James W. Dalling - US grants

Despite current interest in tropical forests as key components of the global carbon
cycle and as centers of biodiversity,we remain largely ignorant of the inter-relationships
among forest attributes (e.g.,dynamics,diversity,structure),and the physical properties
of the environments in which these forests grow.In recent years datasets have become
available describing growth,mortality and recruitment of approximately 10%of the
global diversity of tropical tree species.These data result from the development of a
network of large-scale forest dynamics plots coordinated by the Center for Tropical
Forest Science (CTFS),and established using methods identical to those used to survey
and census the 50-ha Forest Dynamics Plot on Barro Colorado Island (BCI),Panama.
Just as data from the BCI plot have played a prominent role in our understanding of the
community-level consequences of ecological processes played out at local scales,we will
show how these plots now provide an unprecedented opportunity to examine a venerable
question in tropical ecology:How do soil-borne resources influence the variation in forest
structure and demographic turnover rates observed among plots,and among habitat types
within plots?
To achieve this objective we propose three sets of measurements.At nine plot
sites we will characterize variation in soil-moisture availability through the year and
throughout the plots using a hydrological model (TOPMODEL).This model predicts
soil-moisture saturation deficit based on rainfall,stream-flow data and local topography.
Second,at five plots currently lacking soils data we will sample soil-chemical properties
within standardized,topographically-defined habitat types.Third,at the same plots we
will perform seedling growth experiments using mycorrhizal and non-mycorrhizal
pioneer species to assess plant-availability of soil nutrients for each habitat type.These
measurements will allow us to address questions at two spatial scales.At the among-plot
scale we will be able to ask how stem density and basal area,as well as community-wide
patterns of growth,mortality and recruitment,correlate with moisture availability and soil
fertility.At the within-plot scale we will ask whether species exhibit specialization to
particular hydrological niches,and whether habitat types differ in soil fertility,in
turnover rates,and in local species richness.Resolution of these broad questions
concerning correlations among soil-borne resources and characteristics of the vegetation
will in turn permit us to refine future hypotheses aimed at understanding the mechanistic
basis for interspecific variation in demographics and distribution.
This study will provide the first standardized large-scale measurements of the
environmental context within which tropical forests grow.Our trans-continental approach
to testing questions of ecosystem function and community organization has rarely been
attempted,but it will be essential if we are to improve our understanding of the
biogeographic and biophysical limits to our ecological generalizations.Understanding
how variation in water and nutrient availability determines forest structure,composition
and dynamics,and potentially influences local diversity through niche partitioning,will
be essential to predicting future vegetation responses to climate change and will also
provide a first step in guiding management to protect forest diversity.Wide dissemination
and application of the results of this project is ensured by the active participation of in-
country collaborators,and by data sharing through CTFS.

Differences in regeneration requirements among species have been the focus of research to explain the high species diversity of tropical forest. However, most studies have concentrated on regeneration strategies related to sexual reproduction, neglecting the contribution of vegetative propagation Even though it has been recognized that a variety of tropical plant species have the potential to reproduce vegetatively, the actual frequency of different modes of reproduction in natural populations remains unknown. Vegetative reproduction can be a particularly important regeneration pathway for small-seeded understory species that have low recruitment success from seeds. The primary goal of this proposal is to determine the relative role that vegetative reproduction plays in the regeneration of species of tropical understory shrubs, and to evaluate the consequences of this mode of reproduction in terms of genetic diversity and mating opportunities. Plants will be mapped and similarities among their genetic fingerprints will be used to distinguish the clones from the genetically distinct individuals in the field. Combining the genetic and the spatial data I will be able to determine dispersion of clones and the frequency of establishment by sexual and vegetative reproduction in the population. This study will improve our understanding of tropical ecology in areas such as colonization, recruitment, gene flow, and population demography and genetics. The support of this grant will provide training in molecular techniques for a Panamanian woman scientist. Furthermore, the database of genetic information, with the geographic locations of individual plants will be made available to the research community to address a myriad of ecological and evolutionary questions, where knowledge of the genotypes and genetic relationships among plants is essential.

Understanding how biodiversity is maintained is a central issue in ecology. One long-standing theory is that species can coexist if they differ in their requirements for limiting resources. For plant species, the availability of soil resources such as water, nitrogen, and phosphorus may be critical in determining which species persist in a given area. This research will examine how a diverse group of palm species can coexist in tropical mountain forest in Panama. A series of seedling transplant experiments will be performed to test the hypothesis that differences in plant performance across a gradient of soil fertility results from differences in resource allocation patterns among species. Specifically, it is predicted that species found on the most nutrient-poor sites will allocate relatively more of their resources to root biomass and to anti-herbivore defenses. These allocation patterns should reflect the increased cost of extracting nutrients from the soil, and increased cost of herbivore damage when nutrients are in short supply. This study will be among the first experimental tests of a prominent hypothesis to explain how tropical tree species coexist. Furthermore, the proposed research will add new information on the relative importance of herbivore pressure and belowground resources in determining the distribution of forest understory palms, a group of species that have been heavily exploited for horticultural use.

