Incorporating temporal dynamics to enhance grazing management outcomes for a long‐lived species

摘要

Abstract Identifying successful management strategies for long‐lived species is challenging, especially in fluctuating environments, because the response of individual vital rates and overall growth rates to management action can vary over time. A demographic approach that isolates the impact of management and environment on individual vital rates, and assesses their importance for growth rate over time, can indicate whether and why a management strategy is successful. Here we take a demographic approach to assess the success of grazing management for the perennial grass Stipa pulchra, California's state grass and a focus on conservation efforts. We tracked S. pulchra populations in paired grazed and ungrazed plots over 6 years, which included a historic drought and an El Niño rain year, and calculated summary transition matrices for the main effects of year, grazing and year × grazing interaction for each site. We averaged these summary transition matrices to calculate population growth rates, λ, for the main effects of year, grazing and year × grazing and used life table response experiment (LTRE) analysis to compare how individual vital rates contributed to differences in λ between grazed and ungrazed plots over time. Overall we found that grazing was a successful management strategy; grazed populations maintained a positive average population growth rate whereas ungrazed populations declined. We found that grazing had a larger positive effect on growth rates in wet years than drought years. This was because grazed populations benefited from a greater contribution of adult growth immediately following the end of the drought compared with ungrazed populations and from adult survival thereafter. Synthesis and applications. Although Stipa pulchra is the focus of many management efforts, the use of grazing for management is contentious, and there have been few studies that take a demographic approach to understanding S. pulchra responses over time. Our results indicate that conservation and restoration efforts should move beyond recruitment to focus on adult growth and survival for overall population success. To this end, moderate grazing can be a successful strategy to maintain S. pulchra populations and is particularly important in wet years to promote plant growth.