Organized by:

Horacio Paz, Centro de Investigaciones en Ecosistemas, Universidad Nacional
Autónoma de México

Lourens Poorter, Forest Ecology and Forest Management Group, Centre for
Ecosystem Studies, Wageningen University

Louis Santiago, Department of Botany and Plant Sciences, University of
California, Riverside

Kaoru Kitajima, Department of Botany, University of Florida
The goal of the symposium is to discuss how functional analysis of species in complex tropical plant communities is allowing us to ask new questions and gain a greater understanding of the mechanisms that underlie patterns of plant community organization.
By functional analysis, we mean multispecific comparative analysis of plant traits that describe how plants use resources and allocate them to growth, storage, defense, and physiological function. The symposium is divided in a morning session on shade tolerance, and an afternoon session on drought tolerance. During the past decades, shade tolerance has been the central paradigm to understand how species partition resource gradients. The morning session focuses on some new approaches (measuring the wholeplant light compensation point) and conceptual ideas (the importance of physical defenses for the growth survival trade-off, ontogenetic changes in light requirements).
The afternoon session focuses on the rapidly developing field of water relationships, and its importance for drought tolerance and species distribution. We end with a discussion whether there is a trade-off between shade and drought tolerance, or whether these represent largely orthogonal axes, thus contributing to species coexistence and a high species diversity in the tropics.
Sean C. Thomas 1 and Jennifer L. Baltzer 2
1
University of Toronto
2
Harvard University.
Email : sc.thomas@utoronto.ca
The concept of "shade tolerance" was originally derived from applied studies on temperate forest trees, with shade tolerance classes reported being essentially subjective.
The recent comparative literature on tropical forests has used a variety of "proxymeasures" of shade tolerance, including demographic, environmental, and physiological parameters. Here we make the case for explicit measures of the whole-plant light compensation point (WPLCP) -- estimated as the X-intercept of relationships between tree growth and integrated measures of light availability -- as the most useful measure of the ability of trees to grow and survive under low light conditions. Our recent studies suggest that WPLCP corresponds closely to subjective shade tolerance classes in well-

studied forest systems, that WPLCP for growth and survival are nearly identical, and that leaf dark respiration rates are the single most important physiological correlate of
WPLCP in tropical trees. WPLCP also shows consistent variation with tree soil preferences in some cases, providing important insights into resource limitations on tree spatial distributions. We conclude that the WPLCP approach provides a critical tool to understanding a broad range of questions in the functional biology of tropical trees.
Keywords : shade tolerance, light compensation, dark respiration, plant resources, physiological ecology
Niels Anten, Roelof Oomen and Galia Selaya
Section Plant Ecology & Biodiversity, Institute of Environmental Biology, Utrecht
University
Email : N.Anten@bio.uu.nl
After large-scale disturbances in tropical rain forest, short- and long-lived pioneers (SLP and LLP, respectively) usually emerge at about the same time; however SLPs soon dominate the top of the canopy. What characteristics enable these SLPs to achieve early dominance and how are the LLP able to co-exist with their shorter-lived neighbors? To address this question we estimated whole-plant carbon gain of three SLP, four LLP and three liana species growing in 1/2, 2 and a 3 years old secondary forest stands. The SLPs
Ochroma and Trema readily dominated the canopy and by the third year were more than two-fold taller than the other species. In the 1/2 y/o stand these two species had higher photosynthetic rates both in absolute terms and per unit leaf- (Pleafmass) and total mass
(Pmass) than the other species. These greater Pmass and Pleafmass values were not so much correlated with a greater efficiency of light capture but rather with a greater photosynthetic light-use efficiency. This suggests that leaf physiological traits, that largely determine this light-use efficiency, are important in giving SLPs an early advantage. LLPs and lianas in turn had three-fold greater leaf longevities than the SLPs.
Overall there was a strong negative correlation between Pleafmass and leaf longevity suggesting that traits that facilitate a high instantaneous efficiency in photosynthesis e.g. a high specific leaf area (SLA) or a high PNUE- are associated with low leaf longevity.
The photosynthesis per unit leaf mass over the life of a leaf was slightly higher in the
LLPs and lianas than in the SLPs. Coexistence between pioneer species during early phases of succession might therefore be associated with a trade-off between plant characteristics that confer a high instantaneous efficiency in photosynthesis and those that confer a long leaf lifespan.
Keywords: co-existence; competition; functional traits; secondar forest; restoration

