July 3, 2015

Plant Phenology and a Changing Climate

Brown MBL PhD student Xi Yang studies plant phenology or “the rhythm of the seasons”. Plant phenology is the timing of events such as leaf-out in the spring and senescence (color-change) in the fall, events which are sensitive to climate change. Phenology can serve as an  important indicator of the impact of climate change on ecosystems. Changes in phenology also provide feedback to the climate system by, for example, changing the surface albedo. Xi uses remote sensing to monitor vegetation phenology at large spatial scales as well as models to predict future changes in phenology. He also uses a digital camera, spectroscopy and chemical analysis to understand the seasonality of plant physiology. 

Xi Yang with his instruments. Photo by Jim Tang.

Remote sensing provides a tool for ecologists to understand large scale phenomena. Plant color changes throughout a  season are easily captured by satellite sensors. Xi used a sensor named MODerate resolution Imaging Spectroradiometer (MODIS) onboard the satellite Terra and Aqua to monitor phenological changes in plants in New England. MODIS takes pictures at a time interval of half days, so Xi was able to create a time-lapse “photo” for his study area — New England, USA.  From those images, he used a computer program to extract the time when the leaf turn green and yellow (or red). 

More important than observing leaf color change is to understand how ecosystem function changes. Leaf biochemical properties such as chlorophyll concentration and leaf biophysical properties such as leaf mass per area are directly linked with ecosystem function. Thus Xi is collecting leaves from the forests of Martha’s Vineyard and Harvard Forest each week throughout the growing season and measuring not only their biochemical/biophysical properties but also the reflectance and transmittance of the leaves. With those measurements, scientists can address questions such as ‘how does the key component of photosynthesis – chlorophyll – change throughout the season?’ and ‘how can the reflected light from the leaves as measured by the spectroradiometer be used to estimate the chlorophyll concentration in the leaves?’ Xi hopes that through his work we can gain a better understanding of the drivers of plant phenology and the consequences of recent phenological shifts. 
Based on the some of the research described above, Xi and colleagues’ recently reported in the Journal of Geophysical Research that spring in New England is arriving about a week earlier than it did fifty years ago. Read more…