Interactive effects of temporal and spatial fire characteristics on the population dynamics of a fire-dependent Cypress species.
Abstract
Current predictions about climate change and its impact on fire regimes have spurred research on how increasing fire frequencies will affect the population dynamics of fire-dependent species. There has been less consideration of changes in the spatial characteristics of fire regimes, despite recognition of the importance of spatial heterogeneity and connectivity in population dynamics. Here, we test the hypothesis that spatial heterogeneity in fire regimes may not benefit obligate seeders with canopy seed bank, due to regeneration constraints into non-burned areas. We develop a size-structured population model for a population of Tecate cypress Callitropsis forbesii (Jeps.) Little, a rare obligate seeder of conservation concern, across a complex landscape in the Santa Ana Mountains (Orange, CA, USA). Combining the population model with field measurements of stand structure and fire history, we test the roles of heterogeneity in fire regime and dispersal among patches with differing fire history on population persistence. The model predicts population persistence at fire return intervals >17 years, the approximate fire return interval at the site over the last century. However, population growth and population persistence increase when subpopulations are connected by dispersal and, within a small window of fire return intervals, when subpopulations have uncorrelated fire dynamics. Contrary to our expectations, spatial heterogeneity in fire regime increased population growth rates at intermediate (15-25 year) fire return intervals. Synthesis and applications. Conservation efforts focused on isolated populations of fire-dependent species should focus on both the temporal (fire return interval) and spatial (heterogeneity in fire occurrence) aspects of fire regimes. For one threatened Cypress species, we find that the interaction between temporal and spatial characteristics of a fire regime can increase the risks of population extinction, particularly under fire regimes characterized by more frequent, larger and more spatially homogenous fire. Species with fragmented subpopulations not well connected by dispersal also have a greater risk of extinction under these fire regimes.