Loss of oak dominance in dry-mesic deciduous forests predicted by gap capture methods

Gap capture methods predict future forest canopy species composition from the tallest trees growing in canopy gaps rather than from random samples of shaded understory trees. We used gap capture methods and a simulation approach to forecast canopy composition in three old oak forests (Quercus spp.) on dry-mesic sites in southern Wisconsin, USA. In the simulation, a gap sapling is considered successful if it exceeds a threshold height of 13–17 m (height of maximum crown width of canopy trees) before its crown center can be overtopped by lateral crown growth of mature trees. The composition of both the tallest gap trees and simulated gap captures suggests that 68–90% of the next generation of canopy trees in the stands will consist of non-Quercus species, particularly Ulmus rubra, Carya ovata and Prunus serotina. Quercus species will probably remain as a lesser stand component, with Quercus alba and Quercus rubra predicted to comprise about 19% of successful gap trees across the three stands. Several methods of predicting future canopy composition gave similar results, probably because no gap opportunist species were present in these stands and there was an even distribution of species among height strata in gaps. Gap trees of competing species already average 11–13 m tall, and mean expected time for these trees to reach full canopy height is only 19 years. For these reasons, we suggest that dominance will shift from oaks to other species, even though late successional species (e.g., Acer and Tilia) are not presently common in the understories of these stands.     

Leaf anatomical responses to light in five tropical moraceae of different successional status

We characterized the leaf anatomical characteristics and maximum assimilation rates of five neotropical Moraceae of different genera and successional positions. Plants were grown under different light levels and transferred to high light, simulating canopy openings. Total blade thickness increased with irradiance among all species, and thicker blades were developed when plants were switched. However, blade thickness, and the extent to which it was modified, was independent of the species’ successional position and did not predict photosynthetic performance. Palisade thickness was a good predictor of maximum photosynthetic rate, but only on a species-specific basis. Overall, leaf thickening with increasing irradiance was associated more with structural than with photosynthetic changes. The early successionals Cecropia obtusifolia and Ficus insipida exhibited similarly high photosynthetic plasticity and acclimation values, but differed in their leaf anatomical traits. The late successional Poulsenia armata produced the most anatomically plastic leaves, but failed to acclimate either anatomically or photosynthetically when transferred to higher light levels.     

Steady and dynamic photosynthetic responses of seedlings from contrasting successional groups under low‐light growth conditions

To test the hypothesis that leaf‐level photosynthetic‐related traits might confer late successionals a competitive advantage over early successionals in low‐light growth conditions, steady photosynthetic assimilation‐ and dynamic photosynthetic induction‐related traits were examined in low‐light‐grown seedlings with contrasting successional status. Compared with the early successionals, late successionals as a group significantly exhibited lower leaf gas exchange rates. While late successionals required a longer time to respond to simulated sunflecks, they had lower rates of induction losses after sunflecks. Such photosynthetic induction traits allowed late successionals to more effectively utilize subsequent sunflecks. It was observed that plants with lower gas exchange rates responded more slowly to simulated sunfelcks, but they had lower rates of induction losses after sunflecks. In addition, the rate of response to sunflecks was positively correlated with the rate of induction loss after sunflecks across the successional status of species. A principal components analysis (PCA) demonstrated that early and late successionals were separated along the first axis of the PCA, and that early successionals were grouped on the right and were associated with higher gas exchange rates, fast responses to sunflecks, and rapid rates of induction loss after sunflecks; late successionals held an opposite pattern. Overall, our results suggest that smaller respiratory carbon losses and lower metabolic costs give late successionals a competitive advantage in low‐light growth conditions, that late successionals have an advantage over early successionals in utilizing sunflecks, and thus that the successional status of species are mainly associated with the leaf‐level photosynthetic‐related traits.     

