Woody plant encroachment (WPE) has been reported to be an on-going trend in many drylands worldwide. While many efforts have been devoted to finding the drivers of WPE, and estimating the consequences of this shift in plant life form on global carbon budgets and climate change, the question of what determines the rates of WPE remains unclear. Here we conduct a meta-analysis of the existing literature to reevaluate the widely-reported and generally-accepted phenomenon of WPE in arid and semi-arid ecosystems in light of this developing theory. More specifically, we address whether global reports of WPE are consistent with our hypotheses that the rates of increase in woody canopy cover are constrained by the "woody cover deficit", defined as (potential woody cover – actual woody cover) / potential woody cover.
Results/Conclusions
Based on a meta-analysis of the WPE literature, we show that woody plant encroachment appears to be a bounded process, which depends on the maximum potential cover supported by the local climate. Rates of woody encroachment are high when actual cover is well below potential cover and approaches zero when actual canopy cover is at or near potential cover. A surprisingly high fraction (~35-40%) of locations in the meta-analysis experienced decreases in woody cover, reflecting biotic, edaphic and anthropogenic disturbances that may initiate WPE as a recovery from prior disturbance.