%0 Journal Article
%T Relationships between GPP, Satellite Measures of Greenness and Canopy Water Content with Soil Moisture in Mediterranean-Climate Grassland and Oak Savanna
%A Shishi Liu
%A Oliver A. Chadwick
%A Dar A. Roberts
%A Chris J. Still
%J Applied and Environmental Soil Science
%D 2011
%I Hindawi Publishing Corporation
%R 10.1155/2011/839028
%X We investigated the impact of soil moisture on gross primary production (GPP), chlorophyll content, and canopy water content represented by remotely sensed vegetation indices (VIs) in an open grassland and an oak savanna in California. We found for the annual grassland that GPP late in the growing season was controlled by the declining soil moisture, but there was a 10¨C20-day lag in the response of GPP to soil moisture. However, during the early and middle part of the growing season, solar radiation accounted for most of the variation in GPP. In the oak savanna, the grass understory exhibited a similar response, but oak trees were not sensitive to soil moisture in the upper 50£¿cm of the soil profile. Furthermore, while we found most VIs to be more or less related to soil moisture, the Visible Atmospherically Resistance Index (VARI) was the most sensitive to the change of soil moisture. 1. Introduction Plant-available soil moisture is a key element in ecosystem functioning, since it links energy balance and hydrological cycles, contributes to vegetation composition and richness, and impacts productivity. California¡¯s Mediterranean climate is characterized by highly variable winter precipitation and prolonged summer drought, and its vegetation communities are strongly affected by the availability of water, resulting in pronounced annual cycles of growth and senescence [1]. For example, the growing season of annual grasslands typically begins in the wet and cool winter after major rain events and extends to the late spring supported by a declining supply of soil moisture. Given the nature of the hydroclimate regime in California, understanding the response of different plant functional types to soil water availability should be a primary objective of any advanced natural resource management system. Semiarid savanna in California, which is composed of widely spaced trees, understory grasses, and forbs, is a common land cover type in California. It has been hypothesized that trees and grasses are able to coexist because of either their differences in resource-acquisition potentials or differences in demographic mechanisms, under such disturbances as fires and grazing [2¨C4]. In general, spatial niche separation in root distributions appears to be more prevalent in arid systems [5], and plant-available moisture, rather than nutrients, may be the main resource limiting plant growth in savannas [6, 7]. Regardless of the ultimate controls on savanna structure, trees and grasses compete for available soil water, and although grasses are typically superior
%U http://www.hindawi.com/journals/aess/2011/839028/