Continuous droughts' effect on herbaceous vegetation cover and productivity in rangelands: results from close-range photography and spatial analysis

摘要

The function and structure of annual vegetation communities in rangeland ecosystems are the first to respond to extreme events such as drought. Changes in productive patch distribution patterns are a common indicator for ecosystem changes. Rarely, however, do realistic studies observe the effect of drought on patch distribution patterns in very high space-time resolution with remotely sensed data. We investigated the dynamics of herbaceous vegetation productive patches in a semiarid rangeland. The persistence of herbaceous vegetation productive patches during a period of drought was studied from two data sources: plant cover obtained from high-resolution vertical photography and productivity from biomass field estimates. This was done between two consecutive drought years (2007-2008 with 200 mm; 2008-2009 with 135 mm; annual average ～300 mm) and along a topographic gradient. The results show that ongoing droughts led to spatial convergence of the productive patch size. Yet, productive and less productive patches persisted while plant cover decreased with rainfall reduction. We conclude that the response of herbaceous rangeland ecosystems to ongoing drought is mainly characterized by changes in the productive patch size and distribution. Moreover, these ecosystems are more resilient than expected, and their productivity might undergo drastic changes and yet maintain productivity under more severe drought scenarios than those currently predicted in the climate change literature.