Rebecca A. Montgomery 1 and Thomas J. Givnish 2
1 University of Minnesota
2
University of Wisconsin-Madison
Email : rebeccam@umn.edu
Shaded environments are not uniformly dark, but punctuated by sunflecks, short bursts of bright light, that carry up to 90% of the photosynthetically active light that strikes plants.
Sunflecks can account for up to 65% of total carbon gain in forest understories.
Theoretically, understory plants should undergo physiological induction and open their stomata rapidly in response to sunflecks, and slowly lose induction and close stomata afterwards, while sun-adapted species should undergo induction more slowly and lose it more rapidly. Such patterns have been observed in a number of studies in both temperate and tropical forests. In almost every study to date, however, comparisons have only been made between two or three species, or two or three categories of species based on their relative distributions along a light gradient. Never has a quantitative relationship been drawn between measures of the light regimes inhabited by several species and the dynamic photosynthetic responses of those species. We used the Hawaiian lobeliads, which have radiated into habitats from open alpine bogs to shaded rain-forest, to examine the relationship of dynamic photosynthetic light responses to dynamic light regimes and to explore how dynamic responses influence daily leaf carbon gain. We found that the rate of photosynthetic induction increased significantly toward shadier sites. Uninduced light use efficiency – actual photosynthesis vs. that expected under steady-state conditions – increased from 10% to 70% across the same gradient. In silico transplants – modeling carbon gain using one species’ photosynthetic light response in its own and an other species dynamic light regimes – demonstrated the adaptive nature of species differences: understory Cyanea pilosa in it’s light regimes outperformed gap-dwelling
Clermontia parviflora, while Clermontia in it’s light regimes outperformed Cyanea . The apparent crossover in daily photosynthesis occurred at about the same PFD where dominance shifts from Cyanea to Clermontia in the field.
Keywords : Dynamic photosynthetic responses, light partitioning
Kaoru Kitajima
University of Florida
Email : kitajima@botany.ufl.edu
Tissue density and biomechanical strength are hypothesized to be important as functional traits underlying the growth-survival trade-offs widely observed among tropical tree species. However, direct quantitative tests of this hypothesis have been rare. Increased allocation to defense is expected to enhance seedling survival especially in low-resource

environments where recovery from tissue loss would be slow, even if it requires growth rate reduction. Here, I summarize the results of recent studies that have quantified interspecific variations in tissue density and fracture toughness of seedling leaves and stems, as well as modulus of elasticity (stiffness) of stems, for neotropical tree species from seasonal moist and dry forests. In all data sets, the wide variation in biomechanical strength among species is strongly correlated with tissue density and fiber contents.
Seedlings that survive well in shaded understories are physically well defended with high tissue density in both leaves and stems, even very early in ontogeny. Tissue density and mechanical strength are positively correlated between stems and leaves, as well as between developmental stages, suggesting concordance of physical defensiveness between organs and ontogenetic stages. Leaf and stem density exhibited a stronger correlation with seedling survival than mechanical strength. Thus, high tissue density provides not only mechanical strength, but also additional ecological advantage for juvenile persistence in the shaded forest understory such as enhanced resistance against pathogens. However, these survival advantages of high tissue density and mechanical strength are apparently in a trade-off relationship with growth rates. Thus, mechanical defensiveness incurs a large opportunistic cost in resource rich environments such as treefall gaps. Possible reasons why slow growth associated with high physical defense include lower photosynthetic efficiency of thicker leaves, costs of allocation to fibers and support biomass, and slower developmental rates required for construction of dense and physically stronger tissues.
Keywords : biomechanics, physical defense, tissue density, trade-offs, tropical tree seedlings
Gerardo Avalos 1,2 , Gabriela Gei Maria 2 , Mauricio Fernandez 2 and Olivia
Sylvester 3
1
The School for Field Studies, Center for Sustainable Development Studies
2
The University of Costa Rica, School of Biology
3
The University of Costa Rica, Sch.
Email : faetornis@yahoo.com
Palms represent one of the most conspicuous groups of Angiosperms in Tropical Forests.
Although their contribution to overall species diversity is small, they influence forest structure and dynamics, and play an essential role in foodwebs. The distribution of palms reflects fine shifts in environmental heterogeneity and light availability. In arborescent species, adaptation to fine gradients is constrained by the capacity to increase in height and provide crown support while maintaining structural stability. The lack of a vascular cambium determines habitat colonization and resource allocation across gradients of light environments, influencing overall palm structure and architecture. We first analyzed the allometry of the stem height vs. stem diameter in eight neotropical palms representing a wide range of habitat and regeneration requirements across different successional sites in

