Responses to Simulated Drought and Elevated ...

Responses to Simulated Drought and Elevated Nutrient Supply among Shade-Tolerant Tree Seedlings of Lowland Tropical Forest in Singapore D. F. R. P. Burslem; P. J. Grubb; I. M. Turner Biotropica, Vol. 28, No. 4, Part B. (Dec., 1996), pp. 636-648. Stable URL: http://links.jstor.org/sici?sici=0006-3606%28199612%2928%3A4%3C636%3ARTSDAE%3E2.0.CO%3B2-Q Biotropica is currently published by The Association for Tropical Biology and Conservation.

Your use of the JSTOR archive indicates your acceptance of JSTOR's Terms and Conditions of Use, available at http://www.jstor.org/about/terms.html. JSTOR's Terms and Conditions of Use provides, in part, that unless you have obtained prior permission, you may not download an entire issue of a journal or multiple copies of articles, and you may use content in the JSTOR archive only for your personal, non-commercial use. Please contact the publisher regarding any further use of this work. Publisher contact information may be obtained at http://www.jstor.org/journals/tropbio.html. Each copy of any part of a JSTOR transmission must contain the same copyright notice that appears on the screen or printed page of such transmission.

The JSTOR Archive is a trusted digital repository providing for long-term preservation and access to leading academic journals and scholarly literature from around the world. The Archive is supported by libraries, scholarly societies, publishers, and foundations. It is an initiative of JSTOR, a not-for-profit organization with a mission to help the scholarly community take advantage of advances in technology. For more information regarding JSTOR, please contact [email protected].

http://www.jstor.org Tue Dec 18 05:54:21 2007

BIOTROPICA 28(4b): 636-648

1996

Responses to Simulated Drought and Elevated Nutrient Supply among Shade-Tolerant Tree Seedlings of Lowland Tropical Forest in Singapore1 D. F. R. P. Burslem2,P. J. Grubb Department of Plant Sciences, Downing Street, Cambridge CB2 3EA, England and

I. M. Turner Department of Botany, National University of Singapore, Lower Kent Ridge Road, Singapore 119075

ABSTRACT Seedlings of three tree species of lowland tropical rain forest in Singapore grown in pots of forest soil at low irradiance were subjected to simulated drought and nutrient addition treatments. The gravimetric soil water contents applied during drought simulations were within the range found during an eight-month survey of soil moisture content at 10 understory sites in primary forest in Singapore. Aspects of plant growth and dry mass allocation were influenced strongly by simulated drought for all three species. Morphological responses to the low-frequency watering regime were similar to those described for herbaceous crop plants subjected to water shortage, and suggest that the tree seedlings used are capable of withstanding substantial periods of low soil water availability. In addition, analysis of covariance showed that height and leaf area growth of seedlings of Antidesma cuspidatum were limited by M g and height growth of seedlings of Vatica maingayi by one or more macro-nutrients. These results suggest that tree seedlings of tropical lowland rain forests may be limited by the availability of soil resources, as well as irradiance, when growing in deep shade.

KT words: drought response; lowland tropicalforest; n ~ h i e n response; t plant physiological ecology; shade-tolerantseedlings; Singapore; tree seedling growth; water relations.

IT IS WELL ESTABLISHED THAT EVEN T H E SEEDLINGS of highly shade-tolerant tree species are strongly limited by the level of irradiance on the floor of tropical lowland rain forest (Whitmore 1984, Chazdon 1988). Other factors potentially limiting the growth of tropical rain forest tree seedlings are the availability of water and nutrients. An irrigation experiment in seasonally dry tropical forest in Panama has shown that the availability of soil water during the dry season may limit the survival and growth of seedlings of Virokz surinamensis in the understory (Fisher et al. 1991), but not the growth of understory shrubs in the same forest (Wright 199 1). N o experiment of this kind has been conducted in the ever-wet, aseasonal tropics (sensu Whitmore 1984), although there has been speculation that the availability of water may sometimes limit the growth of trees in such areas o n the basis of climatic data (Briinig 1969) and water balance calculations (Baillie 1976). The climate of Singapore is representative of Received 23 June 1994, accepted 9 August 1995. Present address: Department of Plant and Soil Science, University of Aberdeen, Cruickshank Building, St. Machar Drive, Aberdeen AB9 2UD, Scotland.

the lowland aseasonal tropics, having no difference between months in mean rainfall determined over a long period (see climate diagram in Walter et al. 1975). However, the absence of any consistent pattern of rainfall distribution may obscure events which cross the artificial boundaries used for recording meteorological information (Briinig 1969, Baillie 1976) and significant short-term variation in patterns of rainfall within months. In fact, longterm meteorological data show that in Singapore a dry spell ( < I m m rainfall every day) longer than two weeks occurs nearly every year o n average and a dry spell longer than four weeks occurs about once a decade (Fig. 1); of these spells a high proportion are absolute droughts ( ~ 0 . m 2 m every day, Fig. 1). During these dry spells, evaporation exceeds precipitation, leading to the development of a considerable soil water deficit (Nieuwolt 1965). The occurrence of frequent unpredictable droughts justifies speculation that limitation by water supply may occur and may interact with limi.tation by other factors. An example of one such interaction is that with nutrient supply: the balance between limitation to g o w t h by N and by P may be influenced by the soil moisture regime (Lloyd & Pigott 1967) because the rate of N mineraliza-

Seedling Responses to Drought and Nutrients

637

STUDY SITE AND SPECIES

o w b a r n o - ~ m t n a b m r n o - ~ m * m - - - - - N N N N N N N N N N m m m m m m

Duration (days)

FIGURE 1. The frequency of dry spells (closed bars) and absolute droughts (open bars) in Singapore for the years 1929-1941 and 1948-1990 (total 56 years). A dry spell is defined as a period of at least 15 days duration during which there is less than 1 mrn of rainfall every day; a drought occurs when there is less than 0.2 rnm of rainfall every day. Data supplied by the Singapore Meteorological Service.

tion is dependent on soil warer status (Marrs et a/. 1991). Bioassay experiments have provided an indication of the extent of limitation by nutrient supply for cuttings or seedlings growing in an andosol taken from tropical rain forest in the lowlands of Costa Rica (Denslow et al. 1987) and in an ultisol taken from coastal hill dipterocarp forest in Singapore (Burslem et al. 1994, 1995). Both studies showed that seedlings of woody species possessing rnycorrhizas were not limited by the availability of P under the experimental conditions used, but in neither case was the particular limiting macro-nutrient identified. Similarly, neither study examined the responses of seedlings to added nutrients at levels of irradiance corresponding to conditions in the shaded understory of natural forest, and potential interactions with soil moisture supply status were not examined. In this paper we address three questions: At an irradiance representative of closed canopy forest, are seedlings of shade-tolerant trees limited by either water or nutrient supply when exposed to soil moisture conditions that simulate, approximately. conditions in the forest? Are there interactions between warer and nutrient supply on seedling growth? For seedlings of Antidesma cuspidatum M.A., shown to be limited by a macro-nutrient other than P by Burslern et a/. (1995), what is the primary limiting macro-nutrient?

