Winter annuals not only escape but also withstand winter droughts : Results from a multi-trait, multi-species approach

Winter annual plants are a dominant life form in drylands. They evade seasonal drought through their life history, but are also exposed to drought within their growing season. Across species, winter annuals differ in traits allowing them to reproduce before a drought occurs (drought escape) as well as in traits minimizing tissue dehydration (drought avoidance) and/or maintaining functioning under drought (drought tolerance). It is yet uncertain how these traits are coordinated and influence winter annuals’ performance responses to drought within the growing season and their distribution along rainfall gradients. Understanding these mechanisms is crucial to predict global change impacts in drylands. We measured 22 traits hypothesized to influence whole-plant performance responses to drought in 29 winter annuals common in the Eastern Mediterranean Basin. We examined trait syndromes and linked species’ strengths of these trait syndromes with their fecundity responses to an experimental within-season drought, their maximum growth rates (in 18 species), and their distribution along a rainfall gradient. Four trait syndromes emerged: Two were largely consistent with drought avoidance and tolerance, while the other two consisted of traits considered to confer drought escape. Both escape syndromes were differently associated with plant size and therefore referred to as small and tall escape syndrome. Species with a pronounced small escape syndrome showed, albeit weakly, higher fecundity losses under experimental drought. Both species with a pronounced avoidance or tall escape syndrome exhibited higher growth rates, but only annuals with pronounced avoidance traits tended to occur in moister conditions. Our findings highlight that winter annuals, despite their common life history, exhibit several trait syndromes conferring them similar ability to cope with drought in the growing season. Consequently, increasing within-season drought with global change may hardly affect community composition of winter annuals.