Simulated Indigenous fire stewardship increases the population growth rate of an understorey herb

Abstract

Understanding how plant populations respond to multiple drivers is increasingly critical for biodiversity conservation under global change. Indigenous knowledge can provide guidance for sustainable management, but the outcome of its application in novel ecosystems is rarely known. Simulating the re-introduction of Indigenous stewardship in contemporary contexts with population models allows for the comparison of different management scenarios and the elucidation of the mechanisms driving population outcomes. Beargrass Xerophyllum tenax is an ecologically and culturally important understorey plant managed through fire and leaf harvest by Native Americans. We collected demographic and abiotic data on beargrass over 3 years across fire severities in nine populations and conducted an experiment to simulate Native American leaf gathering. These data were used to build integral projections models (IPMs) with soil moisture and light availability as covariates. With these IPMs, we simulated stochastic population growth rates across future fire and leaf harvest scenarios. We then decomposed our simulation results using stochastic life table response experiments (SLTREs). The ‘no fire’ and ‘business as usual’ (180-year fire return interval, 58% probability of high-severity fire) scenarios resulted in lower population growth rates than ‘Indigenous fire stewardship’ (10-year fire return interval, 10% chance of high-severity fire). SLTREs revealed that Indigenous stewardship led to higher beargrass population growth rates due to greater fire frequency, higher adult survival and increased vegetative reproduction. Fire also interacted with harvest in the simulations; leaf harvest increased population growth rate only in combination with Indigenous fire stewardship. Synthesis. Stochastic and retrospective population dynamics tools combined with an understanding of Indigenous management practices allow for the comparison of future socio-ecological scenarios as well as mechanistic understanding of differences between scenarios. Simulated Indigenous stewardship supported the long-term persistence of X. tenax populations while business as usual and no fire did not. The benefits of Indigenous stewardship to population dynamics, and the complexity of interactive effects of multiple drivers, provide further impetus for collaboration across Indigenous and western knowledge systems. Xerophyllum tenax is presented as a model system to explore the influence of Indigenous stewardship, or its absence, on population dynamics.