Environmental heterogeneity promotes floristic turnover in temperate forests of south-eastern Australia more than dispersal limitation and disturbance

Abstract

Australia’s temperate forest landscapes encompass broad topographic and edaphic ranges, and are regularly disturbed by fire. Nonetheless, relative contributions of environmental heterogeneity, disturbance regimes, and dispersal limitations to plant species turnover remain poorly understood. To evaluate the relative influences of deterministic (environmental, disturbance), and stochastic (spatial) processes on plant species turnover [beta-diversity (β diversity)] in natural forest landscapes, and how such influences vary among plant functional types and vegetation strata. We assessed the environment and species composition of 81 forest stands, representing a range of structures and fire histories across contiguous landscapes in south-eastern Australia, and examined the potential to explain β diversity using variance partitioning and distance-decay analyses. Explanatory variables accounted for 34–55% of β diversity of multiple plant functional types, with environmental heterogeneity explaining the greatest proportion (10–25%). Stand structural variables (e.g., leaf area index, height coefficient of variation) accounted for 8–14% of β diversity in understorey life forms and 5% in canopy species, far greater than a single direct descriptor of disturbance history such as time-since-fire which explained just 2% of tree and shrub β diversity. β Diversity increased with increasing geographic distance for all functional types. Dispersal limitation accounted for 5–11% of β diversity, and distance-decay rates varied among plant functional types. Landscape-scale conservation of forest biodiversity will require representation of a broad environmental range as well as metrics that fully capture site disturbance histories, including stand structural complexity as a potential proxy for fire regimes.