Over the past 35 years, a robust theory of plant defenses has been developed to explain why plant species vary in the toxicity and physical toughness of their vegetative tissues. Application of this theory has broadened our understanding of plant-herbivore interactions, yielding new insights into plant ecology and evolution. This project will now extend the theory to cover defenses of seeds against fungi in the soil. Seeds have only a limited capacity to adjust resource acquisition and allocation to track a changing environment or to combat individual threats. Fungi that attack and can kill seeds should therefore create strong selection on suites of characteristics, or seed defense syndromes. This project will quantify the chemical and physical defenses of seeds of 18 tree species that exhibit a broad range of seed traits and germination behavior in lowland tropical forest in Panama. Experiments will determine the time course of seed survival, the efficacy of defenses at different times following dispersal onto the soil, and the ways in which particular defenses correlate with others to form syndromes of seed defensive traits. The study will offer a new perspective on the ecological and evolutionary importance of seed dormancy and a first step towards formalizing seed defense syndromes for an array of ecologically important plants.

The project will provide unique, cross-disciplinary training in tropical ecology, molecular biology, and fungal biology at the undergraduate, graduate, and post-doctoral levels and will support research and publication by students from the United States and Latin America. Research protocols and data sets will be posted on a bilingual project website, and results will be disseminated through the public outreach programs of the Mycological Herbarium of the University of Arizona.

Tropical rainforests support a rich diversity of life including over 50,000 tree species. One key to this diversity appears to be that different species grow on different soils. This project will explore the features of plant function that cause species to divide up the rainforest habitat in this way. Research will test the hypothesis that the ability of a tree species to store wood carbohydrates and nutrients in wood and then move them back out again for use in other parts of the tree determines where a species is found in the forest along a gradient from low to high soil fertility, a hypothesis not yet tested in tropical forests. The two main objectives are to compare 1) the stocks of carbohydrates and nutrients in wood in trees in different natural communities along parallel gradients of soil fertility in lowland and montane tropical forests in Panama, and 2) the ability of the saplings of related tree species that grow on different soils to remobilize stocks of nutrients in response to experimental defoliation.

This project will train a female Ph.D. student from the U.S. and expand international scientific collaboration with students and faculty from a local public university in Panama. As part of this collaboration, the Ph.D. student will mentor an undergraduate Panamanian student on a senior thesis project and serve as the teaching assistant for an undergraduate field course in Panama. In the U.S., the Ph.D. student will share her knowledge of plant biology as a volunteer in the Plants iView program, a six-week, after school, outreach experience at Urbana Middle School in Illinois. Besides this educational training and public outreach, the project will also benefit society by increasing understanding of how to maintain biodiversity.

Many mountains around the world share a characteristic vegetation. This consists of a forest canopy dominated by oaks or related species, and an understory layer that is filled by dense thickets of bamboo. The deep shade cast by these bamboo thickets, coupled with competition from bamboo for water and nutrients, greatly limits the regeneration success of tree seedlings. However, most bamboos also share an unusual trait ? they flower once in their lives and then die. At intervals of between 20 and 120 years, depending on species, an entire bamboo population at a site flower together. Following this episode of mass-flowering and mass mortality, light penetrates to the forest floor stimulating tree seedling growth for a few years before the bamboo returns. The decay of dead bamboo, however, also stimulates the growth of soil bacteria and fungi which compete with plants for nutrients in the soil. This competition reduces the availability of nitrogen, a key resource for plant growth. This project will examine how a bamboo die-off that began in 2020 in western Panama affects the regeneration of oak forest, an endangered ecosystem of critical conservation value. Specifically, it will test the hypothesis that bamboo mortality preferentially favors the regeneration of oaks, because these tree species form a specific type of association with fungi in the soil called ectomycorrhizas. Ectomycorrhizal fungi benefit oaks by providing access to a source of nitrogen that is unavailable to other competing plant species. Understanding how mountain oak forests function is essential to their management, and to predicting whether lower elevation tree species will be able to migrate upslope in a warming world. In addition, this project will provide training in forest ecology and plant-soil interactions for US undergraduate and graduate students.

The project will take advantage of an existing network of plots established in oak-bamboo forest on the slopes of Volcan Barú National Park. Bamboo, oaks and other tree species have been mapped in these plots, tree growth rates are being monitored annually, and soil nitrogen availability has already been measured. In addition to determining how bamboo die-off affects seedling recruitment, and juvenile and adult tree growth in these existing forest plots, a nitrogen addition experiment will be established to test whether oak recruitment is selectively favored due to their ectomycorrhizal association. Replicate patches of forest will be located that have similar tree community composition, but that have three classes of understory bamboo: live bamboo, dead bamboo, and no bamboo. Seeds and seedlings of the two locally dominant ectomycorrhizal oak tree species Quercus salicifolia and Q. costaricensis, and four similar non-ectomycorrhizal tree species will be transplanted into plots in these patches. Additionally, nitrogen fertilization and control treatments will be established in each plot and seedling growth monitored for one year. If ectomycorrhizal nitrogen supply favors oaks during bamboo decay then the seedling growth advantage of oaks versus non-oaks will be greatest in plots with dead bamboo and no additional nitrogen, and smallest in the nitrogen fertilized no bamboo plots.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.