The study site was a 71 -ha fragment of mostly primary coastal hill dipterocarp forest on Bukit Timah in the Republic of Singapore (103"501E, 1°20'N). The nutrient status of the forest on Bukit Timah has been described by Grubb et al. (1994), and the tree flora by Corlett (1990). The parent material at Bukit Timah is granite, which gives rise to a very acidic ultisol (pH 3.5-4.2 at 0-10 cm) with low concentrations of total N (0.70-1.88 mg g1at O10 cm) and total P (0.05-0.08 mg g-I at 0-10 cm). The Diprerocarpaceae dominate the tree flora of Bukit Timah in terms of basal area, and Shorea curtisii Dyer ex King and Dipterocarpus caudatus Foxw. ssp. penangzanus (Foxw.) Ashton are particularly abundant (Wong 1987, Corlett 1990). In this respect Bukit Timah is typical of the coastal hill variant (Burgess 1975) of the hill dipterocarp forest type of Wyatt-Smith (1963). Singapore has mean monthly temperatures in the range 26.427.8"C (Dale 1963) and about 2400 m m of annual rainfall (Watts 1955). No month has a mean rainfall less than 140 mm (Corlett 1990), but the distribution of rainfall varies a great deal from year to year, and potential evaporation may be in excess of precipitation for periods of up to six months (Nieuwolt 1965). The species used were Antidesma cuspidatum M.A. (Euphorbiaceae), Hopea gr%fithii Kurz and Vatica maingayi Dyer (both Dipterocarpaceae). All three are shade-tolerant as seedlings and grow in primary forest in Singapore. Previous use in experiments justified the inclusion of Antidesma cuspidatum, while the two dipterocarps were selected o n the basis of availability of seedlings and the commercial importance of the family. Antidesma is a much smaller tree than the two dipterocarps (mature height 10 m cf 40-45 m) and has a smaller fruit (dimensions of drupe of Antidesma 2-5 x 25 rnrn cJ 7 X 5 m m for nut of Hopea, 7 x 7 m m for nut of Vatica). Voucher specimens have been deposited in the herbarium of the National University of Singapore (SINU).

METHODS Moisture content and loss-on-ignition were investigated for soil from 10 high forest sites chosen to reflect the range of slope conditions within the 'core' area of the reserve (i.e.,each site greater than 50 m from the edge) over an eight-month period in 1990. These 10 sites were the same as those used for determinations of the amount of litter at Bukit

638

Burslem, Grubb, and Turner

Timah in August 199 1 (Grubb et al. 1994). Within each site a 1 m2 quadrat was selected at random and top-soil (0-10- cm) samples taken at fourweekly intervals on 27 April-12 October and on 16 November 1990. Samples were double-wrapped in polythene and transferred to the laboratory on the same day, sub-samples oven-dried to constant mass and weighed. O n the first two sampling dates the dried samples were then transferred to a muffle furnace at 550°C for 2 - 4 hours, cooled in a desiccator and reweighed. - Moisture content and loss on ignition are expressed on the basis of wet and dry mass respectively. These methods follow the recommendations of Allen et al. (1986). Seedlings of Antidesma cupidatum and Hopea grzfithii were collected from the understory of primary rain forest at Bukit Timah Nature Reserve in August 199 1 and the Singapore Botanic Gardens' Jungle in October 1991 respectively. Within a day of collection all seedlings were transplanted singly (PVC) tubes (20 cm tall X into p~lyvin~lchloride 5 cm diameter) containing Bukit Timah forest soil and then stored before use in the shade-screen where the ex~erimentwas to be conducted. The Antidesma seedlings were of unknown age, but collected from the same seedling carpet used by Burslem et al. (1995), while those of Hopea were six months old, being derived from a fruiting of late March 199 1. Seedlings of Vatica maingayi were obtained from fruit produced in August and September 1990 by a single tree in Bukit Timah Nature Reserve. Seeds were germinated on the surface of a tray of Bukit ~ i m a hforest soil in deep shade beneath a glasshouse bench. In November 1990 the seed t r a i was transferred to the shaded understory of primary forest at Bukit Timah, and transferred back to the glasshouse in August 1991. Seedlings were transplanted singly into tubes of soil (as above) in September 1991 and then stored in the experimental shade-screen until use. The soil used in this experiment was taken from beneath a primary forest canopy at Bukit Timah in August and September 199 1. Soil from 10-20 cm in the profile was transferred directly into the bottom half of the 20 cm PVC tubes, while that from 0-10 cm was sieved through a 5 x 5 mm mesh and mixed thoroughly befor: being transferred into the top half of each tube. t this The irradiance and redlfar red ~ u o t i e n in experiment were manipulated to reflect conditions in the forest understory. The shade-screen was constructed using a combination of mesh netting and green polyester filter ('Primary Green', LEE Color-

