Manipulating disturbance regimes and seeding to restore mesic Mediterranean grasslands

Question: Can managing disturbance regimes alone or in combination with seeding native species serve to shift the balance from exotic towards native species? Location: Central coast of California, USA. Methods: We measured vegetation composition for 10 yr in a manipulative experiment replicated at three sites. Treatments included no disturbance, grazing and clipping at three frequencies with and without litter removal. We seeded eight native species into clipped plots and compared cover in comparable plots with no seeding. Results: Regardless of frequency, clipping generally shifted community dominance from exotic annual grasses to exotic annual forbs, rather than consistently favoring native species. At one site, perennial grass cover decreased in no‐disturbance plots, but only after 4 yr. Litter removal had minimal impact on litter depth and plant community composition. Grazing had a highly variable effect on the abundance of different plant guilds across sites and years. Seeding increased abundance of only two of eight native species. Conclusions: Managing disturbance regimes alone is insufficient to restore native species guilds in highly‐invaded grasslands and seeding native species has highly variable success.     

The local distribution of a perennial bunchgrass: biotic or abiotic control?

The present study was designed to investigate the relative importance of biotic versus abiotic factors in determining the distribution of species among heterogeneous environments. The composition of 70 quadrats of 0.25-m², located in a 13-m × 40-m region of a mown field in Durham, North Carolina, was defined through two sets of principal component scores representing soil and vegetation gradients in the study area. The first principal component in each of the analyses exhibited a monotonic increase in value along an elevational gradient characterizing the field. This represents a one-dimensional correlation structure between the background soil and vegetation variables. Secondary soil and vegetation gradients, represented by the remaining principal component axes, were for the most part uncorrelated.Regression analyses were used to study the relationships coupling the distribution of Danthonia sericea, a bunchgrass species, to the distribution of associated soil and vegetation gradients. The correlation of the distribution of Danthonia to the dominant soil/vegetation gradient was highly significant. In addition, strong, secondary correlations with the background vegetation remained after removal of variance associated with soil composition; the converse was, in general, not true. One exception involved a minor zinc gradient that exhibited a negative correlation with the distribution of Danthonia. A strong correlation was found coupling an interaction term associated with the primary soil/vegetation gradient to the distribution of Danthonia. The results suggest that the present distribution of Danthonia is modified to a large extent by competitive interactions with other plant species. However, the relative significance of the soil-by-vegetation interaction term also indicates that soil composition must be explicitly considered to properly understand the role of biotic interactions. Any one factor, biotic or abiotic, may modify the distribution of a species, but the way in which it modifies the distribution will depend directly upon the composition of the other factors making up the biotic and abiotic, environmental background.Abbreviations Danthonia = the species Danthonia sericea - PC = principal component - PCA = principal component analysis - SS = sum of squares