Canopy and age structures of some old sub-boreal Picea stands in British Columbia

Abstract. 14 old, unlogged, Picea-dominated stands in the moist cool Sub-Boreal Spruce biogeoclimatic subzone of central British Columbia, Canada, were sampled to describe canopy heterogeneity, regeneration patterns and tree population age structures. These stands are composed of Picea engelmannii × glauca hybrids, Abies lasiocarpa and lesser amounts of Pinus contorta and Populus tremuloides, and had survived 124–343 yr since the last stand-destroying wildfire. Canopy cover was patchy and highly variable (ranging from 30.5 % to 86.4 %) but was not significantly related to stand age. Vertical canopy structure was less variable, reflecting the shade-tolerance and live crown ratios (length of live canopy expressed relative to tree height) of component species: 18.8 % for Populus, 20.2 % for Pinus, 46.7 % for Picea and 51.4 % for Abies. Individual stands varied considerably in their population structures and in their stand development trajectories, yet some patterns are evident. Survivors of the initial post-disturbance cohort of trees took 51 to 118 yr (mean = 80, s.d. = 20) to establish. Some stands had all tree species present during stand initiation, while other stands indicated early successional roles for Populus and Pinus, or a late successional role for Abies. Abies recruitment, while often slow in the beginning, occurs uniformly throughout the history of most stands, reflecting the high shade-tolerance of this species. Picea is often recruited in high densities early in stand development, and then (after long periods of exclusion) may be displaced by Abies in some stands but maintains itself in others. Minor, single-tree disturbances (due to bark beetles, root rot, and windthrow) were important in accelerating the reinitiation of Picea in the understory. Results thus suggest that stands from this region can be self-perpetuating in the absence of fire. Yet, post-fire tree populations still clearly dominate these spruce-fir forests, for only the oldest stand had greater basal area in the replacement cohort than in the initial cohort.     

Distinct male reproductive strategies in two closely related oak species

Reproductive strategies of closely related species distributed along successional gradients should differ as a consequence of the trade‐off between competition and colonization abilities. We compared male reproductive strategies of Quercus robur and Q. petraea, two partly interfertile European oak species with different successional status. In the studied even‐aged stand, trees of the late‐successional species (Q. petraea) grew faster and suffered less from intertree competition than trees of the early‐successional species (Q. robur). A large‐scale paternity study and a spatially explicit individual‐based mating model were used to estimate parameters of pollen production and dispersal as well as sexual barriers between species. Male fecundity was found to be dependent both on a tree's circumference and on its environment, particularly so for Q. petraea. Pollen dispersal was greater and more isotropic in Q. robur than in Q. petraea. Premating barriers to hybridization were strong in both species, but more so in Q. petraea than in Q. robur. Hence, predictions based on the competition–colonization trade‐off are well supported, whereas the sexual barriers themselves seem to be shaped by colonization dynamics.     

Effects of Quercus emoryi on herbaceous vegetation in a semi-arid savanna

Ten trees (5–70 m² canopy area) were selected to determine effects of tree size (crown area) on herbaceous species composition and biomass in a Quercus emoryi savanna in southeastern Arizona. Consistent with most studies in temperate savannas, herbaceous biomass was reduced beneath the canopy relative to grassland areas. However, tree size appeared to exert no influence over herbaceous biomass. In contrast to most temperate savannas, Q. emoryi trees did not affect distribution of herbaceous species.     