[ran International, Ladbroke Hall, London W10 5 H H , England), and was isolated from adjacent screens by black polythene covering two opposite sides. Six PAR sensors (Didcot Instrument Co. Ltd., Abingdon, Oxon, OX14 3LD, England) were arranged systematically across the shade-screen and allowed to integrate over one week in October 199 1. The procedure was repeated using eight sensors integrating over two days in April 1992 to determine the effect of deterioration of the polyester filter. Comparison with a similar sensor on the roof of the Botany Department building yielded values (mean ? 1 SEM) of 0.9 ? 0.03 percent and 0.5 ? 0.07 percent of full daylight transmission (PAR) for the October and April determinations respectively. The difference may have occurred because weather conditions differed during the two measurement intervals: the incidence of cloudiness affects the daylight factor for shadescreens in Queensland (P. J. Grubb, pers. obs.). A single spot reading of the redlfar-red quotient (sensor manufactured by Skye Instruments Ltd, Units 516, Ddole Industrial Estate, Llandrindod Wells, Powys, LD1 6DF, Wales) at the center of the shade-screen during sunny conditions on 29 April 1992 gave a value of 0.59. The experiment consisted of a bioassay study of nutrient limitation for Antidesma under two different watering regimes and a factorial combination of nutrient addition- and watering treatments for Hopea and Vatica. For Antidesma there were seven nutrient addition treatments under a frequent watering regime: 1, unfertilized control; 2-6, with additions of NH4N03, Na2HP04, KC1, MgS04.7H20 and CaC12.2H20respectively; 7, with additions of all nutrients in treatments 2-6. Treatments 1, 3 and 7 were repeated under a low-frequency watering regime (treatments 8, 9 and 10 respectively). The treatments applied to Hopea and ktica were identical to numbers 1, 7, 8 and 10 of the Antidesma experiment, i.e., the four possible combinations of presence or absence of complete fertilizer addition and the two watering regimes. A more balanced combination of treatments was not possible given the constraints of available seedlings and space in the shade-screen; those chosen reflected the outcome of previous experiments and our interest in the effects of the moisture regime on nutrient limitation to growth. There were 10 replicates of each treatment. The height, stem diameter and dimensions of all leaves were measured for each seedling on 1823 October and pots arranged in a completely randomised design on 24 October. Nutrients were added on three occasions: 25-26 October 1991


639

TABLE 1. Summary of the characteristics of the watering treatments imposed during the experiment. Infrequent watering treatments (above the dashed line) are characterired individually, while the features of the frequent watering treatments (below the dashed line) did not dzffer between nutrient-addition heatWIentS or species (see text). T/aluesare mean (and range) of time to jrst watering (days), the length of interval between subsequent waterings and the p-avimetric soil water content at subsequent waterinfs.

Species

Treatment

Number of times watered

Time to first watering

(days)O

Mean intervals between subsequent (days) First

Second

Mean % soil water content at wilting

First

Second

Antidesma

Control 3 (1-7) 83 (36-136) 26 (11-63) 18 (9-29) 18.1 (15.1-20.2) 17.5 (15.1-19.4)

+P 4 (0-6) 100 (72-134)* 15 (10-21) 19 (6-32) 18.3 (14.7-21.8) 18.3 (14.7-21.8)

+All 4 (1-10) 95 (64-140) 17 (10-23) 16 (6-29) 19.1 (17.0-21.1) 19.1 (17.6-20.6)

Hopea Control 5 (2-6) 67 (56-78) 16 (11-22) 18 (11-30) 19.5 (17.0-23.3) 19.4 (17.0-22.5)

+All 4 ( 1 6 ) 79 (61-108) 18 (6-26) 25 (15-51) 19.3 (17.6-22.7) 18.6 (16.5-21.3)

T/atica Control 3 (0-6) 90 (76-120)' 25 (8-32) 23 (7-46) 17.8 (13.2-21.4) 17.5 (13.2-21.0)

+All 3 (1-6) 92 (72-119) 20 (7-26) 20 (12-28) 18.8 (16.0-20.8) 18.9 (17.6-20.0)

..................................................................................................... ............................... ------------------

All three Frequent watering

70

2

2

* Excluding one plant not watered throughout the experiment.

(the start of the experiment), 6-7 November 1991 and 3-6 February 1992. T h e three applications of nutrient solutions resulted in total additions of 72.6 mg N, 80.2 mg P, 56.3 mg K, 16.4 mg Mg, and 15.0 mg Ca per pot for plants receiving all nutrients, and the same total amounts of the individual component of this mixture for plants in the single nutrient-addition treatments. The concentrations of nutrient solutions added on the first two occasions were the same as those used by Denslow et al. (1987) and Burslem et al. (1995), while concentrations were increased by a factor of 12.5 on the last occasion in order to make possible the addition of smaller volumes of liquid. The soil in all pots was brought to field capacity (35 g water per 100 g wet mass; N = 180) by thorough watering on 26 October and 7 November 1991 after the completion of nutrient additions on those dates. Between the first two sets of nutrient additions, and thereafter until the end of the experiment, all plants in the frequent-watering treatments were watered in a similar way every second day using a fine-mist sprayer adjusted to deliver a mild jet. Plants in the low-frequency watering treatment were not watered between the two sets of nutrient additions; thereafter they were monitored daily and watered only when the plant was visibly 'wilting' (defined as an inclination of the leaves at an angle >90° to the perpendicular). When this occurred the pot was weighed and water applied until the mass had increased by 30 g after 10 minutes of free drainage. This regime was selected, following preliminary trials, because it en-

.

abled the ~ l a n t sto re-establish and maintain horizontal leaf inclination for several days in all three species; it represented one-quarter to one-third of the change in water content of the soil between field capacity and soil water content at which the plants 'wilted'. As all pots were being monitored individually they received water on different days and on different numbers of occasions (Table I), a technique that was necessary because of the variation in the sizes of plants which existed at the beginning of the experiment: the Vatica seedlings were about 14 months old, while those of Hopea were 6 months old. The aim of applying a uniform soil water status between and within species was ~ a r t i a l achieved; l~ analysis of variance showed that there was no significant difference in the mean gravimetric soil water content at which the first or second wilting occurred for comparisons between the three species (for comparison of soil moisture contents at first wilting: F = 1.57; P > 0.05) or between the two nutrient treatments common to all three species (at first wilting: F = 1.22; P > 0.05) (Table 1). Soil water content was calculated from measurements of the pots made during the experiment and by obtaining the mass of soil in each pot after drying to constant mass at 80°C at the end of the experiment. The mass of the pots was 100 ? 5 g and that of the plants was assumed to be negligible ( 6 g). Plants were re-randomised within the shadescreen on 30 November 1991, 3 January 1992, and 31 January 1992, re-measured on 26 March-4 April and harvested on 30 March-7 April 1992.


640

TABLE 2. Matrix of correlation coeficients between mean soil moisture content measured monthly over the period April-November 1990, mean losson-ignition measured in April and May I990 and the standing crop of leaf litter and total litter present in August I99I for I 0 sites in closed canopy forest in Bukit Timah Nature Reserve. Degree of signifcance indicated asfollows:: P < 0.05; *: P < 0.01. MoisLossture on-ignicontent tion ,

"

A

,

I

M

I

J

,

J

,

A

,

S

I

O

,

N

Time of year

FIGURE 2. Changes with time in the gravimetric water content (on a % wet mass basis) of soil at 0-10 cm for 10 understory sites in Bukit Timah Nature Reserve, Singapore, during 1990. The methodology employed followed Allen, Grimshaw and Rowland 1986.

