Competition, Drought, Season Length? Disentangling Key Factors for Local Adaptation in Two Mediterranean Annuals across Combined Macroclimatic and Microclimatic Aridity Gradients.

Competition in mesic sites and drought stress combined with short growing seasons in drier sites are key environmental factors along macroclimatic aridity gradients. They impose a triangular trade-off for local adaptation. However, as experiments have rarely disentangled their effects on plant fitness, uncertainty remained whether mesic populations are indeed better competitors and drier populations better adapted to drought stress and short season length. Aridity differs also at microclimatic scale between north (more mesic) and south (more arid) exposed hill-slopes. Little is known whether local adaptation occurs among exposures and whether south exposures harbor conspecifics better adapted to drier climates that could provide adaptive reservoirs under climate change. We sampled two Mediterranean annuals () in 15 sites along a macroclimatic aridity gradient (89-926 mm rainfall) on corresponding north and south exposures. In a large greenhouse experiment, we measured their fitness under drought stress, competition, and short vs. long growing seasons. Along the macroclimatic gradient, mesic populations were better competitors under benign conditions. Drier populations performed no better under drought stress per se but coped better with the short growing seasons typical for drier macroclimates. At microclimatic scale, north exposure plants were slightly better competitors in in , south exposure plants coped better with drought under short season length. We demonstrate that local adaptation to drier macroclimates is trading-off with competitive ability under benign conditions and vice-versa. Drought escape via short life-cycles was the primary adaptation to drier macroclimates, suggesting that intensified drought stress within the growing season under climate change challenges arid and mesic populations alike. Moreover, the drier microclimates at south exposures exhibited some potential as nearby reservoirs of drier-adapted genotypes. This potential needs further investigation, yet may assist populations to persist under climate change and lessen the need for long-distance migration.