A hyper-arid environment shapes an inverse pattern of the fast-slow plant economics spectrum for above-, but not below-ground resource acquisition strategies

The fast-slow plant economics spectrum predicts that because of evolutionary and biophysical constraints, different plant organs must be coordinated to converge in a unique ecological strategy within a continuum that shifts from fast to slow resource acquisition and conservation. Therefore, along a gradient of aridity, taxa with different strategies will be expected to be successful because selection pressures for slow resource acquisition become stronger as the environment becomes drier. In extremely arid and seasonal environments, however, a slow strategy may become disadvantageous because slow traits are costly to maintain. Additionally, as the availability of water decreases, selection pressures increase, reducing the variation in ecological strategies. Using shrub assemblages along an aridity gradient in the Atacama Desert, we test the hypothesis that selection pressures imposed by hyper-aridity act simultaneously on the variation and coordination of trait attributes, leading to an inverse pattern in the fast-slow plant economics spectrum, where strategies shift from slow to fast as the environment becomes drier. We established 20-22 plots at each of four sites along the gradient to estimate plant community structure and functional variation. For all species recorded, we quantified a set of leaf, stem, and root traits. Results revealed an inverse pattern of the fast-slow economics spectrum for leaf and stem traits, but not for root traits; that is, as aridity further increased, above-ground traits exhibited a shift from a slow to a fast strategy with some level of coordination. Below-ground traits, however, did not shift accordingly with our prediction, rather they showed more complex pattern of shift and coordination with above-ground traits along the gradient. We also found that trait variation showed an idiosyncratic pattern of variation along the gradient, indicating that ecological strategies are driven by local processes within sites. Synthesis. Our results increase our understanding of the fast-slow plant economics spectrum by showing that environmental gradients, as well as local process can simultaneously shape different below- and above-ground resource acquisition strategies in extremely poor resource environments.