湖南栎类天然次生林幼树更新特征及影响因子

以湖南典型栎类天然次生林为研究对象,基于51块样地的调查数据,采用k-means聚类分析划分林分类型,研究湖南不同栎类天然次生林幼树更新特征,分析了湖南不同栎类天然次生林幼树更新指标(幼树密度、幼树平均地径、平均高以及平均冠幅)与环境因子、林分因子的相关性,旨在阐明环境因子、林分因子对幼树更新的影响,以期为湖南不同栎类天然次生林的恢复与经营管理提供理论依据。结果表明:(1)利用聚类分析可将研究区内栎类天然次生林划分为5个类型,包括甜槠锥栗混交林(CC)、亮叶水青冈多脉青冈混交林(FC)、石栎樟树混交林(LC)、枹栎甜槠混交林(QC)、青冈栎混交林(CG)。(2)不同类型栎类天然次生林更新幼树优势种分化明显,物种丰富度差异显著(P0.05)。5种不同栎类次生林幼树密度均未超过500株/hm~2,更新情况较差;幼树数量差异显著(P0.05),为亮叶水青冈多脉青冈混交林石栎樟树混交林青冈栎混交林枹栎甜槠混交林甜槠锥栗混交林;生长情况差异显著(P0.05),为青冈栎混交林亮叶水青冈多脉青冈混交林枹栎甜槠混交林甜槠锥栗混交林石栎樟树混交林。(3)相关分析结果显示,不同类型次生林幼树更新的主要影响因子存在差异。甜槠锥栗混交林中幼树密度与腐殖质厚度呈显著负相关(P0.05);幼树平均高与灌木盖度呈显著正相关(P0.05);幼树平均地径与草本盖度、灌木盖度呈显著正相关(P0.05)。亮叶水青冈多脉青冈混交林中幼树密度与海拔、腐殖质厚度、枯落物厚度呈显著正相关(P0.05),与草本盖度呈极显著正相关(P0.01);幼树平均地径与郁闭度呈显著负相关(P0.05);幼树平均高、幼树平均冠幅与坡位呈显著正相关(P0.05)。石栎樟树混交林中幼树密度与坡向、土壤厚度呈显著正相关(P0.05),其余因子对幼树生长无显著影响。枹栎甜槠混交林中幼树密度与郁闭度、乔木密度呈极显著正相关(P0.01),与坡位呈显著负相关(P0.05);幼树平均冠幅与坡度呈显著负相关(P0.05)。青冈栎混交林中幼树平均地径与土壤厚度呈显著正相关(P0.05),与乔木密度呈极显著正相关(P0.01);幼树平均冠幅与灌木盖度呈显著正相关(P0.05)。     

Primary ecological succession in vascular epiphytes: The species accumulation model

Epiphytes are integral to tropical forests yet little is understood about how succession proceeds in these communities. As trees increase in size they create microhabitats for late‐colonizing species in both small and large branches while maintaining small tree microhabitats for early colonizing species in the small and young branches. Thus, epiphyte succession may follow different models depending on the scale: at the scale of the entire tree, epiphytes may follow a species accumulation model where species are continuously added to the tree as trees increase in size but at the scale of one zone on a branch (e.g., inner crown: 0–2 m from the trunk), they may follow the replacement model of succession seen in terrestrial ecosystems. Assuming tree size as an indicator of tree age, I surveyed 61 Virola koschnyi trees of varying size (2.5–103.3 cm diameter at breast height) in lowland wet tropical forest in Costa Rica to examine how epiphyte communities change through succession. Epiphyte communities in small trees were nested subsets of those in large trees and epiphyte communities became more similar to the largest trees as trees increased in size. Furthermore, epiphyte species in small trees were replaced by mid‐ and late‐successional species in the oldest parts of the tree crown but dispersed toward the younger branches as trees increased in size. Thus, epiphyte succession followed a replacement model in particular zones within treecrowns but a species accumulation model at the scale of the entire tree crown.     

Canopy manipulation as a tool for restoring mature forest conifers under an early‐successional angiosperm canopy

Low‐light environments in early‐successional forests that have established after abandonment of farming often restrict the establishment of later successional species resulting in an arrested succession. This 6‐year study tested the potential of different canopy manipulations to facilitate the establishment of a light‐demanding canopy tree species, tōtara (Podocarpus totara), within a regenerating kānuka (Kunzea robusta) stand. Results highlighted the effectiveness of artificial gaps over other methods (ring‐barking and edge‐planting) in accelerating the growth of planted tōtara. Seedlings under gaps grew consistently taller and faster over time indicative of an improved understorey light environment. Ring‐barking did not have a significant effect on tōtara growth because only a portion of the treated trees died, and after 6 years dead trees remained standing with intact branches resulting in insignificant increases in light transmission. At the forest edge sites, tōtara growth was highly variable. Although some seedlings grew as tall as in the gaps, others did not. Survival was also lower in the edge sites than in other treatments, which was likely due to enhanced herbivory from ungulates which impacted some plants at these sites. Gap creation is likely to be an important tool for restoring late‐successional canopy species in regenerating stands both through providing ideal sites for the growth of light‐demanding species such as tōtara and through natural establishment of other future canopy trees into the gaps.     

