Stem and root diameter growth of European beech and Norway spruce under extreme drought

Current and future climate change will be accompanied by more frequent and more severe drought events, with potential impacts on tree growth and forest stand productivity. Tree growth response may depend on its water status and on the competition or facilitation of the neighbouring trees. We analysed inter- and intra-annual diameter increments of European beech and Norway spruce trees within a mixed forest stand for two treatments, i.e., with and without rainfall exclusion and for two neighbourhood competition situations of the two climatically contrasting years 2014 and 2015. Rainfall exclusion by roofs at about 3 m height induced soil drought under 116 trees, particularly supported in the year 2015 by hot and dry weather conditions. The effects of extreme drought was examined at three levels, i.e. at two stem heights and at the main coarse root of 48 trees with inter- and intraspecific neighbourhood. We found species specific diameter growth performances during the year at all three levels. Compared to beech trees annual diameter increments of spruce trees were in most cases significantly higher in the year 2014, while in the drought year 2015 no significant differences between the two species were obvious. Under the heavy drought by the rainfall exclusion experiment and by the hot year 2015 diameter increments were significantly smaller for spruce trees at all three levels whereas for beech trees at all levels no significant differences were obvious. The diameter growth differences between inter- and intraspecific neighbourhood were in most cases small and not significant. In the drought year 2015 beech trees in intraspecific neighbourhood grew better at all levels compared to interspecific neighbourhood, with a significantly higher growth rate at the upper stem level. Reasons for the species specific reactions patterns on drought were discussed (phenology, water balance, species specific physiological reactions).