What should European foresters do about climate change?

Climate change models predict that central Europe will become hotter and drier, especially during the summer growing season. Inevitably, that will suit some tree species and be bad for others, but what steps might European forest managers take now to prepare for the forecast changes? That’s the question behind the research conducted by Tomáš Hlásny and his colleagues, supported by the FORGER project.

The team studied a region of 491,000 km2 that encompassed Wood decay fungi on Fagus sylvatica, Central Apennines, Italy. Photo: B.Vincetithe Czech Republic, Hungary and Slovakia as well as parts of Austria, Croatia, Germany, Italy, Poland, Romania, Serbia, Slovenia and Ukraine. Climate records from the past 60 years and the output of 10 different model projections for regional climate enabled them to estimate how temperature and precipitation are expected to change across the study area in the period from 2071 to 2100. The primary difference from the current climate is a decreasing gradient of precipitation from the northwest to the southeast of the study region. There would be substantial drying in southern Slovakia and throughout Hungary, with the prospect of droughts spanning several years. Temperatures will also increase, adding to evaporation and transpiration and so making water stress worse. It all adds up to critical conditions for drought-sensitive tree species, most notably European beech (Fagus sylvatica), a species of high ecological and commercial importance in central Europe.

Dryer, hotter conditions affect forest trees in many ways. They can change the competitiveness of different species, so the forest composition changes. They can change the pattern of pests and diseases, which in turn can make drought-induced dieback worse. Fire regimes may change too, opening the way to different pioneer species and colonisers. If those changes are not anticipated they can result in direct economic losses in the timber sector and indirect impacts through the loss of non-productive services. For that reason, Hlásny and his colleagues looked in detail at possible adaptation measures to reduce the impact of drought-related risks.  

Beech is liable to suffer most from drought. The drought of 2000–2003 caused considerable direct damage to beech, particularly in Hungary. Pests also caused unprecedented damage, with large-scale defoliation caused by gypsy moth (Lymantria dispar) and damage by beech bark beetle (Taphorychus bicolor). This kind of pest outbreak is likely linked to warmer and drier climate, and might get worse in future.  

Norway spruce (Picea abies), which is very widely planted in central Europe, is already prone to pests and diseases by virtue of being outside its natural range. These outbreaks can be triggered by relatively minor stressors, including drought, and warmer weather might also result in more generations of destructive bark beetles each year. As a result, Norway spruce may see the greatest change among European forest species, especially at lower and middle altitudes, where it is possible that it might be replaced by broadleaved species. Local oaks (Quercus spp) are more resistant to drought and less susceptible to pests and diseases than most other species. Thus oaks might take the place of beech and even spruce in some areas. In addition, climates suitable for Mediterranean oaks might appear, adding to the pool of drought-tolerant species in the region.

Turning to adaptation measures, the research points out that deliberately changing species composition in anticipation of climate change is a good option. Planting oaks among Norway spruce in the Alps, for example, reduces damage by bark beetles. Oaks could also reduce the impact of drought, because they use less of the available water. 

Foresters have two primary tools at their disposal to change species composition: harvesting and regeneration. Future harvests, for example, should be smaller in extent, because large clear cuts act as heat islands and exacerbate the effects of changing climate. Smaller cuts also permit natural regeneration by pioneer species. Along with more selective harvesting, this will tend to make the forest as a whole more diverse in terms of species and in the age structure of the individual species populations, both of which tend to confer resilience and promote adaptation.

In regeneration, the researchers suggest that assisted migration – bringing seeds or saplings from further afield – could have an important role to play by ensuring that the progenitors of future generations are drawn from populations that are already pre-adapted to future conditions. They do caution, however, that it might not be a good idea to bring in species not native to Central Europe, and that any attempts to promote assisted migration will have to contend with the prevailing wisdom among some foresters and environmentalists that “local is always best”. Nevertheless, if the seed source is carefully selected, assisted migration could help local forests to adapt to climate change more rapidly than they otherwise might.

Because trees tend to be relatively slow-growing and long-lived, any natural adaptation to climate change is likely to be a race against time. The long lead time for any effort to assist with or speed adaptation means that society ought to start thinking seriously about the options soon, but climate change is not currently a factor in forestry planning. As the researchers note: “All forest management decisions are taken in face of uncertainty about the future, and delaying the implementation of adaptation measures in expectation of improved knowledge will likely lead to perpetual inaction.”

So while more research, including all that generated by FORGER, will help, we also need to act.

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