Variation in crown light utilization characteristics among tropical canopy trees

BACKGROUND AND AIMS: Light extinction through crowns of canopy trees determines light availability at lower levels within forests. The goal of this paper is the exploration of foliage distribution and light extinction in crowns of five canopy tree species in relation to their shoot architecture, leaf traits (mean leaf angle, life span, photosynthetic characteristics) and successional status (from pioneers to persistent). METHODS: Light extinction was examined at three hierarchical levels of foliage organization, the whole crown, the outermost canopy and the individual shoots, in a tropical moist forest with direct canopy access with a tower crane. Photon flux density and cumulative leaf area index (LAI) were measured at intervals of 0.25-1 m along multiple vertical transects through three to five mature tree crowns of each species to estimate light extinction coefficients (K). RESULTS: Cecropia longipes, a pioneer species with the shortest leaf life span, had crown LAI <0.5. Among the remaining four species, crown LAI ranged from 2 to 8, and species with orthotropic terminal shoots exhibited lower light extinction coefficients (0.35) than those with plagiotropic shoots (0.53-0.80). Within each type, later successional species exhibited greater maximum LAI and total light extinction. A dense layer of leaves at the outermost crown of a late successional species resulted in an average light extinction of 61% within 0.5 m from the surface. In late successional species, leaf position within individual shoots does not predict the light availability at the individual leaf surface, which may explain their slow decline of photosynthetic capacity with leaf age and weak differentiation of sun and shade leaves. CONCLUSION: Later-successional tree crowns, especially those with orthotropic branches, exhibit lower light extinction coefficients, but greater total LAI and total light extinction, which contribute to their efficient use of light and competitive dominance.     

Plasticity and acclimation to light in tropical Moraceae of different sucessional positions

Summary We evaluated both the photosynthetic plasticity and acclimation to light of seedlings of five co-occurring tropical tree species in the Moraceae,Cecropia obtusifolia, Ficus insipida, Poulsenia armata, Brosimum alicastrum, andPseudolmedia oxyphyllaria. Distinct differences in the species' abilities to respond to increasing irradiance correlated with their known habitat breadths and successional status. The early successinalsCecropia andFicus exhibited the highest photosynthetic rates and conductance values in high light. There was a several-fold difference in assimilation across light regimes, consistent with a high physiological plasticity. When individuals grown at low light were transferred to higher irradiances, seedlings of bothCecropia andFicus produced leaves which photosynthesized at rates as high or higher than those of plants continuously grown in high light, indicating a high photosynthetic acclimation potential. In contrast, the late successionals were characterized by both a more restricted physiological plasticity and acclimation potential. Higher light levels resulted in only moderate increases in assimilation among the late successionals, and onlyBrosimum acclimated fully to increased irradiances. NeitherPoulsenia norPseudolmedia increased appreciably their photosynthetic rates when transferred to high light. This suggests that acclimation potential cannot always be inferred from plasticity responses, and calls for a reevaluation of arguments developed solely from plasticity studies. Finally, differences between the early and late successional species in the allocation of nitrogen into RuBP carboxylase and thylakoid nitrogen pools or non-photosynthetic compounds are suggested by the distinct relationships between maximum photosynthetic capacity and nitrogen content.     

