Increasing carbon discrimination rates and depth of water uptake favor the growth of Mediterranean evergreen trees in the ecotone with temperate deciduous forests

5 years ago

Increasing carbon discrimination rates and depth of water uptake favor the growth of Mediterranean evergreen trees in the ecotone with temperate deciduous forests

Adrià Barbeta, Josep Peñuelas

Tree populations at the low-altitudinal or -latitudinal limits of species' distributional ranges are predicted to retreat toward higher altitudes and latitudes to track the ongoing changes in climate. Studies have focused on the climatic sensitivity of the retreating species, whereas little is known about the potential replacements. Competition between tree species in forest ecotones will likely be strongly influenced by the ecophysiological responses to heat and drought. We used tree-ring widths and δ13C and δ18O chronologies to compare the growth rates and long-term ecophysiological responses to climate in the temperate-Mediterranean ecotone formed by the deciduous Fagus sylvatica and the evergreen Quercus ilex at the low altitudinal and southern latitudinal limit of F. sylvatica (NE Iberian Peninsula). F. sylvatica growth rates were similar to those of other southern populations and were surprisingly not higher than those of Q. ilex, which were an order of magnitude higher than those in nearby drier sites. Higher Q. ilex growth rates were associated with high temperatures, which have increased carbon discrimination rates in the last 25 years. In contrast, stomatal regulation in F. sylvatica was proportional to the increase in atmospheric CO2. Tree-ring δ18O for both species were mostly correlated with δ18O in the source water. In contrast to many previous studies, relative humidity was not negatively correlated with tree-ring δ18O but had a positive effect on Q. ilex tree-ring δ18O. Furthermore, tree-ring δ18O decreased in Q. ilex over time. The sensitivity of Q. ilex to climate likely reflects the uptake of deep water that allowed it to benefit from the effect of CO2 fertilization, in contrast to the water-limited F. sylvatica. Consequently, Q. ilex is a strong competitor at sites currently dominated by F. sylvatica and could be favored by increasingly warmer conditions. In this forest ecotone, growth rates of the Mediterranean tree were surprisingly similar to the temperate deciduous tree species and higher than in sites with typical Mediterranean conditions. The Mediterranean species' carbon discrimination rates were enhanced in the last years in response to warmer temperatures and thanks to a deeper water uptake, as shown by the more depleted and less variable tree-ring δ18O. In contrast, the physiology of the temperate species was limited by a tighter stomatal regulation. Source water appeared as the paramount factor determining tree-ring δ18O.

Publisher URL: http://onlinelibrary.wiley.com/resolve/doi

DOI: 10.1111/gcb.13770