Complex bioclimatic and soil gradients shape leaf trait variation in Embothrium coccineum (Proteaceae) among austral forests in Patagonia
Patterns of trait variation may be adaptive when vary in relation to an environmental gradient. In particular, leaf traits can affect productivity and competitive ability. We identify patterns of leaf size and shape variation with environmental heterogeneity in one of the most widespread tree species within temperate South America: Embothrium coccineum (Proteaceae). We collected leaf specimens and composite soil samples from 35 populations between 38° and 55° S latitude in Patagonia, covering a wide range of mean annual precipitation (MAP) and mean annual temperature (MAT). At each location, we measured nine morphological traits, some of which were cross correlated hence we focus on a smaller number of representative traits. We hypothesized that leaf area (LA), dry mass (DM), and specific leaf area (SLA) would increase, and that leaf shape (SF) would be more elongated, with increasing temperature, precipitation, and soil nutrient availability. We also expected growing season climate to be more closely associated with leaf traits than mean annual metrics. We used bivariate and backward stepwise multiple regressions to analyse the dependence of morphological traits with climatic and edaphic metrics. LA and DM increased with increasing summer rainfall or winter temperature, as hypothesized. Opposite to our hypothesis, LA and DM decreased with increasing summer temperature suggesting that in terms of leaf size, E. coccineum may sense summer conditions largely as an increasing aridity stressful gradient. Surprisingly, SLA increased with increasingly warm or dry summers. SF was related positively to MAT and negatively to MAP, suggesting that under more benign western climate regimes E. coccineum leaves tend to be elongated. Across sites, LA and DM increased with soil organic carbon and available phosphorus, and decreased with soil nitrogen and exchangeable cations. The opposite pattern was observed for SLA. Biologically meaningful climate metrics and soil nutrient conditions are useful predictors for leaf size and structure in the widespread E. coccineum. The SLA patterns probably resulted from lower values in long lasting leaves, in addition to increasing soil nitrogen, so leaves in the south are thicker. Alternatively, it could be consequence from non-isometrical scaling of LA and DM, so larger leaves such as those under oceanic western climates have lower SLA. Patterns of multiple leaf trait variation along complex environmental gradients may become uncoupled from each other, differing from what is suggested in the literature for traits that vary along simple environmental gradients.