Changes in stand composition and structure between 1981 and 1996 in four Front Range plant communities in Colorado

We measured leaf distribution, crown shape, and biomass allocation patterns in open-grown saplings of three tree species (Cedrela odorata, Cordia alliodora, and Hyeronima alchorneoides) that differed in above-ground architecture and successional status. The objectives were to: (1) analyze the relationship between crown structure and biomass allocation in free-growing juveniles and (2) assess whether predictions that relate crown structure and biomass distribution of adult individuals can be applied to noncompeting saplings. Significant differences in crown structure and biomass allocation were found among the three species. Mean dry biomass was 990 g in Cedrela, 665 g, in Cordia, and 1281 g in Hyeronima. Cedrela allocated a greater proportion of biomass into roots than the other two species. We observed no distinct pattern of foliage angle, and no evidence of decrease in crown construction efficiency with tree size. Predictions stated by other authors for mature individuals partially explained the relationships between biomass allocation and morphological variables of non-competing saplings. We proposed two alternative biomass allocation patterns: (1) in open conditions saplings of early successional species exhibit structural characteristics more adapted for high light levels than gap-dependent species, and (2) early successional species have lower cost-benefit ratio for biomass construction and higher foliar efficiency than late successional ones. In general, crown morphometric characteristics and biomass distribution patterns of the three species studied corresponded with the two proposed patterns.     

不同生境栓皮栎天然更新幼苗植冠构型分析

栓皮栎存在于秦岭南坡的多种林分中,生活在不同生境中的个体往往形成不同的树冠形态和构型特征。为了说明不同生境条件下栓皮栎幼苗的植冠构型变化,采用典型抽样法,对秦岭南坡3种生境中(林冠下、林隙、林缘)的栓皮栎天然更新幼苗的侧枝、叶片特征及其空间分布进行了调查分析,结果表明:3种不同生境中栓皮栎幼苗植冠形态发生了明显的可塑性变化,(1)林冠下的幼苗明显为开阔型树冠,林隙和林缘处的幼苗树冠相对紧密;(2)幼苗的1级侧枝密度与分枝角度在3种生境下均差异显著(P0.05);从Ⅰ到Ⅳ层,林冠下幼苗的分枝角度在冠层内变化幅度不到5°,而林缘处幼苗的分枝角度变化高达40°;发生5个以上1级侧枝的概率以林冠下最大,为0.6;(3)从林缘、林隙到林冠下,幼苗的叶长、叶宽、单叶面积、叶面积指数逐渐降低,数量叶密度和比叶面积则逐渐增大,与其它两种生境相比,林冠下幼苗的叶片逐渐向树冠上层集中,且以更高序的侧枝为主要着生枝条;(4)林隙中栓皮栎幼苗的树高、地径明显优于林缘和林冠下,缩短了苗木进入主林层的时间,林隙对栓皮栎种群更新有利。在今后栓皮栎林的经营中,可以通过适当间伐来增加林隙数量,为森林更新和结构的优化的提供有利条件。     

Succession in the southern part of the Canadian boreal forest

Forest succession following fire in a forest mosaic of northwestern Quebec has been studied in order to: (1) describe the successional pathways using communities of different ages and (2) evaluate convergence of successional pathways and possible effect of fire suppression on the establishment of steady-state communities. As a first step, ordination and classification techniques were used in order to remove changes in forest composition which are related to abiotic conditions. Then, ordinations based on tree diameter distributions were used to study shifts in species composition in relation to time since the last fire.Even under similar abiotic conditions, successional pathways are numerous. However, regardless of forest composition after fire, most stands show convergence toward dominance of Thuja occidentalis and Picea mariana on xeric sites and dominance of Abies balsamea and Thuja occidentalis on more mesic sites. Stable communities of >300 yr occur on xeric sites while on mesic sites directional succession still occurs after 224 yr. Nearly all species involved in succession are present in the first 50 yr following fire. Only Abies balsamea and Thuja occidentalis increase significantly in frequency during succession. Following initial establishment, successional processes can generally be explained by species longevity and shade tolerance. Early successional species may be abundant in the canopy for more than 200 yr while the rapid decrease of Picea glauca, a late successional species could be related to spruce budworm outbreaks. Considering the short fire rotation observed (about 150 yr), a steady-state forest is unlikely to occur under natural conditions, though it may be possible if fire is controlled.     

