Local and global patterns of shrub proliferation - changes in autocorrelation in an arid rangeland ecosystem from 1937-2008.

TitleLocal and global patterns of shrub proliferation - changes in autocorrelation in an arid rangeland ecosystem from 1937-2008.
Publication TypeConference Proceedings
Year of Publication2010
AuthorsBrowning D.M., Laliberte, Andrea S., Rango A.
Conference NameUS - International Association for Landscape Ecology
Date PublishedJune 25-30, 2010
ARIS Log Number249627
Abstract

The proliferation of trees and shrubs in grassland and savanna ecosystems has been widely observed, although future trajectories remain difficult to ascertain. Spatial manifestations of shrub proliferation bear relevance on surface hydrology and the spread of disturbance. We quantify changes in spatial autocorrelation in shrub cover in an arid grassland ecosystem to test the validity of a conceptual framework highlighting the role of shrub patch dynamics in the shrub encroachment process. We build upon the framework to suggest how the encroachment and stabilization phases manifest spatially. We suggest that indicators of spatial autocorrelation illustrate where local interactions are more prevalent, thereby functioning as an indicator of biotic regulation and stabilization in shrub cover. We analyzed binary maps of shrub cover from 1937 to 2008 derived from time series aerial photography. Spatial analyses entailed resampling 1-m depictions of shrub patches to percent shrub cover within 20-m X 20-m quadrats to examine the spatial autocorrelation with Moran’s I using GeoDa software. We decomposed the global index of spatial association into contributions from local neighborhoods using Local Indicator of Spatial Association, which quantifies the extent to which cells are similar to or different from their neighbors. Shrub cover in 1937 was highly heterogenous and became more dispersed and less patchy over the 71 years. As shrub cover increased, it became increasingly positively autocorrelated with Moran’s I increasing from 0.383 in 1937 to 0.742 in 2008. We found no clear evidence of self-organization, although we did detect clear evidence of local effects that resulted in broad scale patterns with hot-spots of low and high cover. The trend highlights the potential for edaphic or geomorphic constraints on shrub cover. We anticipate collecting field data on vegetation structure, cover, and soil properties to discern whether segregated patterns in autocorrelation are related to land surface conditions.