Geographical variation in population demography and life history traits of Tecate cypress (Hesperocyparis forbesii) suggests a fire regime gradient across the USA–Mexico border

Abstract

Plant adaptations to fire often display spatial heterogeneity associated with geographical variation in fire regime. We examined whether populations of the Tecate cypress (Hesperocyparis forbesii Adams) in southern California and northern Baja, Mexico, exhibited spatial heterogeneity in cone serotiny, in other life history traits associated with fire-adaptation, and in population demographic structure, to assess a putative difference in fire regime across the USA–Mexico border. Demographic data, tree life history data, and tree ring series were used to compare the demographic structure and life history traits of three populations in southern California with three populations in northern Baja California. In Baja populations, a greater number of tree size classes were present (χ ² = 12,589; P < 0.05), cone serotiny was more facultative (Mann–Whitney U = 58, P < 0.05), and young adult trees had a higher reproductive output (Mann–Whitney U = 2.65, P < 0.05), suggesting that a difference in fire regime between southern California and northern Baja has existed long enough (ca 8000 years) to drive microevolutionary divergence between the two sets of populations, and is not solely the result of 20th century differences in fire management policies across the international border. The transitional area between the two different fire regimes does not appear to coincide with the border itself but may lie in a zone of ecological transition south of Ensenada. The range of phenotypic variation observed within the Tecate cypress metapopulation suggests this species has the capacity to adapt to future environmental changes.