Effects of size,neighbors, and site condition on tree growth in a subtropical evergreen and deciduous broad‐leaved mixed forest,China

Successful growth of a tree is the result of combined effects of biotic and abiotic factors. It is important to understand how biotic and abiotic factors affect changes in forest structure and dynamics under environmental fluctuations. In this study, we explored the effects of initial size [diameter at breast height (DBH)], neighborhood competition, and site condition on tree growth, based on a 3‐year monitoring of tree growth rate in a permanent plot (120 × 80 m) of montane Fagus engleriana–Cyclobalanopsis multiervis mixed forest on Mt. Shennongjia, China. We measured DBH increments every 6 months from October 2011 to October 2014 by field‐made dendrometers and calculated the mean annual growth rate over the 3 years for each individual tree. We also measured and calculated twelve soil properties and five topographic variables for 384 grids of 5 × 5 m. We defined two distance‐dependent neighborhood competition indices with and without considerations of phylogenetic relatedness between trees and tested for significant differences in growth rates among functional groups. On average, trees in this mixed montane forest grew 0.07 cm year^?1 in DBH. Deciduous, canopy, and early‐successional species grew faster than evergreen, small‐statured, and late‐successional species, respectively. Growth rates increased with initial DBH, but were not significantly related to neighborhood competition and site condition for overall trees. Phylogenetic relatedness between trees did not influence the neighborhood competition. Different factors were found to influence tree growth rates of different functional groups: Initial DBH was the dominant factor for all tree groups; neighborhood competition within 5 m radius decreased growth rates of evergreen trees; and site condition tended to be more related to growth rates of fast‐growing trees (deciduous, canopy, pioneer, and early‐successional species) than the slow‐growing trees (evergreen, understory, and late‐successional species).     

Some autecological characteristics of early to late successional tree species in Venezuela

The breadth of the continuum concept of strategy with respect to succession was tested on 21 tree and shrub species common in either unlogged or logged stands, respectively, in the Forest Reserve of Caparo, Venezuela, by examining morphological, physiological and population characteristics. Based on a preliminary abundance analysis, `early', `mid' and `late' successional species as well as `generalists' were distinguished. Early successional species, i.e. Ochroma lagopus, Heliocarpus popayanensis and Cecropia peltata were similar in many autecological aspects, e.g. monolayered leaf arrangement, orthotropic architectural models, no adaptive reiteration, clumped distribution, but differed in gap association and distribution along a drainage gradient. Mid-successional species established themselves both in large and small gaps (> 300 m[sup2 ]; 80–300 m[sup2 ]) and showed a clumped to regular distribution pattern in logged areas; they exhibited more diverse crown and leaf characteristics than early successional species. Late successional species established themselves only in small gaps and understorey, and showed a regular spatial pattern in undisturbed areas. All late successional species displayed architectural models with plagiotropic lateral axes and showed a multilayered leaf arrangement. Adaptive reiteration was a common feature of late successional species which could be further subdivided into large, medium-sized and small trees, indicating different light requirements at maturity. Generalists were common treelet and shrub species in both disturbed and undisturbed sites where they are also capable of completing their life cycle. The light compensation point (LCP) of an individual plant was strongly influenced by its crown illuminance. Large late successional species showed the widest range of LCP values, reflecting the increasing light availability with increasing height in mature forest. On the basis of many autecological characteristics, it was found (i) that there is in fact a continuum of species strategies with respect to succession even among early and mid-successional species and (ii) that the latter group of species showed the widest breadth of autecological traits, reflecting the heterogeneous environment in which they establish and mature.     

