Climate warming delays and decreases seedling emergence in a Mediterranean ecosystem

Temperature and moisture impact strongly on the early stages of a plant's life cycle. Global climate change is altering the environmental cues that seeds receive resulting in compromised seedling emergence and changes to seedling performance. Here, we investigate how temperature and moisture affect these early stages of plant development in four Banksia species collected from a longitudinal climate gradient in southwest Western Australia. A common garden was used to examine the between-species and among-population variation in seedling emergence, growth and leaf traits under two soil temperature regimes and three levels of precipitation. We predicted that reduced moisture and increased temperature would delay and reduce total seedling emergence and negatively affect seedling performance. Furthermore, we expected that within species there would be geographically structured variation in response to the treatments. Species differed significantly in all measured traits. Soil warming resulted in strong impacts on regenerative traits, significantly slowing seedling emergence in two species and reducing total seedling emergence in three species. In addition, warming altered seedling performance with significant reductions to the above-ground leaf biomass ratio of three species. In contrast, response to soil moisture manipulation was minimal across all species but possibly due to issues regarding implementation of an effective moisture treatment. The species that showed the greatest decline in emergence under warmed conditions (B. quercifolia) also showed the smallest vegetative shift; the species with the smallest decline in emergence (B. coccinea) showed a relatively large vegetative shift. Among-population differences were significant for many traits, however, trait differentiation was inconsistent across species and, contrary to our hypothesis, the variation we observed was not clearly associated with the climate gradient. As these among-population differences in traits are not easy to predict, we caution the use of simple rules for choosing seed populations for conservation and restoration.