How effectively can forests cope with drought and what can forestry operators do to improve their resistance and resilience? To find out, Swiss Federal Institute for Forest, Snow and Landscape Research (WSL) researchers drew on a unique long-term experiment by the Forest Research Institute of Baden-Württemberg (FVA) in southwestern Germany: at six study sites with Norway spruce and silver fir forests, a special form of forest management has been researched since the 1970s, namely the irregular group shelterwood regeneration method (Femelschlag). As the name suggests, this means that groups of trees are left in place when logging. New trees will then grow in the resulting gaps, leading to the emergence of an uneven-aged forest.
Precise measurements were taken over time for every individual tree on the various sites. Conclusions were then drawn for Switzerland's forests based on a comparison with more than 300 study sites investigating forest growth (as part of the Experimental Forest Management (EFM) project, some of which WSL has been observing for over 100 years.
Norway spruce and silver fir are the most important tree species for the forest industry: spruce is the main source of timber, but drought and bark beetle infestations mean that in many locations its days are numbered. Silver fir is regarded as a potential substitute for the spruce, as it has deeper roots, making it better adapted to water scarcity. "Both of them are currently hot topics in European forestry, from both an ecological and an economic perspective," says Andreas Rigling, head of WSL's Forest Dynamics Research Unit.
Tall, large trees more sensitive to drought
Thanks to the long-term data from southwestern Germany, together with new measurements at these sites, the international research team was able to investigate the effects of drought on spruce and fir trees. Measurements of the trees' growth and condition showed that spruce suffered from even a mild drought, while silver fir was still able to access enough water and so could even benefit from higher temperatures.
During extreme droughts, for example in 2003 and 2011, fir was also ailing, but still less than spruce. The density and size of the trees – both factors that can be influenced by forest management – played a role here. "Large and tall trees are more sensitive to water scarcity because they have to suck the water up to the crown," explains the study's lead author, Alessandra Bottero, from WSL's Ecosystem Ecology Research Group.
Managing forests in the face of drought stress
Thus, there are a number of ways that forest managers can reduce forests' susceptibility to drought: thinning out the tree stock; promoting silver fir and drought-tolerant hardwood species in dry and warm locations; and preventing the trees from getting too large. Bottero points out that these findings agree with those from WSL's long-term studies in Switzerland, such as the thinning and irrigation experiments in the Pfynwald pine forest. "It is encouraging that we are making similar observations in forests with different management forms and species compositions."
What interventions forestry operators should perform, and how often, depends on the tree species composition and also on the site, the development phase and the function of the forest – for example, whether it will be harvested for timber or is intended to provide long-term protection against natural hazards. The researchers have communicated the new findings at workshops and will also publish the project results in journals for forestry practitioners.