Temporal dynamics of shrub proliferation: Linking patches to landscapes

Authors: Browning, Dawn; LALIBERTE, ANDREA - New Mexico State University; Rango, Albert

Submitted to: International Journal of Geographical Information Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/28/2010
Publication Date: 8/1/2011
Citation: Browning, D.M., Laliberte, A.S., Rango, A. 2011. Temporal dynamics of shrub proliferation: Linking patches to landscapes. International Journal of Geographical Information Science. 25(6):913-930.

Interpretive Summary: The proliferation of woody plants in historically grass-dominated rangeland ecosystems has consequences for sustainability of land use practices (e.g., livestock grazing), functioning of hydrologic and biogeochemical (i.e., carbon, nitrogen), and the status of ecosystem services (e.g., carbon sequestration). Because shrub proliferation occurs over large areas, a remote sensing method for quantitatively assessing shrub cover is needed. We devised a novel approach to monitor changes in shrub cover and patch density over 71 years using time series aerial photography by coupling landscape-level analysis with detailed monitoring of individual patches. We argue that such patch-based monitoring methods will become increasingly important in these actively managed ecosystems as broad-scale measures of percent cover approach and fluctuate about a dynamic equilibrium. Our findings will benefit researchers working to formulate science-based land management strategies; e.g., members of the National Resource Conservation Service and Bureau of Land Management agencies responsible for monitoring public lands; scientists seeking to understand the mechanistic underpinnings of the drivers of shrub encroachment; and state agencies involved in monitoring and assessment of ecosystem services.

Technical Abstract: Shifts in vegetation composition and cover are generally characterized by processes acting at different levels such as landscapes, hill slopes, or plant interspaces. Object-oriented analytical approaches are based upon the inherently hierarchical nature of complex systems and are well-suited to research applications conducted at a range of spatial scales. We quantified long-term vegetation dynamics from two perspectives (i.e., landscape and vegetation patches) in a Chihuahuan Desert ecosystem in southern New Mexico, U.S.A. from 1937 to 2008. Our multi-scale approach comprised object-based analysis of time series aerial photography with automated image analysis at the landscape scale and manual delineation of shrub image objects at the patch scale. We sought to identify mechanisms associated with changes in shrub patch density and percent cover by classifying the fate of individual shrub patches from one image to the next in the time series. The classification scheme captured colonization by new shrub patches, growth or decline in patch area, and patch stability (i.e., change in size of less than 15%). Patch growth was categorized as growth by coalescence with neighboring patches or canopy expansion. Similarly, patch decline was distinguished as either loss of patch area due to fragmentation of conglomerate patches or canopy die-back. Interpretations of change in patch density based solely on shrub establishment and mortality can be too simplistic. Increases in patch density can result from an influx of new patches or fragmentation of patches into its constituent patches; conversely, decreases in density may be due to mortality of patches or coalescence of existing patches. We illustrate that patches grew in size at the beginning of the study period in conjunction with increases in shrub cover (0.5% in 1937 to 11% in 1960) and patch density (4 patches ha-1 in 1937 to 80 patches ha-1 in 1960) increased during the initial encroachment phase of shrub proliferation. Shrub cover remained stable at 7% from 1967 to 1989 and over this period, patch dynamics were broadly characterized by maintenance of patch area with roughly equal proportions of mortality and establishment. Shrub cover increased linearly from 8% in 1989 to 14% in 2008, approaching a maximum projected shrub cover of 18% based on mean annual precipitation of 230 mm from Sankaran et al. (2005). Shrub patch fate over this period constituted growth and maintenance of patch area whereas appearance of new patches remained relatively stable. Patch dynamics signify a shifting mosaic in which shrub patch establishment, growth, and mortality wax and wane. Monitoring patch dynamics will become increasingly important in these actively managed ecosystems as broad-scale measures of percent cover approach and fluctuate about a dynamic equilibrium.