Diversity is the insurance of the forest

As part of the BEF China experiment, the development of trees in differently composed forest pieces is continuously recorded. (Photo: Stefan Trogisch)

It’s all in the mix: Forests with a high biodiversity are best protected against extreme weather conditions in the wake of climate change, a study shows. If individual tree species grow less in a year due to their respective susceptibility to moisture or drought, others can compensate for this failure. As a result, the fluctuations in productivity in species-rich forests are comparatively low and the bottom line is that the performance of the overall system increases. The findings thus show how forestry can strengthen forests for the challenges of the future, say the researchers.

Sometimes it rains all the time and then there is another year of drought – according to forecasts, such problematic climatic capers are to be expected more and more frequently due to climate change. This means stress for the forests: Extreme moisture or drought cause problems for trees – they grow weaker or even die. As a result, ecosystem services such as water storage, temperature regulation and, above all, biomass production decline. Stressed forests also absorb less carbon while at the same time emitting more greenhouse gas – effects that are fueling climate change again.

Studies have already fundamentally shown that natural systems cope better with stress than human systems. In the case of our subtropical forests, naturalness means societies made up of several tree species. Despite the various ecological advantages of mixed forests, however, monocultures from rapidly growing tree species characterize forestry. Because they are considered to be more productive and easier to use. However, previous studies have already questioned whether this calculation will also work out in the future. The new study results by an international team of researchers now contribute to this and show more precisely which factors determine the functioning of forests under climatic stress.

Mixing effects on the track

The insights are based on data from the “BEF-China” project. In this long-term outdoor experiment, scientists in eastern China are investigating how the biodiversity of tree species affects subtropical forest ecosystems. Hundreds of thousands of trees in different species compositions were planted on a large test area – from monocultures to strong mixtures. Over the years, the researchers have continuously recorded the development of the different parts of the forest under the respective conditions. In a recently published study, they were able to clearly demonstrate the beneficial effect of the mixtures: “Planted forests with a high diversity of tree species with different properties achieve a higher productivity than forests with a low diversity,” summarizes Xiaojuan Liu from the Chinese Academy of Science in Beijing.

In the latest study, the team is now shedding light on the background to the favorable mixing effect. According to this, it is becoming apparent that forests with a high biodiversity are best insured against stress: the annual wood growth fluctuates less with mixed stands, which ensures the productivity of the overall system. The stabilization is characterized by the different characteristics of the tree species. They lead to favorable asynchronicity, explain the researchers: if one species grows well in a wet year and another in a dry year, the bottom line is that the loss of the other is balanced out. The following applies: The more different these properties and the more dynamic the resulting exchange relationships between the species, the more pronounced the stabilizing asynchronicity of the tree species community.

Diversity in wet and dry tolerance

Specifically, the study results showed that differences in drought tolerance as well as the water conduction and evaporation capacity of the trees are decisive: the more different a forest community was in these properties, the more stable the communal biomass production rate was under fluctuating climatic conditions. The most even growth was achieved by tree communities that were characterized by a variety of drought tolerance or water use strategies.

“Such relationships between biodiversity and growth stability and the mechanisms responsible for them could only be demonstrated in grassland experiments. We are now proving this for the first time under experimental conditions for subtropical forest ecosystems, ”says first author Florian Schnabel from the German Center for Integrative Biodiversity Research (iDiv). “We were also able to prove once again that species-rich stands also produced more wood compared to monocultures”.

The researchers hope that the findings from the BEF-China project will be put into practice: global forestry and initiatives to offset CO2 through forests should focus on the establishment, maintenance or restoration of species-rich forests. This is the only way to maintain and increase their growth stability and productivity in the face of climate change. Co-author Christian Wirth from iDiv concludes: “We urgently need to protect the functionality of the forests. A decisive component for this is a change in forest management: away from monocultures towards diverse mixtures. This is the best insurance for the forests themselves, and of course also for their users. “

Source: German Center for Integrative Biodiversity Research, specialist article: Science Advances, doi: 10.1126 / sciadv.abk1643

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