Interspecific differences in sapling performance with respect to light and aridity gradients in Mediterranean pine-oak forests: implications for species coexistence

The relative abundance of two codominant Mediterranean tree species, shade-tolerant Quercus ilex L. and shade-intolerant Pinus halepensis Mill., is inversely correlated along aridity gradients, but this pattern is not explained by seedling responses to water or light availability, suggesting that subsequent life history stages may explain forest composition. To test this hypothesis, we calibrated statistical models of sapling growth and height-diameter allometry as functions of light availability and climatic variation as well as models of sapling mortality as a function of growth history. Contrary to the expectation of a sun-shade growth trade-off, P. halepensis grew faster than Q. ilex saplings at both low and high light levels. Low precipitation and aridity suppressed sapling growth rates, but no evidence of a shade-drought growth trade-off was found either. Pinus halepensis sapling mortality was strongly growth dependent, exhibiting high mortality rates at low growth, but the mortality of Q. ilex saplings was not. Height-diameter allometric variation was higher in low- than in high-light environments and was more pronounced with respect to changes in light than climatic water. Our results suggest that interspecific differences in sapling mortality and plasticity, rather than growth, may control species distributions at the mesic end of the aridity gradient.