Seasonal Effects on Vegetation Greenness of California Grassland During Five Years of Global Change Treatments

The response of ecosystem net primary production to global environmental changes can vary among ecosystems, across years, and as a result of multi-year feedbacks. To better understand the response of California grassland production to four globally occurring environmental changes, we studied the response of vegetation "greenness" indices, such as the normalized difference vegetation index (NDVI), using a hand-held vis-nir-swir spectrometer during five growing seasons of the Jasper Ridge Global Change Experiment (JRGCE). The JRGCE studies the response of California annual grassland to four global change treatments--warming, nitrogen addition, elevated CO₂, and increased precipitation--applied in a full-factorial, randomized block design. Across the five years, NDVI followed a characteristic seasonal pattern, rising steeply in the fall at the start of the Mediterranean-type growing season, leveling off or dropping slightly during mid-winter, then rising to an annual maximum in mid-spring, and finally dropping sharply with the onset of summer drought. Each of the global change factors modified this pattern with specific seasonal effects on NDVI. Warming increased NDVI during January and February, except during the one year when these months were unusually warm; warming also tended to decrease NDVI in late spring, indicating accelerated senescence of the vegetation. In all years, watering increased NDVI from the time of peak standing biomass until shortly after two post-season waterings. In addition, during two growing seasons with mid-winter droughts, watering increased NDVI during winter and early spring. In all years, elevated CO₂ decreased NDVI during late winter and early spring; in some years there was a compensatory increase in NDVI in late spring or early summer. In the spring of each year, added nitrogen increased vegetation greenness, but in alternating years there was also a lag in greening during fall and winter. Seasonally integrated measures of NDVI mirrored interannual changes in the response of aboveground net primary production to the global change treatments.