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

Abstract

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 (Brachypodium hybridum, Hedypnois rhagadioloides) 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 H. rhagadioloides; in B. hybridum, 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.

Keywords: Brachypodium hybridum; Hedypnois rhagadioloides; aridity gradient; competition; drought stress; local adaptation; season length; slope exposure aspect.

FIGURE 1

Left: Map of mean annual precipitation in the study region and the location of 15 sampling sites along the macroclimatic rainfall gradient from northern to southern Israel. Right: Illustration of the microclimatic difference between north and south‐exposed hill‐slopes in three exemplary sites. Note the sparser vegetation on south exposures as indication of their drier microclimate. Photo credit: J. Metz.

FIGURE 2

Portraits of the study species Brachypodium hybridum (a) and Hedypnois rhagadioloides (b) and a view of the greenhouse setup (c). Photo credit: M. Winter (a,c) and F. Gade (b).

FIGURE 3

The fitness (quantified as reproductive biomass per plant in all panels) of Brachypodium hybridum and Hedypnois rhagadioloides populations under combinations of long vs. short season length (vertical columns) and three treatments (control, drought stress, and competition; horizontal rows). To ease comparison, the uppermost panels (a–d) summarize the mean trends per treatment across the macroclimatic rainfall gradient (always on x‐axis), i.e. averaged across north and south exposures. The remaining panels (e–p) display the same responses per treatment, yet detailed for north and south exposures and using adjusted y‐axes.