I am interested in the role of diversity (species, communities, functional types) in ecosystem functions, which has important implications for both the accurate prediction of ecosystem response to future global change as well as the conservation of diversity.  I seek a mechanistic understanding of ecosystem responses to climate variability based on functional groups (dry vs. wet tropical forests, C3 vs. C4 grasslands).


Environmental controls on the distribution and phenology of C3 and C4 grasses

  

            

A number of studies have demonstrated the ecological sorting of C3 and C4 grasses along temperature and moisture gradients.  However, comparing species in different and relatively unrelated lineages may obscure our understanding of the ecological differences imposed by photosynthetic pathway.  I am currently investigating the distribution and phenology of C3 and C4 grasses in Hawaii in a phylogenetic context (i.e., comparing lineages and closely-related sister taxa) using a combination of remote-sensing data, herbarium records of species localities, and climate data. 


Photo: Panicum hillebrandii (C3 grass) from www.hear.org


Investigating the seasonal and interannual influence of clouds on tropical forest phenology


Seasonal variation in light has been shown to control the timing of phenological events in tropical forests.   Whereas the phenology of seasonally dry tropical forests are expected to be water-limited, rainforests receive plenty of water and thus are expected to be light-limited.  This study uses a new globally gridded satellite dataset, the NOAA NCDC GridSat-B1 data, to examine the role of clouds and light limitation in the rainforests of Luquillo, Puerto Rico (LTER) in comparison to the seasonally dry tropical forest at Barro Colorado Island (BCI), Panama (CTFS). 

                                                      

                                                                                                  Photo of BCI: Christian Ziegler, Wikimedia Common         

                        

The response of ecosystems to El Niño-Southern Oscillation (ENSO) provides a useful analogy for how these ecosystems may respond to future climate change.  I used satellite NDVI over the past decade to map the spatial and temporal response of rainforests and dry forests in the Hawaiian Islands to show that they exhibit asynchronous responses to an El Niño-driven drought.  Whereas the dry forests showed a strong “brown-down”, the rainforests showed a “green-up” response to El Niño. The divergent response was related to cloud cover in the rainforest indicating a strong role of light-limitation,. 

Photo: SST anomalies from TOPEX/Posiedon

www.visibleearth.nasa.gov



The diversity of Hawaiian dry forests and their role as a signal for climate change


The tropical Pacific plays an important role in global climate variability and there is currently a lack of high-resolution climate records from that region.  I completed a high temporal resolution paleoecological study (fossil pollen and charcoal) of a lake sediment core, from Maui, Hawaiian Islands that provided strong evidence that a shift from dry to wet conditions marked the beginning of the Medieval Climate Anomaly (also known as the Medieval Warm Period).  This wet climate interval coincided with rapid Polynesian population growth, which resulted in a loss of diversity of native forests.    

       


  
               

NDVI is increasingly used to for species distribution modeling and to predict community richness patterns.  However, the mechanisms underlying the relationship between species diversity and NDVI are not well established.  I used extensive field data from Hawaiian dry forests, combined with over a decade of satellite images, in structural equation models (SEMs) to show that underlying the simple correlation between NDVI and species richness was the indirect of precipitation and the direct effect of structural complexity.