At harvesting all plants in low-frequency watering treatments and corresponding treatments of those watered frequently were measured for xylem pressure potential using a pressure-bomb apparatus (Scholander et al. 1965; Ritchie & Hinckley 1975). Pre-dawn and midday determinations were made on plants harvested at 0430-0700 h and 12001500 h respectively. Measurements on half of the surviving plants for any given nutrient addition1 watering treatment were made in random order during each of the two periods on a single day, but the different species were harvested on different days. The harvesting schedule was organised so that plants of the frequently-watered treatments were always at the end of their two-day watering cycle, i.e., when xylem pressure potential was likely to be at a minimum. Plants in the infrequently-watered treatments were at different stages in their watering cycles when harvesting was carried out. T h e measurements were made by separating the base of the shoot from the root and sealing the entire inverted shoot in the pressure bomb (Model 650, PMS Instrument Co., 2750 N.W. Royal Oaks Drive, Corvallis, Oregon 97330). After harvesting, the plants were separated into stem, leaves, primary roots and lateral roots, leaf areas determined (Delta-T Devices Ltd., Cambridge, CB5 OEJ, England) and dl plant parts oven-dried and weighed. Mass ratios of leaves (LMR), roots (RMR) and stems (SMR) and specific leaf area (SLA) were calculated as described by Evans (1972). Lateral root mass ratio (LRMR)

Moisture content

Loss-on-ignition Leaf litter Total litter

0.837" 0.634* 0.799"

Leaf litter

-

0.632' 0.675* 0.867"

Total litter

,

-

was calculated as the proportion of total root dry mass allocated to lateral roots, and leaf area per unit lateral root dry mass as the quotient of leaf area and lateral root mass. Statistical analysis was by analysis of covariance (ANCOVA) for measures of plant size, analysis of variance (ANOVA) for dry mass allocation or non-parametric techniques for those variates not satisfying the assumptions underlying ANOVA; mean values presented are adjusted for the covariate where appropriate (Sokal & Rohlf 1981).

RESULTS SOIL CHARACTERISTICS.-Soil moisture content in the forest varied both spatially and temporally over the range 14.8-35.6 percent (Fig. 2). For plants in pots 'wilting' occurred in the range 13-23 percent soil moisture content, and field capacity was 35 percent (Table 1). Direct comparisons of the two data-sets are not appropriate because changes in soil structure during potting may have led to differences in water potential at a given water content, but the results do suggest that the low-frequency watering treatments were broadly comparable to the lower end of the range of soil moisture conditions the seedlings would have been exposed to in the forest during this period of 1990. Mean soil moisture content in 1990, mean loss-on-ignition in 1990 and the amount of litter on the forest floor in August 1991 were all positively correlated, suggesting a parallel degree of coarse patchiness in the three factors at Bukit Timah (Table 2). Mean loss-on-ignition of soil in the pots was not obtained, but is likely to have been typical for Bukit Timah soils as a result of the way


641

Antidesma cuspidatum

Frequent watering Infrequentwatering

2 -c.a

-

-

Predawn Midday Time of measurement

Vatica maingayi

Hopea grifithii

-

-2.5

-2.5 Predawn Midday Time of measurement

Predawn Midday Time of measurement

FIGURE 3. Untransformed pre-dawn and midday mean xylem pressure potential (MPa) for seedlings of Antidesma cuspidatum, Hopea grzfitbii and Vatica maingayi watered frequently (closed bars) or infrequently (open bars). Nutrientaddition treatments are combined within watering treatments following a lack of statistical significance for effects of nutrient treatments: bars reoresent one standard error of the mean.

in which the soil was collected and potted (see Methods). E x P E R I M E N T . - A ~ ~ ~ofs ~variance s showed significant main effects of different species ( F = 53.2, P < 0.001), watering treatments ( F = 126, P < 0.001) and time of measurement ( F = 37.7, P < 0.001) and significant two-way interactions between all these factors on xylem pressure potential at the end of the experiment (Fig. 3). In contrast neither the main effect nor any interaction terms involving nutrient treatments was statistically significant. Under low- and high-frequency watering treatments mean xylem pressure potential was -0.38 and -0.13 MPa respectively for seedlings of Hopea and - 1.76 and -0.40 MPa respectively for Vatica (Fig. 3); the values for Antidesma were intermediate between these extremes. Therefore xy-

lem pressure potential of Vatica when watered frequently was less than that of Hopea when watered infrequently (Fig. 3). For seedlings of Antidesma analyses of covariance on height, stem diameter, leaf number and leaf area at the end of the experiment (using height, stem diameter, leaf number and estimated leaf area at the beginning of the experiment as covariates respectively) were conducted using the subset of three nutrient addition treatments carried out under the differential watering frequency treatments. A reduction in the frequency of watering reduced leaf number at the end of the experiment ( F = . 12.5, P < 0.001) and had similar, but non-significant, effects o n height ( F = 3.19, P = 0.080) and leaf area ( F = 3.70, P = 0.060); addition of all macro-nutrients (but not P alone) increased height growth relative to the control treatment ( F = 4.39,

642

Burslem, Grubb, and Turner Arltidesrna cuspidaturn

**

C

+K

+P

+N

+Ca +Mg +All

Frequent A--

+All

Frequent

Control

+P

+All

Infrequent

1

Hopea griffithii

Control

C

+All

Infrequent

Vatica rnairigay!

Control

+All

Frequent

Conbol

+All

Infrequent

FIGURE 4. Adjusted mean height (cm) of seedlings of Anticie,mn rir,pi~lhtzim,Hupen gl-%fithi;and I/ntiiz mni~ignyi fbllowing growth in pots of Rukit Timah fbresr soil without nutrient additions (C, Control), or with additions o f N ( l N ) , I' ( l I ' ) , K (+K), Ca (+Ca), Mg (+Mg) or N, !L K, Ca and Mg (+All) dnd ~vateredeither frequently (Frequent) or infrequently (Infrequent) as described in the text. For Atztidecmn, significance of difference benveen a nutrientaddition treatment mean and its corresponding C:ontrol mean is indicated as fbllows: ', P < 0.05; '*, P < 0.01.

