Understanding the distribution of plant productivity is vital for understanding the spatial variability of ecosystem functions. This study evaluates microtopographic controls (1m-12m) on plant productivity on three rolling hills in Sedgwick Natural Reserve, located in south-central California. Specifically I evaluate the relationship between topographic metrics and plant biomass production through space and time.

Biomass was measured using destructive harvests and seasonal Airborne Visible Infrared Imaging Spectrometer (AVIRIS) data. Eighty-three 1x0.5m2 quadrats of aboveground plant matter at peak biomass (ANPP) were harvested for the 2012 growing season. For the 2009 growing season, AVIRIS derived Normalized Difference Vegetation Index (NDVI) was used to estimate biomass at roughly monthly intervals from March to August. I evaluate whether seasonal changes in growing degree days (GDD) was a better predictor of plant phenological events than cumulative days since first soil moisture increase. To characterize topography, I used a 1m resolution digital elevation model derived from terrestrial lidar data to calculate curvature, aspect, and the Compound Topographic Index (CTI) - an index that integrates the flow accumulation area and slope. Using GDD, I found that ecosystem productivity was not temperature limited early in the growing season. Using webcam images I was able to remotely monitor phenological events quantitatively, but was not able to calculate NDVI because I lacked appropriate spectral bands.

Plants growing on north facing slopes consistently had higher ANPP than those on south facing slopes, due to lower temperatures, hence greater preservation of soil moisture. No correlation was found between CTI or curvature and ANPP across the 83 sampled points in 2012, potentially because it was a dry year and there was limited water redistribution to lower positions in the landscape. Although a relationship between topography and soil moisture is probably valid for humid areas, it is subject to considerable variability in semiarid grasslands because during low rainfall years, evapotranspiration consumes much of the moisture that could be available for downslope movement. In these settings it is important to consider the amount and duration of water input as well as soil properties in understanding the distribution of soil moisture as it might influence plant productivity.