Abstract
Predicting how changes in climate and land use jointly impact populations is a pressing task in ecology. Microclimate plays a key role in species' local persistence by modulating regional weather effects. However, empirical evidence remains limited regarding the relative effects of landscape structure and microclimate conditions on intraspecific trait variation. Using a spatially and temporally replicated demographic dataset, we tested the relative effects of landscape structure (area and connectivity of remnant habitat fragments), microclimate (heat load), and annual weather conditions (study year) on intraspecific variation in plant traits. We also investigated whether local heat load modulated the weather effects on the traits studied. We performed repeated measurements of stem height, leaf area, number of stems, main inflorescence length, and number of primary side inflorescences of 569 permanently marked individuals of the grassland specialist Salvia nemorosa L. We sampled 13 populations encompassing microhabitats exposed to different heat load levels over three consecutive years. Mature individuals had fewer stems in isolated and taller stems in small habitat fragments. High levels of heat load and dry years affected negatively all measured traits, and the negative effects of exposure to high heat load were generally exacerbated in dry years. Our results suggest that exposure to strong environmental stressors may complicate the detection of the real effect of human impact on plant populations. Effective landscape planning for the conservation of dry grassland species should prioritize not only improved habitat connectivity but also the maintenance of habitats with heterogeneous microclimates capable of buffering weather extremes.