I' = 0.018), but had no effect o n the other measures of plant growth. I11 no case was there a significant interaction between watering and nutrientaddition treatments. In order to identify the particular macro-nutrient limiting height growth of Atltidesnzn seedlings in this experiment, single classification analysis of covariance was performed for the entire set of treatments and adjusted treatment means compared with their appropriate control mean (Fig. 4). When partitioned in this way the increase in response to the addition of all nutrients under the frequent-watering treatnlent was particularly marked (I' = 0.005), and the only other treatment to influence height growth was the addition of MgSO, (I' = 0.015; Fig. 4). Stem diameter also increased in response to the addition of hIg (I' = 0.040) and leaf area in response to Mg (P= 0.014) and Ca (I' = 0.026), but in neither case xvas there a statistically significant effect of all nutrients.

Growth did not increase in response to either nutrient-addition treatment carried out under the low-frequency watering treatment. T h e mechanism by which MgSO, increased growth of the Antidesm n seedlings may be determined by examination of the data on dry mass distribution (Table 3), 31though firm conclusions are constrained by lack of statistical significance in many cases. It is notable from Table 3 that addition of nutrients tended to increase leaf mass ratio (LMR) in Antidrsinn; for plants watered frequently the largest effect occurred in response to the addition of all macro-nutrients, and the greatest response to a nutrient added singly was to Mg. rwo-way analysis of variance revealed significant effects of the watering treatments on RMR ( F = 6.02, P = 0.018) and, correspondingly, LMR ( F = 7.01, P = 0.0 11): the increase in h V R fro111 0.50 in high- to 0.58 in low-frequency watering trratnienrs was matched by ,I decrease in I.MR


643

TABLE 3. Mean (and standard error) root mass ratio (RMR), leaf mass ratio (LMR), stem mass ratio (SMR), lateral root mass ratio (LRMR) and total sample size (N) for seedlings of Antidesma cuspidatum, Hopea griffithii and Vatica maingayi grown in pots of Bukit Emah forest soil under frequent (FWJ or infrequent (IFW) watering regimes and various nutrient addition treatments. Species Antidesma

FW/ IFW

Nutrient treatment

FW

Control +N +P

RMR

LMR

SMR

LRMR

N

+K

IFW

Hopea

FW IFW

Vatica

FW IFW

+Ca +Mg +All Control +P +All Control +All Control +All Control +All Control +All

from 0.33 to 0.25 (Table 3). The only effect of nutrient addition on allocation was a tendency for P to influence SMR, but in opposite ways under different watering regimes (Table 3). The ratio of lateral- to total root dry mass shifted to an emphasis on more lateral roots in plants from the lowfrequency watering treatments ( F = 8.04, P < 0.01); there was no effect of nutrient treatments (Table 3 ) . Specific leaf area (SLA) in Antidesma declined from 321 to 286 cm2 g-l, comparing high- with low-frequency watering treatments ( F = 1 1.1, P < 0.01); no difference due to nutrient addition occurred in either watering regime (Fig. 5 ) . Comparison of SLA among all treatments for Antidesma yielded a significant increase in response to the addition of K ( P < 0.05) and no other treatment, although M g and Ca also tended to increase mean SLA (data not shown). The effects of watering frequency and nutrient addition were tested for Hopea and Vatica using analyses of covariance on the same four measures of growth as used for the Antidesma. The response to watering treatments varied between species: for Hopea there was a lower mean stem diameter ( F = 7.89, P = 0.009) and leaf area ( F = 4.97, P = 0.034) in lowthan high-frequency watering treatments, and a similar trend for height ( F = 3.50, P = 0.071); for Vatica all four measures of growth were lower in the low-frequency watering treat-

ment than in well-watered controls (Figs. 4 and 6 ) . Nutrient addition did not influence growth of Hopea, but increased height growth of the Vatica seedlings ( F = 12.5, P = 0.001; Fig. 4 ) . For neither species was there a significant interaction between nutrient-addition and watering treatments. The lower leaf area in response to the lower frequency watering treatment reflected overall reductions in dry mass allocation to leaves (for Vatica in absolute terms as well as relative to total dry mass), which occurred despite parallel increases in the allocation of dry mass per unit leaf area (Table 3 cf: Fig. 5 ) . Leaf mass ratio decreased from 0.53 to 0.48 and from 0.51 to 0.44 in Hopea and Vatica, respectively, in response to infrequent watering (Table 3 ) . For both species there was a redistribution of this material to both stems and roots. There was no significant effect of nutrient addition on dry mass allocation in either species (Table 3). In Hopea SLA was lower for plants in lowthan high-frequency watering treatments (242 cm2 g-I vs 260 cm2 g-l, F = 11.9, P = 0.002; Fig. 5 ) . Specific leaf area in Vatica did not respond to reduced water supply in the absence of nutrient additions, but showed a reduction from 212 to 196 cm2 g-I in the presence of additional nutrients (Fig. 5). Neither of the main effects was statistically significant (interaction F = 3.96, P = 0.054). The simultaneous trends toward lower leaf area production (i.e. reduced surface area for tran-


644

A. Specific leaf area (crn2 i') Antidesrna cuspidaturn 400

Hopea gnffithb

Vabca rna~ngayi

4001

1

Control

+P

+All

Control

+All

Control

+All

B. Leaf area per unit lateral root dry mass (mm2 m i ' ) Antidesrna cuspidaturn

Control

+P

Vatica rnaingay~

Hopea gnffithu

+All

Control

+All

Control

+All

FIGURE 5. Untransformed mean (A) specific leaf area (cm2 g-I) and (B) leaf area per unit lateral root dry mass (mm2 mg-') of seedlings of Antidesma cuspidatum, Hopea grzfitbii and ktica maingayi following growth in pots of Bulut Timah forest soil without nutrient additions (Control), with additions of P (+P) or with additions of N, P, K, Ca and Mg (+All) and watered eirher frequently (closed bars) or infrequently (hatched bars). Bars represent standard errors of the mean; for sample sizes see Table 3.

spirational water-loss) and increased development of lateral roots (i.e. potentially greater length of root for water uptake) under conditions of restricted water availability resulted in major changes in the quotient of these factors in response to watering treatments. Leaf area per unit lateral root dry mass was about 60 percent lower under lowthan high-frequency watering conditions in Antidesma ( F = 67.2, P < 0.001) and about 40 percent lower in Hopea (F = 13.4, P < 0.001) and Vatica ( F = 24.8, P < 0.001; Fig. 5). There were also striking differences between species, resulting in very little overlap (Fig. 5). Leaf area per unit lateral root dry mass also responded to nutrient treatments in Antidesma (F = 5.13, P = 0.010), being on average 38 percent greater in plants to which all nutrients had been added than in the control (Fig. 5).

