A case study of grassland responses to fuel reduction treatment on the mount Hamilton range of California

In many grasslands, mowing and grazing are common management methods that can decrease fuel loads and alter community composition to achieve specific targets, such as increasing diversity or reducing grass dominance. Communities may respond with changes in their taxonomic, phylogenetic, and functional diversity, species turnover, functional shifts, and individualistic species responses. We address these in a 4-yr case study of grazing and mowing in a California grassland located in the Mount Hamilton Range of Santa Clara County. We hypothesize that both grazing and mowing will reduce grass cover and residual dry matter, allowing competitive release of a more diverse range of forbs, likely increasing functional and phylogenetic diversity. On the other hand, these treatments may impose filters on the community (e.g., mowing may eliminate tall species), reducing functional diversity and shifting the community towards particular functional compositions.

In contrast to our expectations, grass cover did not respond strongly to either treatment, and there were weak effects on residual dry matter. However, there were still strong compositional responses. Mowing led to a reduction in tall, large-seeded, and perennial species and generally higher functional and phylogenetic diversity. The most pronounced response in grazed communities was a marked increase in nitrogen-fixing species, paired with reductions in functional and phylogenetic diversity. Similar results have been observed in previous studies of grazing and mowing impacts at other Californiagrasslands sites, providing a basis for predicting species responses to these treatments. Both can be useful tools to filter the community for particular species compositions, but showed no benefit for commonly used restoration targets such as increasing species richness or native cover. Continued monitoring of the sites will track potentially slow compositional changes and investigate the interactions between treatments and interannual weather variation.

Methods
These data are from grassland vegetation surveys at Blue Oak Ranch Reserve, in the Mount Hamilton Range of Santa Clara County, California. We surveyed seven sites in the spring of 2021-2024. Each site had 3 (or, in one case, 4) transects, with 4 1x1 m sampling plots every 10m along each transect. Within each plot, we estimated the percent cover of each vascular plant.

In addition to the resulting community matrix, we also provide a variety of plot-level summaries of the communities. These include species diversity, functional diversity, phylogenetic diversity, and functional structure.