using fine-grained sandpaper. This permitted us to more easily distinguish individual growth rings. The most recent 5 year radial growth increments were marked on each core. A binocular microscope was used when growth rings were too small to readily distinguish. BA increments were used to determine BA growth rate, because it is a more accurate measure of total tree growth than growth ring width or radial increment (Poage and Tappeiner 2002); if tree growth remains constant over time, radial increment will decrease as the tree diameter increases. The measurements from the three cores for each tree were averaged together. The average 5 year increment and current radial length (inside bark) were used to calculate annual BA growth rate. rðyÞ 2 rðy5Þ 2 ½1 Annual BA growth rate ¼ 5 where r(y) equals current radial length and r(y–5) equals the radial length minus the last 5 years of growth. Leaf area was determined based on the full allometric equations for total tree leaf area (m2) from Monserud and Marshall (1999), as follows: ½2
Douglas-fir total leaf area ¼ b0 X1b1 X3b3
where X1 equals the sapwood BA at breast height (cm2), X3 is the crown ratio, and b0 = 0.65990, b1 = 1.0191, and b3 = 1.1475. ½3
Ponderosa pine total leaf area ¼ b0 X1b1 X4b4
where X1 equals the sapwood BA at breast height (cm2), X4 is the crown depth (m), and b0 = 0.12860, b1 = 0.5166, and b4 = 1.1527. We measured the total height and height to the base of the live crown for each tree using a clinometer or an Impulse 200 Laser Rangefinder (Laser Technology Inc., Englewood, Colorado). When the canopy was uneven, two canopy base heights were recorded, corresponding to the edge-facing and interior-facing orientations, and the average canopy depth was used. Sapwood BA was calculated from the averaged bark thickness and DBH measurements at 1.3 m above the ground, as follows: ½4
Sapwood BA ¼ r 2 rðhÞ 2
where r equals the radial length from pith to cambium and r(h) equals the length from the pith to the sapwood. Growth efficiency has been defined as the grams of wood produced per square metre of leaf area (Waring 1983). BA growth efficiency incorporates both the leaf area and growth rates of individual trees and allows for comparison of efficiency among individuals with differing amounts of leaves. #
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We calculated BA growth efficiency by dividing the BA growth rate (cm2year–1) by the leaf area (m2) of each tree (Nagel and O’Hara 2001). Stand density and site productivity measurements Competition for resources increases as tree density increases (Oliver and Larson 1996) and can influence tree growth rates (Long and Smith 1984). Therefore, relative stand density was measured with respect to each tree. Circular plots with an 8.0 m radius (0.02 ha) were centered on each tree, and DBH of all live trees greater than 4.5 cm was recorded. Stand density index is a common measure of relative stand density that is based on the number of trees per unit ground area if all trees present had a DBH of 25 cm. It was originally proposed by Reineke (1933) for use in even-aged stands. We used a modification of the original stand density index formulation designed for use in uneven-aged stands (Stage 1968) as follows: XDBHi 1:6 ½5 Stand density index ¼ 25 where DBHi is the DBH of each tree (cm). Forest habitat type series (Cooper et al. 1991) was recorded at each sampling point to characterize site quality and associated variation. Habitat type series identify different biophysical settings that are named for the most shadetolerant and moisture-demanding tree species that is present and reproducing successfully. The majority of the sampling sites belonged to ponderosa pine, Douglas-fir, or grand fir habitat series, with the western redcedar series recorded at only three sampling plots. We compared the habitat series with site index at a subset of the sampling sites and found that site index related well to habitat series (unpublished data), indicating that habitat series is a reasonable proxy for differences in site productivity. A relationship between site index and habitat type has also been previously documented in this region (Monserud 1984). Data analysis To test for differences in tree canopy or growth variables between edge and interior locations and among edge trees at different types of sites, we fit three sets of univariate statistical models using the linear mixed-effects model (lme) function of the open-source statistical language R (Pinheiro and Bates 2000; R Development Core Team 2006). Mixed-effects models include random effects that can account for correlated error among observations. In this case, because of the nested structure of our data, residuals from data in the same sampling site, transect, or point may be correlated. We included three nested random effects in each model: site (23), transect nested within site (2), and point nested within transect (2–4 samples per point). Separate models were fit with each level of nested random effects and with no random effects, and analysis of variance (ANOVA) was used to determine which model had the lowest Akaike’s information criterion and thus best fit (Burnham and Anderson 1998). F tests from a sequential ANOVA were used to test for statistical significance at = 0.05, using the model with the best fit. First, we fit models to test for differences in leaf area, BA growth rate, and BA growth efficiency between edge and in-
2099 Table 1. Differences in leaf area, basal area (BA) growth rate, and BA growth efficiency at forest edge and interior locations; sequential ANOVA p values from mixed-effects models are presented.
Predictor variables Tree species Habitat-type series Stand density index Edge–interior location Species: edge–interior location Model diagnostics R2 analog (fixed effects) ICC site ICC transect ICC point
Leaf area (m2)
BA growth rate (cm2year–1)
BA growth efficiency (cm2m–2)