DISCUSSION RESPONSES TO REDUCED SOIL WATER SUPPLY.-The numerous ways in which plants adapt in response

to water shortage, described in detail for herbaceous crop plants by Bradford and Hsiao (1982), were expressed to varying degrees by the shadetolerant tree seedlings we used. Bradford and Hsiao (1982) described the temporal sequence of adaptive responses of plants to the gradual development of water shortage; they emphasized the potential role of morphogenetic responses and osmotic adjustment, acting in the early stages of water shortage, in reducing the necessity for subsequent stomata1 closure. It is likely that responses such as these will be particularly important for plants growing in deep shade because of the restricted potential for positive carbon gain at low irradiance. The morphogenetic responses of crops described were a restriction of canopy development and an increase in root growth relative to shoots. The effects of these responses are clear: a restricted leaf development reduces water loss by evapotranspiration, while an increase in root biomass, in absolute terms or relative to shoot biomass, potentially increases the supply of water to the transpiring surfaces (Begg 1980). Both processes occurred for the three species in this ex-

Seedling Responses to Drought and Nutrients Antidesma cuspidatum

Antidesma cuspidatum 401

+P

Control

l2

I

t n

12

-

10

-

+All

"1

+All

+P

40-

"E-

30-

g

20-

-

+All

Hopea griffithii

Control

Vatica maingayi

8-

Control

Hopea griffithii

Control

645

+All

Vatica maingayi

.

m

a9

1

'

10

-

07 Control

+All

Control

+All

FIGURE 6 . Adjusted mean (A) leaf number and (B) leaf area (cm2) for seedlings of Antidesma cuspidatum, Hopea grzjithii and Vatica maingayi following growth in pots of Bukit Timah forest soil without nutrient additions (Control), with additions of P (+P) or with additions of N, P, K, Ca and Mg (+All) and watered either frequently (closed bars) or infrequently (open bars). Bars represent standard errors of the mean; for sample sizes see Table 3.

periment (Fig. 6, Table 3). The low-frequency watering treatment lowered mean leaf area relative to the control in all three species, and mean leaf number in Antidesma and Vatica (Fig. 6); under similar conditions RMR increased significantly for Antidesma (+8% of total dry mass) and Vatica (+5%) and non-significantly for Hopea (+2%). Similar changes in the partitioning of dry mass

between above- and below-ground parts in response to variation in soil water supply have been observed in tree seedlings of temperate forest Uarvis & Jarvis 19,63, Mazzoleni & Dickmann 1988, Kolb et al. 1990) and tropical forest (Okali & Dodoo 1973). Likewise, shortage of water is known to inhibit leaf growth in tree seedlings (Pope & Madgwick 1974, Mazzoleni & Dickmann 1988, Metcalfe et al.

646


1990), although in some species this is reversible following re-watering (Metcalfe et al. 1990). In these studies almost all the species investigated were probably gap-demanding in primary vegetation (the exception being Khaya iworensis, Okali & Dodoo 1973); in contrast, this paper is concerned with the responses of shade-tolerant species. The tree seedlings in this experiment became adapted to infrequent watering in ways that were not cited by Bradford and Hsiao (1982), presumably because they do not occur, or have not been recorded, in crop plants. O n e example was the decline in specific leaf area in all three species in response to the low-frequency watering treatment; the magnitude of the decline was greater in species with higher mean SLA under full-watering conditions (Fig. 5). Specific leaf area exerts an influence on plant water relations through its effects on the efficiency of water use (Nobel 1980). An increase in leaf thickness will tend to increase the ratio of mesophyll area available for the absorption of C 0 2 per unit leaf area. Since water use efficiency is measured by the amount of C 0 2 assimilated per unit water lost, it will tend to increase in parallel with increases in leaf thickness for a given transpiration rate. The same rationale has been used to account for the decline in the mean SLA of leaves of a spectrum of Eucalyptus species along an increasing aridity gradient in S.E. Australia (Mooney et al. 1978), although a decrease in SLA under dry conditions is not a consistent outcome of experiments on the effects of reduced soil water supply on SLA in woody plants (Okali & Dodoo 1973, Pope & Madgwick 1974, Kolb et al. 1990, Davis et al. 1992). Another response of the seedlings in this experiment to infrequent watering was the change in allocation of root dry mass to laterals, toward a relatively greater emphasis on lateral roots in plants watered less frequently in all three species, especially Antidesma (Table 3). Tree seedlings of some species are known to be flexible in root morphology in response to soil conditions (Toumey 1929, Wright et al. 1992), but an increase in the laterall total root dry mass ratio in response to a drying treatment was the opposite of results obtained for seedling of Pinus banksiana in response to simulated drought (Wright et al. 1992). The effects of an increase in lateral root dry mass are corresponding increases in root surface area available for water uptake and in the total volume of soil exploited by roots, both of which are likely to be beneficial under drought conditions, and especially when combined with a reduction in leaf area. The index amalgamating these effects, leaf area per unit lateral

root dry mass, changed in the direction that would be predicted from a consideration of the functions of leaves and lateral roots in plant water relations (Fig. 5) and in a manner analogous to the role of interspecific differences in root length per unit leaf area in determining the outcome of competition for water, nutrients and light in dry calcareous grassland (Mortimer 1992). Comparisons between the three species in terms of xylem pressure potential in response to the watering treatments should be made with caution because determinations were made for different species on different days and the seedling were of varying age and size. Nevertheless, the differences within the group of species selected by us for their tolerance of shade were remarkable (Fig. 3) and suggestive of differences in mechanisms for coping with reductions in soil water availability Figure 3 shows that Hopea maintained a high and consistent xylem pressure potential in response to soil drying; under similar conditions Vatica exhibited a much lower xylem pressure potential and greater diurnal fluctuations. In the terminology of Kramer (1980) Hopea and Vatica were displaying the characteristics of plants which postpone and tolerate tissue dehydration respectively, and Antidesma some characteristics of both. This study does not enable us to determine the actual mechanisms involved. It is likely that these processes lead to a spectrum of response to climatic droughts of different degrees of severity, although a longer and more detailed study of growth under varying soil water potential would be required to test this idea. TO NUTRIENT ADDITION.-A~~~C the efRESPONSES fects of initial plant size had been accounted for, plant height growth confirmed the result of our previous experiment that seedlings of Antidesma cuspidatum are limited by one or more macro-nutrients when grown in deep shade in pots of Bukit Timah forest soil (Fig. 4; cf: Burslem et al. 1995). Seedlings of Vatica maingayi were also limited by the supply of macro-nutrients, while those of Hopea grrfithii were not (Fig. 4). T h e wider set of treatments for the Antidesma show that the nutrient solution most limiting growth was M g S 0 4 under the conditions we used (Fig. 4). To our knowledge this is the first record of a primary limitation by the supply of Mg for plants growing in soil taken .from relatively undisturbed natural vegetation. In the north temperate zone limitation by Mg has been reported for forest plantations on sandy soils (Tamm 1964) and for polluted natural forests in response to recent acidic deposition (Roberts et al.