Costa Rica (Euterpe precatoria, Prestoea decurrens, P. acuminata, Geonoma edulis,
Chamaedorea tepejilote, Cryosophila warscewiczii, Iriartea deltoidea, and Socratea exhorriza), and then evaluated overall palm structure by considering stem diameter, stem height, number of leaves, and the structure of the stilt root cone using a Principal
Component Analysis (11 morphological variables). We found a strong logarithmic relationship between stem diameter and height. With the exception of the understory palms G. edulis and C. warscewiczii, shaded palms under 1.5 m had significantly lower slopes relative to palms that escaped the light-limited understory, which grew 4-9 times faster. The first principal component (71.4% of the variation) had a homogeneous contribution of most of morphological variables, with the exception of the number of functional leaves, which dominated the second principal component (11.7%). Our results showed that species that differ significantly in size and light requirements shared a common solution to structural problems. Understanding these allometric relationships provides insight in determining how physical limits to morphological diversity influence the distribution and abundance of tropical palms.
Keywords : Tropical Forest, Palm Ecology, Costa Rica, Palm Allometry, Shade
Tolerance
Lourens Poorter
Forest Ecology and Forest Management Group, Centre for Ecosystem Studies,
Wageningen University
Email : lourens.poorter@wur.nl
Functional traits are important drivers of successional processes and the assembly of plant communities. It is generally assumed that functional traits are closely linked to the regeneration niche because of the high selection pressures in the seedling stage, but recent studies have challenged this view. In this study I use cross species and phylogenetic correlation analysis between leaf traits and light environment to evaluate whether species are adapted to the regeneration niche, adult niche, or both. Leaf chemistry, morphology, physiology and crown exposure were quantified for up to 58
Bolivian tropical moist forest tree species that differ in their regeneration and adult light niche. Multiple regression analysis shows that leaf traits of seedlings, saplings, and trees are most strongly related to the regeneration niche, and once that this is taken into account, adult niche does not significantly explain any of the remaining variation in leaf traits. This suggests that, although the regeneration phase is short, it has a long-lasting effect on the form and shape of plant species.
Keywords: adaptation, functional traits, light, maximal size, tropical rain forest

Louis Santiago
University of California, Riverside
Email : santiago@ucr.edu
Drought tolerance and avoidance represent two main ways in which plants deal with water limitation. Loss of leaves through dry season deciduous phenology is a major mechanism of drought avoidance in tropical trees. Tropical forests vary along a gradient from seasonally dry forests with a high proportion of canopy species that lose their leaves during the dry season, to continuously wet forests dominated by evergreen canopy species. Data from a precipitation gradient in Panama demonstrates that deciduous species minimize water loss and respiration costs during seasonal drought, but have relatively high photosynthetic rates, allowing them to maintain high rates of carbon gain when water is available. In contrast, evergreen species have lower photosynthetic rates, but have a potentially longer photosynthetic season. Evergreen species also appear to reduce demand on soil nutrients required to replace leaves. Although seasonally dry tropical forests are appreciated for their preponderance of dry season deciduous species, which represent a drought avoidance strategy, these forests often have an evergreen component which represents the drought tolerant strategy. Data from an extremely dry forest in Mexico suggests that there is a marked divergence in strategies in which species show strong deciduous or evergreen phenology under extreme water stress. In subtropical arid ecosystems of southern California, these tradeoffs are yet more pronounced, with thick-leaved, evergreen chaparral shrubs and cacti representing extreme drought tolerance strategies, and drought deciduous coastal sage scrub and desert annuals representing extreme drought avoider strategies.
Keywords : drought tolerance, phenology, California, Mexico, Panama
Amy Zanne 1 , David Ackerly 2 , David Coomes 3 and Ian Wright 4
1
NESCent
2
University of California, Berkeley
3
Cambridge University
4
Macquarie University
Email : aezanne@gmail.com
Moisture availability is believed to exert strong selective pressure on plant traits, especially with respect to wood anatomy and hydraulics. We have been investigating coordination of wood anatomical and hydraulic traits with leaf traits at regional scales in
Australia and California and global scales. These relationships have also been investigated along gradients in precipitation. We predicted that species with higher wood density would have lower hydraulic conductivity and be found in areas with lower precipitation. Additionally, we expected that hydraulic traits would be better coordinated with traits related to amount of leaf area deployed per sapwood area (LASA) than to leaf

