Quercus robur survival at the rear edge in steppe: Dendrochronological evidence.

Climate change is altering forest ecosystems worldwide, particularly in steppe landscapes, where the rare tree communities are challenged with steadily increasing droughts. In the steppe of Eastern Europe, amid dry conditions, Quercus robur occupies mostly riverine habitats and ravines. Here we study the climate sensitivity and drought vulnerability of a Q. robur population located at the rear edge of the species range, in the steppe of Ukraine. The population occupies two adjacent but clearly contrasting in their microclimatic conditions sites: a river floodplain and a steep-sloping river bank. We develop tree earlywood, latewood, and total ring width site-level chronologies and evaluate their relationship with regional climate variables and the local river's water level using response function analysis. We find that trees growing in the floodplain and at the steep river bank have exhibited slightly different growth patterns. The trees at the flooded site have benefited from water proximity, which facilitated their earlywood growth. These trees have responded positively to the current May and previous September precipitation and previous and current May temperatures. At the non-flooded site, the trees have experienced warm temperatures and the lack of precipitation in June. The extreme drought episodes have triggered a decrease in the latewood and total ring width in trees from both sites. We established that oak growth in the floodplain had been depressed by an unknown stressor around 1900, therefore limiting our ability to identify the more beneficial steppe habitat for Q. robur conclusively. Nevertheless, our results provide a dendrochronological evidence of Q. robur survival in a dry steppe environment and lend new insights into local microclimatic factors contributing to it. [ABSTRACT FROM AUTHOR]