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Patterns of species diversity across tropical environmental gradients:
consequences for global climate change.
My dissertation will examine the broad questions of elevational patterns
in diversity in tropical montane rainforest species and will specifically discuss
the biogeographic patterns of elevational species distributions, experimental
cloning of vegetative plant material, acquisition of mycorrhizal associations in
cuttings, and physiological responses to temperature and desiccation and the
consequences of these for global climate change.
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 midelevations, 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 though 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
almost all my work has been undertaken 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.
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Field sites, Costa Rica
Study species: Family Rubiaceae
Elevational species distributions
Soil and plant nutrient gradients
Experimental cloning of vegetative plant material
Acquisition of mycorrhizal associations in cuttings
Physiological responses to temperature and the consequences of these for
global climate change
Physiological responses to desiccation and the consequences of these for
global climate change
Field sites, Costa Rica
My proposed 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.
This site has 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
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.
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Study species, Family Rubiaceae
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 genus in question, Psychotria (Rubiaceace), is a member of 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.
It is pantropical in distribution with an estimated 1700 species worldwide
including 120 species recorded for Costa Rica, permitting future comparative
studies across the tropics. Members of this genus are largely are restricted to
primary rainforest understories, enabling the investigation of temperature
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 genus
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).
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Elevational species distributions
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 middomain model and climate and community overlap hypotheses.
Objective: Mapping the geographic range of the Family Rubiaceae and
describing the of species richness and range-size distribution.
Method: Field studies plots 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. The species
richness curve supports a mid-elevational peak in diversity. Montecarlo
simulations using empirical range data in RangeModel supported 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.
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Soil and plant nutrient gradients
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 nutrients. 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 (assumed 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.
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Experimental cloning of vegetative plant material
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.
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Acquisition of mycorrhizal associations in cuttings
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).
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Physiological responses to temperature and the consequences of these for
global climate change
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 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.
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Physiological responses to desiccation and the consequences of these for
global climate change
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.