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Research Interests
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Tropical rainforests contain some of the world’s richest plant communities, but our current understanding of tropical plant diversity, species distributions and geographic ranges is still fragmentary. Studying complex interactions of evolutionary constraints with physiology, and biogeographic factors such as species range-size, population dynamics and community interactions, has aided understanding of what limits species distributions, a central question in ecology. My thesis provides a case study, using tropical floristic data, which addresses two broad questions: 1) Are there discernable patterns in species diversity in relation to environmental gradients, and if so, what is the nature of such patterns? 2) How will species respond to global climate change and the current and future predicted increases in temperature and decreases in rainfall? I investigate patterns of species diversity in montane tropical forests, which are known hotspots of biodiversity and endemism due to the compaction of climatic zones along elevational gradients. This climatic compression results in a highly heterogeneous environment where species diversity peaks at mid-elevations, making this system highly susceptible to global climate change. The impact of small shifts in temperature and rainfall in the tropics may be significant and result in altered geographical patterns and ecosystem functions. A substantial and rapid warming since the mid-1970’s translates into a predicted warming of tropical rainforest regions of between 2 to 5°C during this century. Precipitation has declined broadly during the 20th century over tropical land masses. Future predictions of global climate change models predict shifts in rainfall patterns together with an increase in the frequency of ENSO events for the tropics. Drought periods, have been associated with increased mortality and decreased growth rates in tropical plants. In Panama, where there is a seasonal rainfall regime, a wide range of species display wilting, varying degrees of desiccation tolerance, and drought acclimation. To understand how rainfall and seasonality affect the spatial and temporal changes in tropical forest composition and diversity, we need to understand the physiological mechanisms by which soil water availability affects tropical rainforest plants and whether any reduction in precipitation may affect tropical forest composition by favoring species with plastic responses to drought periods or desiccation tolerance. My research interests are biogeographically focused, and through out my Ph.D. I have developed a suite of physiological skills and techniques that allow me examine individual, population, species, and community level responses to current and future changes in our global climate. My research has focused on montane and lowland tropical rainforest in Costa Rica (see field sites, below) and much of my work has been conducted using the highly specious family of understory plants in the Family Rubiaceae. Many of these projects have developed from each other and are currently being prepared as manuscripts for publication and my doctoral thesis submission.
My study site consists of a 35 km long elevational transect located in primary rainforest within the Parque Nacional Braulio Carrillo (PNBC) in Heredia Province, Costa Rica. The transect runs continuously from 40m at the La Selva Biological Station to the Volcan Barva summit at 2906m in elevation. PNBC, and its protection, are fundamental in the supply of drinking water in the Central Valley and Sarapiqui area. The park is also a substantial part of the Area de Conservacion Coridillera Volcanica Central, a presently fragmented corridor of protected areas across the high altitude areas of Costa Rica. Threats to the park include poaching of forest products such as bush meat and palmito and increased fragmentation and isolation due to extensive cutting of forest on the fringes and in the surrounding area. http://www.costarica-nationalparks.com/centralvolcanicconservationarea.html
This study site displays little marked seasonality in rainfall and temperature, receiving very high rainfall that peaks at mid elevations ranging from 3500 - 4000mm/year at La Selva (100 m), 8000 mm at Rara Avis (700 m) and 3300 m close to the Barva summit. The mean annual temperatures at La Selva is 24°C decreasing in a linear fashion with increasing altitude to 10.5°C near the summit. The temperature lapse rate for this gradient has been estimated to be 6°C per 1000 m above sea level. The soil parent material is basaltic and andesitic lavas of Pleistocene age and studies of the gradient’s soils and litter chemistry have been conducted that indicate mineralization and decomposition rates decrease with altitude and total nitrogen and carbon and micronutrients increase with altitude. In addition, patterns of plant distributions and diversity have been document for trees over DBH 10cm, epiphytes, ferns, and for arthropods.
