Influence of landscape-scale variables on vegetation conversion to exotic annual grassland in southern California, USA

In California, USA, coastal sage scrub (CSS) vegetation is being converted to exotic annual grassland, and several causes have been suggested. In order to investigate the importance of environmental variables in the conversion and recovery of CSS, particularly nitrogen deposition within the context of historical fire intervals, we employed an information theoretic approach. Prior studies have not assessed both conversion and recovery, and did not analyze nitrogen critical load for vegetation type conversion. We included measures of climate, topography, vegetation, land use, nitrogen deposition, and fire in our analysis, and found that 34% of CSS study sites were converted to exotic grassland between 1930 and 2009. Converted sites had higher nitrogen deposition with a critical load of 11 kg N ha-1 yr -1, also had shallower slopes, and were more west-facing. A smaller number of sites (24%) recovered to CSS, and these sites had about 2.5 times more CSS and 4.5 times less grassland in the surrounding landscape. CSS conservation and restoration efforts are most likely to be successful when focused on sites with <11.0 kg N ha-1 yr -1 and low invasion of exotic grasses. Analyses such as this that identify important threats may be useful in region-wide plans to conserve unique vegetation types.