size. At the regional-scale in Australia, we have measured wood density, vessel anatomy, leaf size, LASA, maximum plant height, and hydraulic conductivity for a total of 32 tree and shrub species at sites contrasting in average annual rainfall (387 vs. 1220 mm).
Additionally, we have assembled a global database (approximately 4000 species) of wood density, vessel size and vessel density for trees and shrubs. In both the regional and global study, we have found that wood density is relatively uncoupled from hydraulic traits. Instead, vessel size and vessel density are scaling isometrically such that total vessel lumen is being maintained. Furthermore, hydraulic conductivity and vessel anatomy are strongly related to the amount of leaf area deployed (LASA) and the maximum height that plants attain. Precipitation was still a strong predictor of hydraulic, wood density, and leaf traits. The higher rainfall sites had higher hydraulic conductivity,
LASA, but lower wood density. These results suggest that wood density is more strongly related to functions other than hydraulics (e.g., mechanical support or storage) and that hydraulic conductivity appears to be a strong determinant of the amount of leaf area carried by a plant.
Keywords : Australia, hydraulic conductivity, precipitation, wood density, vessel anatomy
Radika Bhaskar 1 , David Ackerly 1 and Alfonso Valiente-Banuet 2
1
Department of Integrative Biology, University of California Berkeley, CA, USA
2 Instituto de Ecología, Universidad Nacional Autónoma de México
Email : radika@gmail.com
Under conditions of drought, the ability to regulate water use is of critical importance to plant performance. To prevent stomatal shut-down and hydraulic failure, plants must balance water supply from the roots with transpirational water loss through the leaf.
Thus, both soil water deficit (soil drought) and evaporative demand (atmospheric drought) present distinct challenges for plants. Comparing communities with contrasting precipitation patterns provides an opportunity to examine how timing of soil versus atmospheric drought influences plant water use and consequently carbon gain. In
California, chaparral shrublands experience hot, dry summers and cool, wet winters, characteristic of Mediterranean environments; thus soil and atmospheric drought coincide. In central Mexico, subtropical shrub communities are morphologically and taxonomically similar to California chaparral, yet climatically distinct: a winter drought is followed by summer monsoonal rains. Sites in Santa Barbara, California, and Tehuacan,
Mexico, were matched for mean annual temperature, precipitation, and drought duration.
Two suites of traits were measured, one related to hydraulic function (e.g. conductivity, minimum seasonal water potential, vulnerability to embolism, wood anatomy), and the other to the leaf economic spectrum (photosynthetic rate, nitrogen concentration, specific leaf area, and carbon isotope composition). Within both communities, species varied widely in hydraulic traits; those with lower leaf-areabased whole plant hydraulic conductance (Kwp) experienced more negative minimum seasonal water potentials.

