Species distribution limits and local adaptation in Acacia along an altitudinal gradient

Understanding how species cope under changing climatic conditions contributes to natural resource management strategies, the conservation of biodiversity, and enhanced primary production. As the climate warms, species current ranges are expected to shift, forcing species to persist through these changes or risk local extinction. Plants that exhibit higher phenotypic plasticity will be better equipped to cope in a changing climate, while those more locally adapted may require translocations to more suitable climates. The altitudinal gradient in the Blue Mountains provides the opportunity to use space for time substitution in climate studies as temperatures are expected to shift up the mountain as the climate warms. A reciprocal transplant was conducted using seeds sourced from low, mid and high altitude populations of Acacia species restricted to lower (warmer) or higher (cooler) altitudes, as well as, species widespread across the altitudinal gradient. These were planted into low and high altitude common garden sites to capture climates within and beyond their current distributions. The aim was to investigate what factors influence species distribution limits along the gradient and determine if plants are locally adapted to the different altitudes. Three hypotheses were proposed for the study. The first, relating to local adaptation predicted that local is best and that seeds planted into sites similar to their climate of origin will exhibit greater fitness (in terms of growth and survival) than those planted further from their originating climate due to local adaptation. The following two hypothesis addressed distribution limits stating that restricted species will have greater fitness than widespread species in their local climate and planting beyond the species distribution will result in greater fitness losses compared to similar distances within their range limits. Results showed that seed was able to establish beyond the current distribution limits of species restricted to either low or high altitudes, indicating that dispersal limitation is a greater determinant of species distributions than environmental factors. Plant fitness (seed emergence, seedling growth and survival) was significantly affected by the planting altitude, where greater emergence, seedling growth and survival were seen at low altitude than high altitude sites. A significant GxE interaction revealed seed from low altitudes had greater fitness at low altitude planting sites. Climate (temperature, humidity and rainfall) was a significant determinant of plant fitness, while edaphic and geographic factors had a negligible effect. Plant fitness declined with increasing climate transfer distance in the low altitude planting sites. Despite the temperature gradient in the Blue Mountains, distribution limits in Acacia species may be determined by historic dispersal rather than climate. Local may still be best in the forward (warmer) edge of species distribution, but as climate warms high altitude population may be outperformed by populations originating from lower altitudes. These findings provide a scientific basis for adaptive management strategies, specifically assisted migration under climate change