Shrub encroachment into grasslands: end of an era?

Background/Question/Methods

Grasslands and savannas characterize much of the world’s dryland ecosystems. Various lines of evidence indicate a destabilization of the grass/shrub balance in these systems in recent decades leading to the proliferation of woody plants. Although increases in woody plant abundance have been widely reported, little is known of the rates, dynamics and spatial pattern of shrub cover change. Shrub encroachment has been on-going in many areas for 100+ years. Has the process gone to completion on such sites? Here we link long-term field (local) and remote sensing (regional) data quantifying the temporal and spatial dynamics of woody cover at a site in the southwestern USA where the shrub Prosopis velutina (velvet mesquite) is known to have proliferated in semi-desert grassland since the early 1900s.

Results/Conclusions

Local field surveys revealed three phases in the overall trend: a rapid expansion phase (1957-1991) followed by short decline phase (1992-1997) and a subsequent stabilization phase (1998-2006). Broad-scale remote sensing assessments during the period 1984-2005 confirm that with the exception of severely disturbed areas or low elevation sites experiencing a proliferation of succulents (cacti), shrub cover has been relatively stable in recent decades. Temporal variation of shrub cover (as indicated by coefficients of variation [CV]) was relatively high on sites that had experienced fire, but contrasting livestock grazing regimes had little influence on woody cover. In areas with no histories of abrupt perturbations (wildfire, brush management), there was a general positive trend of increasing woody cover with increasing elevation, presumably reflecting an orographic effect. Contrary to expectations, there were no consistent relationships between woody cover, its CV or its change (± % cover y^-1) with topoedaphic settings. This study, combining observations of different scales, confirms that recent woody cover trends in semi-arid environments are not uni-directional. Overall, shrub cover at this site in southeastern Arizona appears to have reached its potential and is in dynamic equilibrium under the current climatic regime. However, local perturbations or regional amplification of drought and elevated temperature can disrupt this equilibrium and hence the stability of aboveground ecosystem carbon pools.