Impact of Climate Trends and Drought Events on the Growth of Oaks (Quercus robur L. and Quercus petraea (Matt.) Liebl.) within and beyond Their Natural Range
Due to predicted climate change, it is important to know to what extent trees and forests
will be impacted by chronic and episodic drought stress. As oaks play an important role in European
forestry, this study focuses on the growth response of sessile oak (Quercus petraea (Matt.) Liebl.) and
pedunculate oak (Quercus robur (L.)) under contrasting climatic conditions. Analyses cover both site
conditions of their natural occurrence (Southern Germany and Northeast Italy) and site conditions
beyond their natural range (South Africa). The sites beyond their natural range represent possible
future climate conditions. Tree-ring series from three different sites were compared and analysed
using dendrochronological methods. The long-term growth development of oak trees appears to be
similar across the sites, yet the growth level over time is higher in the drier and warmer climate than
in the temperate zone. When compared with previous growth periods, growth models reveal that oak
trees grew more than expected during the last decades. A recent setback in growth can be observed,
although growth is still higher than the model predicts. By focusing on the short-term reactions of the
trees, distinct drought events and periods were discovered. In each climatic region, similar growth
reactions developed after drought periods. A decline in growth rate occurred in the second or third
year after the drought event. Oaks in South Africa are currently exposed to a warmer climate with
more frequent drought events. This climatic condition is a future prediction also for Europe. In view
of this climate change, we discuss the consequences of the long- and short- term growth behaviour of
oaks grown in the climate of South Africa for a tree species selection that naturally occurs in Europe