Fire and herbivory at the chaparral-grassland ecotone: What conditions maintain these habitat boundaries?
The fire-prone California Mediterranean landscape is characterized by a mosaic of vegetation types, including grassland and chaparral shrub. The maintenance of the chaparral-grassland ecotone has been a source of contentious debate in ecology. The “bare zone” between grassland and chaparral is typically devoid of vegetation or hosts plants that don’t commonly occur in either the grassland or chaparral. The accepted working hypothesis is that the bare zone is maintained by concentrated herbivory from small mammals dwelling under the cover of dense chaparral. At Mt. Diablo State Park (Contra Costa County, California), the classic halo of bare ground occurs around chaparral vegetation dominated by Adenostoma fasciculatum. In September of 2013, Mt. Diablo experienced wildfire that burned many chaparral and grassland areas. Because intense fire temporarily removes chaparral vegetation cover, it is expected that herbivores will be displaced, releasing the chaparral-grassland ecotone from maintenance by herbivory. In July of 2014, I erected an array of herbivore exclosures in the chaparral and the bare zone of burned and unburned sites to address two research questions: (1) does herbivory maintain the bare zone at Mt. Diablo in mature stands, and (2) does herbivory maintain the bare zone after disturbance by fire?
Preliminary results suggest that herbivory does limit the growth of herbaceous understory in mature chaparral and in the adjacent bare zone, with significantly higher plant densities, species richness, and percent cover in excluded plots (March 2015 data collection). A surprising result has been the presence of A. fasciculatum seedlings in excluded plots of unburned sites. While A. fasciculatum is thought to be almost exclusively fire-germinated, this finding suggests that herbivory may actually play a role in limiting the advance of chaparral vegetation in the absence of disturbance by fire. As predicted, herbivore exclosures have not resulted in differences in percent cover in burned sites, suggesting that chaparral fire results in a temporary release from concentrated herbivory at the grassland border, which may facilitate the advance of grasses in the years following fire. Further data collection throughout the summer will yield insight into the strength of these patterns as seasonal drying causes herbaceous plants to become scarcer. Species and functional group analyses will suggest plant traits that mediate these patterns.