1989). The effect of acid pollution is to increase the concentrations of toxic Al3+ ions in soil solution (Ulrich 1989). Deficiencies in the nutrient cations develop in plants because Al3+ ions compete with Mg2+ and Ca2+ for exchange sites in the apoplast of the root cortex (Godbold et al. 1988). The soils at Bukit Timah are inherently highly acidic (Grubb et al. 1994), and the concentration of Al3+ ions in soil solution would be expected to be high also. It may be significant, therefore, that most species of Antidesma are known to be Al-accumulating, in common with many other plant species of the lowland tropics (Chenery 1948). If a link between Al-accumulation and Mg deficiency in seedlings of Antidesma is confirmed, the incidence of primary limitation by one or more of the major nutrient cations may be higher than previously anticipated among plants of tropical lowland rain forest. In contrast, there is no evidence of limitation by the availability of P for mycorrhizal seedlings or cuttings growing in soil from either of the two tropical lowland sites where bioassay experiments have been conducted (Denslow et al. 1987, Burslem et al. 1995), despite the undoubted poverty of both soils in available P (Vitousek & Denslow 1986, 1987; Grubb et al. 1994). CONCLUSIONS.--Thisstudy provides further evidence of the importance of mineral nutrient availability for g o w t h of some shade-tolerant tree seedlings in pots, even at low irradiance (Fig. 4). The relevance of these findings needs to be determined for plants in the forest, where the topsoil contains roots of fully-grown trees as well as those of seed-

647

lings (Grubb et al. 1994). Reduced watering frequency decreased g o w t h in all three species (Figs. 4 and 6) and had marked effects on plant form (Fig. 5) and dry mass allocation (Table 3), although the significance of the correspondence between soil water content in forest top-soil in 1990 (Fig. 2) and during drying treatments in our experiment (Table 1) is not easily interpreted in the absence of a soil moisture characteristic curve for Bukit Timah soil. Notwithstanding this limitation, the seedlings expressed a range of morphological responses which would be considered adaptations to drought if they were observed in agricultural crops (cf: Bradford & Hsiao 1982). We cannot know whether the responses to water-shortage evolved under conditions virtually identical with those experienced now in 'evenvet aseasonal forest' or whether they evolved during the Glacial periods, when conditions were drier in many tropical regions (Flenley 1979). However, we suggest that there is strong selection at the present time for the persistence of responses to water-shortage which minimize the chance of mortality during dry conditions. New experiments are needed to test this possibility.

ACKNOWLEDGMENTS We are grateful to the National Parks Board for permission to conduct research in Bukit Timah Nature Reserve and to G. Jones, Dr. O n g Bee Lian, J. Ong, Soong Beng Ching and Chua Keng Soon for assistance with the research. Financial support was provided by the Natural Environment Research Council (studentship to DFRPB), the National University of Singapore (research grant RP890339) and the University of Cambridge (travel grants to PJG).

LITERATURE CITED ALLEN,S. E., H. M. GRIMSHAW, AND A. l? ROWLAND.1986. Chemical analysis, pp. 285-344. In l? D. Moore and S. B. Chapman (Eds.). Methods in plant ecology. Blackwell Scientific Publications, Oxford, England. BAILLIE, I. C. 1976. Further studies on drought in Sarawak, East Malaysia. J. Trop. Geogr. 43: 20-29. BEGG,J . E. 1980. Morphological adaptations of leaves to water stress, pp. 3 3 4 2 . In N. C. Turner and l? J. Kramer (Eds.). Adaptations of ~ l a n t sto water and high temperature stress. Wiley & Sons, New York. K. J., AND T. C. HSLAO.1982. Physiological responses to moderate water stress, pp. 263-324. In 0 . L. BRADPORD, Lange, l? S. Nobel, C. B. Osmond, and H . Ziegler (Eds.). Encyclopedia of plant physiology new series volume 12B: physiological plant ecology I1 water relations and carbon assimilation. Springer-Verlag, Berlin. BRUNIG, E. F, 1969. O n the seasonality of droughts in the lowlands of Sarawak (Borneo). Erdkunde 23: 127-135. P. F. 1975. Silviculture in the hill forests of the Malay Peninsula. Malaysian Forestry Department Research BURG~SS, Pamphlet 66. AND l? J. GRUBB.1994. Mineral nutrient status of coastal hill dipterocarp forest BURSLEM, D. F. R. P., I. M. TURNER, and adinandra belukar in Singapore: bioassays of nutrient limitation. J. Trop. Ecol. 10: 579-599. , l? J. GRUBB, AND I. M. TURNER. 1995. Responses to nutrient addition among shade-tolerant tree seedlings of lowland tropical rain forest in Singapore. J. E'col. 83: 113-122. CHAZDON, R. L. 1988. Sunflecks and their importance to forest understory plants. Adv. Ecol. Res. 18: 1-63. CHENERY, E. M. 1948. Aluminum in the plant world. Kew Bull. 3: 173-183. C O R L E R. ~ , T. 1990. Flora and reproductive phenology of the rain forest at Bukit Timah, Singapore. J. Trop. Ecol. 6: 55-63.

648


DALE,W. L. 1963. Surface temperatures in Malaya. J. Trop. Geogr. 17: 57-71.

