Water availability and species identity control shrub colonization in abandoned semiarid steppes

The increase in shrub cover and density is a widespread phenomenon in drylands worldwide. Its causes and consequences are complex and strongly idiosyncratic. Detailed knowledge of the rate of shrub colonization, and its abiotic and biotic drivers is crucial to fully understand and manage these areas. We used a set of aerial photographs (1956, 1978, 1985, 1998 and 2009) and field data to relate shrub patch dynamics, and abiotic and biotic properties of 27 abandoned steppes in SE Iberian Peninsula. Shrub patch density in 2009 ranged from 11 to 143 patches ha−1, and increased by an average of ∼180% between 1956 and 2009. Temporal changes in shrub patch density followed non-linear functions (monomolecular, logistic and exponential) describing early, gradual and late recruitment, respectively. Differences in shrub patch colonization rate were related to the identity of patch-forming species, but contingent on water availability. Total increases in shrub patch density were larger when patches were dominated by late colonizer species. Overall shrub patch dynamics was mostly related to the identity of patch-forming species and climatic conditions, and to a lesser extent physical factors such as aspect and rock cover. Given the sensitivity of patch-forming species to climatic conditions, new challenges for steppe management will emerge over the next decades. Species able to thrive under harsh conditions may increase their dominance at the expense of species adapted to less demanding conditions.