April 20 2010

12:00 BSRB 154

EcoLunch Series

Graduate Student Presentations by Jeff Wolf, Christine Scoffoni, Christopher Johnson


Jeff Wolf
"Spatial heterogeneity, resource availability and species diversity in a tropical tree community"

According to resource-based competition (R*) theory, diversity and abundance of species populations should respond in predictable ways to spatial heterogeneity and availability of limiting resources (Tilman, 1982). Tropical trees all require a similar set of resources — light, H2O, CO2 and 14 mineral nutrients that are obtained primarily from the soil. R* theory predicts that greater spatial heterogeneity of limiting soil nutrients should result in greater numbers of co-occurring species. Also, that diversity should peak in habitats with intermediate supply rates of these limiting nutrients. There is evidence for an important role for mineral nutrients structuring tropical tree communities. About 40 % of the trees species populations in the 50 ha plot on Barro Colorado Island (BCI), Panama — where > 300 tree species coexist and there is > 3-fold variation in species richness amongst 20 by 20 m quadrats — are non-randomly distributed along essential nutrient gradients (John et al., 2007). Resource-based competition is one explanation for these findings (Hubbell et al. submitted). In this talk, I will review R* theory and describe the methods I am using to test its predictions for a tropical tree community.

Christine Scoffoni
“The rapid light response of leaf hydraulic conductance: new evidence from two experimental methods”

Previous studies have shown a rapid, up to several-fold enhancement in leaf hydraulic conductance (Kleaf) from low to high irradiance (from < 10 to >1000 μmol photons m-2 s-1), using the high pressure flowmeter (HPFM), for seven of 14 tested woody species. Such a response would have substantial physiological and ecological implications. However, theoretical suggestions have been made that this response might arise as an artifact of the HPFM, and no previous studies have tested if the response is robust across other measurement methods. We tested the Kleaf light response for six evergreen species using refined versions of the rehydration kinetics method (RKM) and the evaporative flux method (EFM). We found new evidence for the rapid light response of Kleaf for three of the six species. The RKM showed that the leaf rehydration time constant was significantly lower in high irradiance relative to darkness. In the EFM, there was a higher flow rate in high irradiance relative to low irradiance, but without a proportional increase of the leaf water potential gradient. This study increased by 50% the number of woody species tested, and those showing a Kleaf light response of two-fold or higher. Combining our data with those of previous studies we report a novel pattern linking the Kleaf light response to leaf morphology. On average, heterobaric species (i.e., those with bundle sheath extensions; BSEs) showed a greater Kleaf light responses than homobaric species (i.e., those without BSEs). Future work should take the Kleaf light response into account when investigating the responses of Kleaf to other internal and external factors. Future work should also focus on further detailed characterization of this substantial dynamic at the nexus of light- and water-relations, its role in whole plant performance, and its linkages with diversity in leaf physiology and morphology, and ecology.

Christopher Johnson
“The stability of nested topologies in mutualist and consumer-resource communities"

The relationship between ecological complexity and stability is an important question in ecology and has stimulated a great deal of debate about the mechanisms that maintain diversity in complex communities. Recently, intriguing research has investigated the topology (structure) of complex ecological communities and found that mutualist communities are highly-nested (with specialists interacting only with subsets of more generalist species) compared to consumer-resource communities. Nestedness has been proposed to increase stability by buffering mutualist communities from perturbations; however, a mechanistic explanation for such a hypothesis is lacking. I use a theoretical model to investigate whether nested communities are robust to species invasion that would change the topology and whether non-nested communities are susceptible to invasion that would build nestedness. Preliminary results indicate that nestedness is a more likely outcome in mutualist than consumer-resource communities. A proposed mechanism for this result is that nestedness increases stability in mutualist communities by mitigating an Allee Effect.















































































































































































































































































































































































































































































































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