In the context of rapid global change, it is essential we understand how environmental drivers are impacting plant population dynamics, yet few demographic studies evaluate multiple drivers or explore interactions. This study tests the impact of fire, soil moisture, canopy cover and their interactions, on vital rates, leaf qualities and population dynamics of beargrass (Xerophyllum tenaxMelanthiaceae), a forest herb in the Pacific Northwest used in basket making by tribal members and traditionally managed through fire. Here we ask 1.) How do fire and abiotic factors influence plant vital rates and leaf qualities?, 2.) What are the long-term population consequences of more-frequent low-severity versus less-frequent high severity fire?, and 3.) How do canopy cover and soil moisture interact to influence these long-term effects? We censused >1000 plants across burn severities in 2015 and 2016 at three high elevation sites in north central Oregon. Field data were used to parameterize regression models of vital rates and covariate predictors. These were then used to build Integral Projection Models to explore the influence of fire regimes and abiotic factors on beargrass vital rates and long-term persistence. Leaf qualities were measured on a subset of plants across plots in 2016.
Results/Conclusions
Plant growth, survival and sexual reproduction depended upon plant size and burn severity. Capsule production increased with soil moisture. Asexual reproduction increased with flowering in the previous year, and depended upon the interaction of size with burn severity and canopy openness. Long-term persistence of beargrass population was highest under low severity fire, moderated by covariates. Leaf qualities relevant to basket making depended upon severity fire, again moderated by covariates. This study demonstrates the importance of considering multiple effects and interactions in order to disentangle the impacts of management and abiotic drivers on plant population dynamics. It also highlights the importance of exploring potential trade-offs between management for plant persistence and management for plant qualities necessary for cultural or economic use by stakeholder groups.