Tradeoffs between growth and function for riparian trees undergoing climatic drying

Background/Question/Methods

Riparian ecosystems in water limited regions are under stress worldwide from river modifications and climate change. Coupled tree-ring and stable isotope analyses provide promising indicators of how environmental changes influence physiological function of riparian trees over decadal scales. We analyzed growth and stable carbon isotope geochemistry in riparian Populus nigra tree rings to identify shifts in tree health and water use linked to environmental stressors along the Drôme River, in the semi-arid zone of southeastern France. Changes over the last 30 years include instream gravel mining that has lowered the floodplain water table, punctuated droughts, and a hotter and drier climate trend during the growing season. We cored 20 individuals at four floodplain sites to analyze the timing and relative ecological impacts of these drivers on tree growth, crown health, and ecohydrological function. 

Results/Conclusions

Analysis of tree-ring chronologies showed divergent patterns among the four sites, with two sites maintaining stable growth and two others with sharp growth declines to <30 cm² annual basal area increment (BAI) over the last decade. Permanent growth declines were initiated in years of meteorological droughts that occurred after, but never prior to, the mining period, and were spatially distributed to suggest that stands on shallow soils were the most vulnerable. In contrast to this signal, the carbon isotope analysis indicated that all trees, regardless of growth pattern, showed an increase in ¹³C discrimination (Δ¹³C), or decreased water use efficiency as they aged. This occurred despite a significant increase in growing season temperature and a decrease in river discharge over the last two decades. The forest stands with permanent growth declines had the greatest number of dead trees, the lowest density of live ones, and the greatest rates of crown dieback among surviving individuals. Together these results suggest that living P. nigra trees in the most stressed sites lost large proportions of their crowns, thus inducing permanent reductions in radial growth, but at the same time maintaining internal water status adequate to survive. The combined results indicate that these semi-arid riparian woodlands are vulnerable to environmental changes, that the severity of impacts is conditioned by interactions between drivers at different scales, and that one way that riparian trees adapt is by reducing water demand by sacrificing major branches and large areas of their crowns.