Effect of propagule pressure on recovery of a California grassland after an extreme disturbance
How is natural regeneration of a patchy landscape affected by within-patch species interactions and among-patch dispersal after an extreme disturbance? Do landscape dispersal processes facilitate the invasion of native-dominated patches by exotic species in adjacent patches?
Irvine Ranch Natural Landmark, Irvine, California, USA.
We monitored plant community composition in paired grassland patches that were initially dominated by native or exotic grasses at eight sites. We followed recovery of native and exotic grassland species over time, starting in a record drought year prior to an intense fire, and then for 3 yr with more typical rainfall patterns after the fire. Additionally, we compared seed rain of native and exotic species across native and exotic patches, quantifying how seed rain influenced species abundance in the following year. Multivariate and regression analyses were used to assess the potential homogenization of the landscape.
Following the extreme drought/fire disturbance, the exotic annual grasses quickly recovered in abundance in patches that they dominated prior to the disturbance. However, the native grass, Stipa pulchra, was not able to recover in the patches it once dominated. As the exotic grasses gradually increased in the native patches, the paired patch types became more similar in composition over time. Exotic grasses produced up to 28 times more seed than the native dominant grass, Stipa; even in the patches initially dominated by Stipa, exotic seed rain was equivalent or greater than the native. Seed rain was positively correlated with the following year's abundance for both exotic and native species.
After an extreme disturbance, recovery of native patches can be stalled by an influx of propagules from neighbouring exotic patches. This exotic seed rain can allow the invasion of areas once dominated by natives, thus inhibiting regeneration. The matrix surrounding remnant native stands can be a critical factor in determining whether an extreme disturbance enhances native diversity vs. increasing its susceptibility to invasion.