COS 99-5 - History of disturbance influences the response of seed banks and plant communities to other disturbances

Abstract: In grasslands, grazing, fire, and drought shape vegetation communities, but little is known about how these disturbances interact to alter community response to future disturbances. In California’s annual grasslands, the grass seedbank has minimal carry-over between years, and is vulnerable to disturbances that decrease seed production or survival. Forbs have a long-lived seedbank, and often emerge in response to disturbances that diminish the grass seedbank. This study investigated how one type of disturbance influences vegetation response to another disturbance. Immediately after a wildfire, we established plots that experienced wildfire, prescribed burn, or no burn. In each fire type, we included plots that had been grazed or ungrazed prior to the fire. Seed bank cores were collected within 1 month of the wildfire, and the number and identity of emerging seedlings were determined in a greenhouse trial. Field vegetation composition was assessed in each of the plots in spring of the 1^st growing season after fire. In ungrazed sites, both wildfire and prescribed burns decreased grass seed germination 7-fold, but only resulted in a slight decrease in grass percent cover in the field. Both burn types decreased dicot germination by 33%, but led to a 3-fold increase in forb cover and a 6-fold increase in legume cover in field plots. Grazing altered the impacts of burns, likely by decreasing fuel load. In grazed plots, burning decreased grass germination by only 50%, leading to a 25% decrease in grass cover. For forbs, plots with a history of grazing and wildfire had 2-fold higher forb germination than in ungrazed wildfire plots, leading to a 25% increase in forb cover.

In the 2^nd growing season after fire, the site experienced a severe drought. Immediately following the drought, we repeated seedbank and vegetation composition measures on these plots to assess whether the vegetation response to drought was influenced by a history of fire and grazing. Forb seedbank and cover were strongly influenced by fire history, with plots that burned (regardless of grazing history) having 2-fold higher forb germination, and 25% increase in forb cover, compared to plots that had been unburned 3 years prior. In contrast, grass germination and cover were impacted strongly by grazing, with grazed plots having 2-fold higher grass germination, and a 20% increase in cover. In summary, disturbance history strongly impacts vegetation response to other disturbances, and grassland management through grazing and prescribed fires can influence vegetation response to drought.