However, species in California, where atmospheric drought was higher, had significantly higher rates of Kwp relative to species in Mexico. The shift observed between sites in
Kwp demonstrates how soil and atmospheric drought may have opposing influences on hydraulic architecture. On functional grounds we expected a link between photosynthetic and hydraulic capacity. However, in contrast with results from more mesic systems, Kwp and maximum assimilation were poorly correlated, suggesting photosynthetic and hydraulic strategies may be independent in subtropical arid communities with strong seasonality of rainfall.
Keywords : hydraulic, soil drought, conductance
Horacio Paz and Fernando Pineda-Garcia
Centro de Investigaciones en Ecosistemas, Universidad Nacional Autónoma de México
Email : hpaz@oikos.unam.mx
In tropical dry forests, temporal and spatial variation in water availability is proposed as the dominant factor affecting species life history and distribution. However, the diversity of water use strategies and the trade-offs involved are still poorly known for this community. For seedlings, we analyzed the patterns of morphological and physiological divergences between a group of 10 congeneric pairs, where one species is specialized to dry habitats and the other to wet habitats in the same community. We predicted that compared to wet specialists, dry specialists will increase surface of water absorption in relation to water loss, water use efficiency, stem capacitance, root depth, stem density, and will exhibit lower vulnerability to embolisms, but at the cost of lower photosynthetic and growth rates. We found that as a general strategy, dry species decreased leaf area ratio, allocated more biomass to fat but not fine roots, suggesting that dry specialists reduced water loss area and did not increase water absorption capacity but probably carbohydrate reserves. Strikingly, most of analyzed traits did not diverge between wet and dry habitats in a general direction as predicted, indicating an important diversity of ways of adaptation to dry habitats. Variation among drought- functional traits seems to be organized along two axes of variation, the leaf life-length, and a trade-off between stem capacitance and vulnerability to embolisms. Also, dry specialists were less plastic adjusting their morphology to the soil water treatment. This differentiation suggests a soil water exploiting strategy for the wet specialists and a more resource conservative strategy for the dry specialists.
Keywords : drought tolerance, seedling morphology, seedling physiology, trade-offs, tropical tree seedlings

Lars Markesteijn and Lourens Poorter
Forest Ecology and Forest Management Group, Centre for Ecosystem Studies,
Wageningen University
Email : Lars.Markesteijn@wur.nl
Water availability is the most important factor determining tree species distribution and differentiation in the tropics, still underlying mechanisms are still not clear. In a tropical dry deciduous and a moist semi-deciduous forest we excavated first year seedlings of 62 tree species at the onset of the dry season. We quantified the variation in morphological root traits and biomass allocation of the seedlings from both forests and assessed functional grouping of species related to drought tolerance on the bases of among trait associations. In a separate study we quantified species ability to survive drought under standardized experimental conditions in order to evaluate how seedling traits affect drought survival, and how drought survival in turn determines species distribution along the rainfall gradient. Seedling traits of both studies were compared in order to validate whether patterns found under standardized conditions are in line with the actual situation as found in the forest.
Keywords: Bolivia, drought tolerance, morphology, survival
Edwin Lebrija-Trejos 1 , Erik Peters 1 , Eduardo A. Perez-Garcia 2 , Jorge A. Meave 2 ,
Frans Bongers 3 and Horacio Paz 4
1
Forest Ecology and Forest Management Group, Centre for Ecosystem Studies,
Wageningen University
2 Facultad de Ciencias, Universidad Nacional Autónoma de México
3
Wageningen University
4
Centro de Investigaciones en Ecosistemas, Universidad Nacional Autónoma de México
Email : edwin.lebrija-trejos@wur.nl
Tropical forest succession involves vertical and horizontal changes in light availability.
Models of succession and species performance have been closely related to these (steep) gradients, with species often arbitrarily assigned to predefined functional groups based on their light requirements for regeneration and adult stages (maximum height). In dry forests, changes in light availability during succession are expected to be reduced due to shorter canopy stature and more patchy development. Hence, other factors such as water availability and day time temperature are expected to increase their role in determining species performance and, consequently, community assemblage changes. To evaluate the importance of these latter factors, we analyzed the relationships between species traits, community dynamics and environmental factors along secondary succession plots in a tropical dry forest of southern Mexico. PAR, soil and air temperature, and water vapor deficits decreased significantly with community development, no clear pattern was observed for soil surface water potential. Species recruitment, survival and abundances

varied along succession, with traits related to transpiration and heat dissipation clearly separating early from late successional species. More in-depth studies of tropical dry secondary and mature forest species traits and trade-offs related to temperature and water stress are needed to properly characterize functional groups, understand niche differentiation and model secondary succession in theses systems.
Keywords : Functional Traits, Pioneers, Drought Stress, Secondary Succession, Tropical
Dry Forest