Tropical understory shrubs provide an excellent model group to investigate patterns in species diversity due to their short generation time, wide distribution and relative high abundance compared to that of many rainforest trees. The family in question, Rubiaceace, is the fourth largest family of flowering plants in the world and one of the most important components of tropical woody vegetation, especially in the rainforest understory. This family is pantropical in distribution with an estimated 13,000+ species worldwide in over 600 genera, including 120 species recorded for Costa Rica, permitting future comparative studies across the tropics. Pollination and seed dispersal of many species in the neotropics is by understory birds, with many bright juice fruits and flowers, making these plants candidates for "keystone species" due to their widespread distribution and long flowering and fruiting periods. Members of this family vary in habit, ranging from high alpine cushion plants to 5-7 m high rainforest trees. Many species in the tropics are restricted to the rainforest understory, enabling the investigation of temperature and rainfall environmental gradients without the interacting affect of species distributed across a light gradient. Herbarium specimens are held for all recorded species at the Instituto Nacional de Biodiversidad (INBio), with replicates at Missouri Botanical Garden (MO) to facilitate species identification. Species of this family have been reported to have very low seed viability, but have been demonstrated to vegetatively reproduce from most aboveground tissue (Gilman unpublished) facilitating manipulative experiments.
Current understanding of tropical plant diversity, species distributions, and altitudinal ranges is fragmentary. Concern due to global climate change and its potential impact on montane biodiversity demands an enhanced understanding of species and their abiotic and biotic interactions to enable more effective conservation strategies. Using a high-resolution quantitative data describing species altitudinal range and richness for a keystone family within rainforest understory communities it is possible to test existing hypotheses of altitudinal patterns in species richness and range size distribution. These hypotheses include the traditional ecological hypothesis that species richness monotonically decreases with increasing elevation, Rapoport’s Rule the mid-domain model and climate and community overlap hypotheses. Objective: Mapping the geographic range of the Family Rubiaceae and describing the pattern of species richness and range-size distribution. Method: Field studies plots of 0.1ha were surveyed (Jan. – June 2004) in primary rainforest in Parque Nacional Braulio Carrillo and at La Selva Biological Station at 100m elevational intervals over a gradient from 40-2900 m. Results of initial data analysis are exciting; preliminary identification of over 1200 plant vouchers indicates 82 species in 21 genera, with the number of species at a single elevation varying from 2 to 23. These collections include at least one new species to science and two new records for Costa Rica. Montecarlo simulations using empirical range data in RangeModel support a mid-domain effect but do not fully explain the species richness pattern. Species altitudinal ranges suggest a continuous community overlap rather than multiple distinct groups. Small range-sizes were more frequently observed than large range-sizes suggesting that plants in the family Rubiaceace are more specialized in their adaptations to environmental factors which limit range-size rather than being community generalists.
Few nutrient survey data sets exist in the world for montane tropical rainforest gradients. It is consequently challenging to investigate species richness patterns and the distribution of species across landscape scales in the absence of key data, such as soil nutrient status. The presence of high levels of nutrients in the soil, however, does not directly indicate availability to the plants present at a given location. To assess potential limitations it is necessary to conduct a fertilization experiment, however the rugged and challenging topography of Braulio Carrillo, mean I’ll leave that one to a more ambitious researcher than I! Instead, an indication of plant nutritional status can be obtained from sampling mature leaves of plants simultaneously collected with soil samples. A simple test of correlation may provide us insight. Objective: To simultaneously collect and analyze soil samples from the common rooting zone for Rubiaceae (20-40cm deep) and mature leaves from Rubiaceae species at the same elevation. Total P and N will be investigated in soil and leaf samples to determine if any correlative relationship exists, in addition to Leaf Specific Area (hypothesized to decrease with increasing elevation). Soil and plant nutrient gradients will be added to the model of species richness patterns over elevational gradients described above.
Vegetation-climate models suggest that montane biota may move higher in altitude in response to global climate warming due to the direct effects of extreme temperatures on plant metabolism. However, it is unlikely that the pattern of change will be so predictable, with paleo-ecological data suggesting that species-specific physiological tolerances, in conjunction with genetic variation and phenotypic plasticity, will result in a significant reassortment of species within plant communities. Such community uncoupling, in response to such rapidly occurring climate changes, is predicted to result in some species extinctions. Empirical descriptions of thermal niches can enhance our understanding of factors that determine range-size and boundaries. Plasticity patterns often vary genetically within natural populations. Species that are genetically highly plastic might be ecological generalists, whereas species that express limited adaptive plasticity might be restricted to narrower, ‘specialist’ ecological ranges. Thus, plasticity itself may determine the ability of a species to tolerate environmental changes that occur at a greater rate than that at which the species can evolve. Objective: To predict the consequences of global climate change on individuals, populations, and species within a clade of understory plants by investigating the relationships between altitudinal range-size, ecotypic variation and phenotypic plasticity. An important and widespread tropical genus of understory shrubs, Psychotria (Rubiaceae) will serve as a model system for this investigation. I propose to perform manipulative field experiments to determine the physiological tolerances to temperature, acclimation ability, and phenotypic variation of species in this genus and to identify potential ecological generalists or specialists within this group. Additionally DNA analysis will establish levels of intraspecific genetic diversity. These experiments will enable me to examine the consequences of these differences in terms of present distribution as well as predicted distribution in response to global climate change.
