Phenotypic plasticity as an index of drought tolerance in three Patagonian steppe

1 Despite general agreement on the adaptive importance of plasticity, evidence 2 regarding the role of environmental resource availability is scarce, at least in plants. The 3 main hypothesis was the traditional interspecific trade-off hypothesis applied to dryland 4 species; those most tolerant to drought have lower potential growth, and therefore also 5 have the lowest plasticity -here specifying that this only occurs in fitness-related traits. 6 Thus, a derived hypothesis proposes that such stability in fitness is only possible 7 through a higher plasticity in underlying traits (i.e. those associated with response 8 mechanisms). Three conspicuous coexisting perennial grasses from a Patagonian steppe 9 were grown under controlled conditions including four levels of steady-state water 10 availability. The most plastic species along this drought gradient was the most sensitive 11 to drought, whereas the least plastic and overall slower growing one was the most 12 tolerant. This negative relationship between tolerance and plasticity was true for fitness13 related traits but was trait-dependent for underlying ones. Remarkably, the most tolerant 14 species had the highest “positive” plasticity (i.e. opposite to the default response to 15 stress) in an underlying trait directly explaining its drought resistance: it increased 16 absolute root biomass. The niche differentiation axis that allows the coexistence of 17 species in this group of perennial dryland grasses, all limited by soil surface moisture, is 18 a functional one of fixed-vs.-plastic responses. 19