Drought and herbivory as modulators of intraspecific differentiation in seedlings of a mountain tree

In mountain ecosystems, plant regeneration might be constrained by multiple factors that change along elevation gradients. Those factors might influence the expression of different traits among populations. Drought and herbivory are strong filters for seedling establishment and, therefore, important selective pressures. Populations of the tree Maytenus boaria support lower soil moisture and higher herbivory pressure at low elevations than at mid-elevations in Córdoba Mountains, central Argentina. Consequently, we expect that populations from the low elevation perform better in response to both factors than populations from mid-elevations. Seedlings from the two elevation origins were exposed to two levels of simulated drought and herbivory in a greenhouse experiment. The selected elevations corresponded to the lowest edge of species distribution (with driest soils and highest herbivory pressure) and the central mid-elevation. Performance-related variables, biomass allocation patterns and several morphological and physiological traits were measured. Seedlings from the low origin showed lower mortality, leaf number and water potential in response to drought than seedlings from the mid-origin. Herbivory affected the performance of seedlings and many plant traits, irrespective of the origin. The interaction between drought and herbivory increased the drought effects on some variables. Seedlings of M. boaria from the low elevation, where conditions are drier and warmer, provide more drought-tolerant offspring than those from the cooler and moister mid-elevation.