New Project: Tropical rainforest community wide extreme temperature tolerances, predicting winners and losers in the fight against global climate change. Email Alex for more information alexgil@ucla.edu
Soil moisture availability in tropical soils may be one of the main factors influencing plant growth, mortality and habitat associations. Yet the responses of tropical rainforest plants to soil water availability and drought have received little attention, perhaps because desiccation tolerance in these communities has been assumed to be poorly developed. Drought periods, have been associated with increased mortality and decreased growth rates in tropical plants. In Panama, where there is a seasonal rainfall regime, a wide range of species display wilting, varying degrees of desiccation tolerance, and drought acclimation. To understand how rainfall and seasonality affect the spatial and temporal changes in tropical forest composition and diversity, we need to understand the physiological mechanisms by which soil water availability affects tropical rainforest plants. Precipitation has declined broadly during the 20th century over tropical land masses. Future predictions of global climate change models predict shifts in rainfall patterns together with an increase in the frequency of ENSO events for the tropics. While the mechanistic understanding of the linkages between global climate warming, ENSO, and precipitation trends remain contentious, any reduction in precipitation may affect tropical forest composition by favoring species with plastic responses to drought periods or desiccation tolerance. Species possessing both a drought avoidance mechanism and a physiological tolerance mechanism to allow drought acclimation would be at an advantage because of increased flexibility in response to changing environmental conditions. Dry forest species are key examples of potential winners in this situation, with their abundance potentially increasing in wet forest communities resulting in a decrease overall of species diversity. It is not clear, however, whether such plasticity would have a negative fitness cost associated with it under stable environmental conditions.
This experiment investigated the consequences of chemical, plant and environmental treatments upon asexual propagation success of cuttings of six species of Psychotria (Rubiaceace) to establish a reliable and inexpensive method of generating clonal populations for forest restoration and experimental research. This project involved over 4,000 cuttings in 20 treatments and was conducted August – December 2003 and 2004. The experiment was extremely successful across all growth forms, with even single leaves rooting from petiole tissue in the absence of nutrients. Data analysis indicates that chemical hormone additions and forest soil treatments were most successful, even though higher nutrient alternatives were provided.
AMF colonize the roots of plants in all terrestrial ecosystem, and the relationship of this interaction is generally considered to be mutalistic. AMF symbionts are known to have numerous effects on plant physiology, and plant community structure and function, even though the specific mechanisms by which AMF interact with their hosts are presently not well understood. The acquisition of AMF associations in a wide range of ecosystems has been demonstrated to result in an increase in growth generally thought to be linked to the enhanced nutrient uptake, particularly phosphorus, from the soil. In tropical rainforests, phosphorus is widely reported to be the principle nutrient limiting tree growth and productivity, and AMF colonization has been shown to aid plant acquisition of phosphorus in acidic mineral deficient soils such as those derived from volcanic parent materials. In the restoration of threatened ecosystems, such as tropical rainforests, an important part of seedling establishment and survival is the relationship between roots, soil nutrients, and soil organisms such as AMF. The establishment of species and their plant growth responses have been demonstrated to relate to the extent of the AMF colonization. Significant biotic variables in many habitat restorations have been reported highlighting the fragmentary status of our knowledge in respect to the role of mycorrhizal fungi in these anthropogenically constructed systems. Enhanced understanding of the relationship between species of dominant and widespread families, such as Rubiaceae, and AMF is essential if we are to be successful in the restoration of areas of tropical rainforest, in addition to some types of experimental research where cuttings are used to generate sets of cloned material for common garden designs. Objectives: Using the ecologically important and widespread understory genus Psychotria as a model system this project will determine the presence of arbuscular mycorrhizal fungi (AMF) associations in two species of Psychotria (Rubiaceae), and quantify the ability of their cuttings, planted in a common garden, to acquire AMF. Additionally, this project will assess the scale of AMF colonization in these cuttings and its relationship with plant growth (biomass addition).
New projects:
Tropical rainforest community wide extreme temperature tolerances, predicting winners and losers in the fight against global climate change.
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