A N D G. F. MIDGLEY. 1992. Growth and gas exchange responses of Leucadendronxanthoconus

DAVIS, G. W., A. I? FLYNN, (Proteaceae) seedlings to different nutrient and water regimes. S. Afr. J. Bot. 58: 56-62. DENSLOW, J. C., I? M. VITOUSEK, A N D J. C. SCHULTZ. 1987. Bioassays of nutrient limitation in a tropical rain forest soil. Oecologia 24: 370-376. EVANS, G. C. 1972. The quantitative analysis of plant growth. Blackwell Scientific Publications, Oxford, England. FISHER, B. L., H. F. H o r n , AND S. J. WRIGHT.1991. Survival and growth of firoh surznamensis yearlings: water augmentation in gap and understory. Oecologia 86: 292-297. FLENLEY, J. R. 1979. The Equatorial rain-forest: a geological history. Butrenvorths, London. 1988. Aluminum toxicity and forest decline. Proc. Nat. Acad. Sci. GODBOLD, D. L., E. FRITZ,AND A. HUTTERMANN. U.S.A. 85: 3888-3892. I? J., I. M. TURNER, AND D. F. R. I? BURSLEM. 1994. Mineral nutrient status of coastal hill dipterocarp forest GRUBB, and adinandra belukar in Singapore: analysis of soil, leaves and litter. J. Trop. Ecol. 10: 559-577. JARVIS, l? G., AND M. S. JARVIS.1963. The water relations of tree seedlings I. Growth and water use in relation to soil water potential. Physiol. Plant 16: 215-235. KOLB,T. E., K. C. STEINER, L. H . MCCORMICK, AND T. W. BOWERSOX. 1990. Growth responses of northern red-oak and yellow-poplar seedlings to light, soil moisture and nutrients in relation to ecological strategy. For. Ecol. Manage. 38: 65-78. ~ M E R I? J., 1980. Drought, stress, and the origin of adaptations, pp. 7-20. In N. C. Turner and l? J. Kramer (Eds.). Adaptations of plants to water and high temperature stress. Wiley & Sons, New York. LLOYD,l? S., AND C. D. PIGOTT.1967. The influence of soil conditions on the course of succession on the chalk of southern England. J. Ecol. 55: 137-146. D. SCOTT,A ND J. PROCTOR.1991. Nitrogen mineralization and nitrification in terra MARRS, R. H., J. THOMPSON, firme forest and savanna soils on Ilha de Maraca, Roraima, Brazil. J. Trop. Ecol. 7: 123-137. MAZZOLENI, S., AND D. I. DICKMANN.1988. Differential physiological and morphological responses of two hybrid Poplrlus clones to water stress. Tree Physiol. 4: 61-70. AND J. S. PEREIRA. 1990. Leaf growth of Eucalyptus globulus seedlings under water J. C., W. J. DAVIES, METCALFE, deficit. Tree Physiol. 6: 221-227. MOONEY, H . A,, I? J. FERRAR, A N D R. 0 . SLAMR. 1978. Photosynthetic capacity and carbon allocation patterns in diverse growth forms of Eucalyptus. Oecologia 36: 103-1 11. MORTIMER, S. R. 1992. Root lengthlleaf area ratios of chalk grassland plants and their importance for competitive interactions. J. Vegn Sci. 3: 665-672. S. 1965. Evaporation and water balances in Malaya. J. Trop. Geogr. 20: 34-53. NIEUWOLT, NOBEL,l? S. 1980. Leaf anatomy and water use efficiency, pp. 43-55. In N. C. Turner and l? J. Kramer (Eds.). Adaptations of plants to water and high temperature stress. Wiley & Sons, New York. OKALI,D. U. U., AND G. DODOO. 1973. Seedling growth and transpiration of two West African mahogany species in relation to water stress in the root medium. J. Ecol. 61: 4 2 1 4 3 8 . POPE,l? E., AND H. A. I. MADGWCK. 1974. The influence of moisture stress on Liriodendron tulipifera L. seedlings. Ann. Bot. 38: 4 3 1 4 3 9 . RITCHIE, G. A,, AND T. M. HINCKLEY. 1975. The pressure chamber as an instrument for ecological research. Adv. Ecol. Res. 9: 165-254. T. M., R. A. SKEFFINGTON, AND L. W. BLACK.1989. Causes of Type 1 spruce decline in Europe. Forestry ROBERTS, 62: 179-222. SCHOLANDER,F., I? H . T. HAMMELL, E. D. BRADSTREET, AND E. A. HEMMINGSEN. 1965. Sap pressure in vascular plants. Science 148: 339-346. SOKAL, R. R., AND F. J. ROHLF.1981. Biometry. 2nd edition. W. H. Freeman and Company, San Francisco. TAMM, C. 0 . 1964. Determination of nutrient requirements of forest stands. Int. Rev. Forestry Res. 1: 115-170. TOUMEY, J. W. 1929. Initial root habit in American trees and its bearing on regeneration, pp. 713-728. In B. M. Duggar (Ed.). Proceedings of the International Congress of Plant Sciences, Ithaca, New York 1926. George Banta Publishing Company, Menasha, Wisconsin. ULRICH, B. 1989. Effects of acidic precipitation on forest ecosystems in Europe, pp. 189-272. In D. C. Adriano and A. H. Johnson (Eds.). Acidic precipitation volume 2: biological and ecological effects. Springer-Verlag, New York. 1986. Nitrogen and phosphorus availability in treefall gaps of a lowland tropical I? M., AND J. S. DENSLOW. VITOUSEK, rainforest. J. Ecol. 74: 1167-1 178. , AND . 1987. Differences in extractable phosphorus among soils of the La Selva Biological Station, Costa Rica. Biotropica 19: 167-170. WALTER, H., E. HARNICKELL, A N D D. MUELLER-DOMBOIS. 1975. Climate-diagram maps. Springer-Vedag, Berlin. WATTS,I. E. M. 1955. Rainfall of Singapore Island. J. Trop. Geogr. 7: 1-71. WHITMORE, T. C . 1984. Tropical rain forests of the Far East. 2nd edition. Clarendon Press, Oxford, England. WONG,Y. K. 1987. Ecology of the trees of Bukit Timah Nature Reserve. Gdns' Bull., Singapore 40: 45-76. WRIGHT,S. J. 1991. Seasonal drought and the phenology of understory shrubs in a tropical moist forest. Ecology 72: 1643-1 657. WRIGHT,R. A,, R. W. WEIN,A N D B. I? DANCIK.1992. Population differentiation in seedling root size between adjacent stands of Jack Pine. For. Sci. 38: 777-785. WYATT-SMITH, 1. 1963. A manual of Malayan silviculture for inland forests. Malayan Forest Records 23.