Phyllostomid bat assemblages in different successional stages of tropical rain forest in Chiapas,Mexico

Due to their role in seed dispersal, changes in the community of phyllostomid bats have direct consequences on ecological succession. The objective of this work was to document changes in the structure of bat assemblages among secondary successional stages of tropical rain forest in Chiapas, Mexico. Bats were mist-netted at ground level during 18 months in 10 sites belonging to 3 successional stages: four sites represented early succession (2–8 years of abandonment), four intermediate succession (10–20 years of abandonment), and two late succession (mature old-growth forest).We captured 1,179 phyllostomids comprising 29 species. Phyllostomid species richness was 17 (58% of all species) in the early stage, 18 (62%) in the intermediate stage and 24 (83%) in the late stage. The late successional mature forest possessed nine species that were exclusively found there, whereas early and intermediate successional stages contained only one exclusive species. Sturnira lilium, Artibeus lituratus, Carollia perpicillata, Artibeus jamaicensis and Glossophaga soricina represented 88% of all captured phyllostomid bats. Frugivores made up more than 90% of the species captured in early and intermediate successional stages and 84% in late successional forest. The Bray–Curtis index of dissimilarity showed a replacement of species through successional stages with the largest dissimilarity between early and late stages, followed by intermediate and late, and the lowest dissimilarity between early and intermediate stages. The number of gleaning insectivore species increased during succession. The carnivorous guild was exclusively found in the late stage (three species). We conclude that the late successional mature forest was the main reservoir for the gleaning insectivore and carnivore guilds; however, early and intermediate successional stages possessed a great diversity of species including many frugivores.     

Differences in xylem and leaf hydraulic traits explain differences in drought tolerance among mature Amazon rainforest trees

Considerable uncertainty surrounds the impacts of anthropogenic climate change on the composition and structure of Amazon forests. Building upon results from two large‐scale ecosystem drought experiments in the eastern Brazilian Amazon that observed increases in mortality rates among some tree species but not others, in this study we investigate the physiological traits underpinning these differential demographic responses. Xylem pressure at 50% conductivity (xylem‐P₅₀), leaf turgor loss point (TLP), cellular osmotic potential (π_o), and cellular bulk modulus of elasticity (ε), all traits mechanistically linked to drought tolerance, were measured on upper canopy branches and leaves of mature trees from selected species growing at the two drought experiment sites. Each species was placed a priori into one of four plant functional type (PFT) categories: drought‐tolerant versus drought‐intolerant based on observed mortality rates, and subdivided into early‐ versus late‐successional based on wood density. We tested the hypotheses that the measured traits would be significantly different between the four PFTs and that they would be spatially conserved across the two experimental sites. Xylem‐P₅₀, TLP, and π_o, but not ε, occurred at significantly higher water potentials for the drought‐intolerant PFT compared to the drought‐tolerant PFT; however, there were no significant differences between the early‐ and late‐successional PFTs. These results suggest that these three traits are important for determining drought tolerance, and are largely independent of wood density—a trait commonly associated with successional status. Differences in these physiological traits that occurred between the drought‐tolerant and drought‐intolerant PFTs were conserved between the two research sites, even though they had different soil types and dry‐season lengths. This more detailed understanding of how xylem and leaf hydraulic traits vary between co‐occuring drought‐tolerant and drought‐intolerant tropical tree species promises to facilitate a much‐needed improvement in the representation of plant hydraulics within terrestrial ecosystem and biosphere models, which will enhance our ability to make robust predictions of how future changes in climate will affect tropical forests.     

Allometry and Shade Tolerance in Pole‐Sized Trees of Two Contrasting Dipterocarp Species in Sabah,Malaysia1

We compared the allometry of two contrasting late‐successional dipterocarp species to test whether a monolayer (shade‐tolerant)–multilayer (shade‐intolerant) model applies to pole‐sized trees. Crown traits of the more shade‐tolerant species (Vatica micrantha) did not conform to either of the familiar monolayer or multilayer models for pole‐sized trees, but instead were consistent with a “persistent multilayer” model. Species differences in crown traits may be influenced more by future rather than present light environments.