Carbon Flux Dynamics from a Coastal Grassland Disturbed by Wildfire and Grazing

Abstract

Native grasslands in California are home to almost 40% of the state's native grass species, but they are also among the most threatened ecosystems in the United States. Human activities like urbanization, agriculture, and the introduction of non-native species have caused significant damage to California's grasslands over the past few decades. In coastal regions of California, advection fog is a common weather occurrence during summer months. It plays a crucial role in reducing heat and water stress in coastal grassland ecosystems. Historically, it has also provided a buffer against intense and frequent wildfires. Our research is centered around the investigation of the intricate relationship between coastal fog in a grazed grassland in Santa Cruz County, which was recently affected by a wildfire. Our primary objective was to comprehend the impacts of fog on carbon and water fluxes. To understand these dynamics, we conducted a comparative study between two sites that vary in characteristics along a presumed gradient of summer stratus. We used the Normalized Burn Ratio index from Sentinel-2A imagery to evaluate burn severity and the Normalized Differential Greenness Index to identify vegetation greenness patterns during the growing season. Using a closed chamber approach, we measured carbon and water fluxes from similar plant communities, which included grasses and forbs, across multiple plots within each site. At each site, we installed passive fog collectors, meteorological stations, and a four component net radiometer to capture data on fog events and microclimate conditions. Our findings reveal intriguing patterns in carbon and water dynamics. The higher elevation grassland site received higher fog water inputs compared to the lower elevation site and exhibited increased plant productivity in terms of gross primary production (GPP). GPP and net ecosystem exchange (NEE) were reduced during fog events throughout the field season. Despite the presence of cattle grazing and the recent wildfire, both sites remained carbon sinks throughout the field season. Our findings support the idea that coastal microclimate, particularly fog events, directly affect carbon and water dynamics in a coastal grazed grassland recovering from wildfire disturbance.