The many different values of a forest increase with the age of the forest
Forests provide us with many ecosystem services such as timber, berries, mushrooms, game and recreation but the number of services and the scope of each service that the forest provides changes as the forest becomes older. This has been shown by researchers from Umeå University, SLU in Uppsala and the University of Gothenburg in a new study published in “Environmental Research Letters”.
Text: Ingrid Söderbergh
Forests provide us with many ecosystem services such as berries and recreation.
The research team has investigated the ecosystem services of tree growth, carbon storage in the soil, the diversity of species in soil vegetation, and the presence of blueberries, food for wildlife and dead wood. While tree growth increased up to the age of 70-100 years, which is when forests are normally harvested, the highest levels of most of the other ecosystem services were found in forests older than 120 years. In addition, those forests were able to deliver a greater number of different ecosystem services at the same time than younger forests could do.
Micael Jonsson, Department of Ecology and Environmental Sciences.
"Our results show that if forests are allowed to grow old, they not only provide more of many different ecosystem services but also, the number of values in that forest increases," says Micael Jonsson, senior lecturer at the Department of Ecology and Environmental Science at Umeå University, lead author of the study.
However, the study shows that the high diversity of ecosystem services in older forests is at the expense of tree growth, which is low in old forests.
"There is a clear trade-off between tree growth and the existence of other ecosystem services. This trade-off seems inescapable, probably because trees in younger, even-aged, managed forests outcompete the plants that are the basis for other ecosystem services,” says co-author Jan Bengtsson, professor at SLU in Uppsala.
One of the conclusions of the study is that the way the forestry industry limits the age of forests by felling stocks when they are 100 years old leads to forests with fewer and lower levels of many of the forest's values, with the exception of tree growth.
"Moreover, the forestry industry is discussing lowering the age when trees are felled, and in this study, we have shown that that could have further negative consequences for many ecosystem services," says co-author Tord Snäll, professor at SLU in Uppsala.
The study also shows that the species of tree in the forest has an impact on the way the values change as the forest ages. Although a middle-aged monoculture of spruce showed some of the highest values of tree growth and low values for other ecosystem services, old forests with only spruce, or spruce with elements of birch, had the highest values for several of the services.
"We could see clearly that the age of the stock is something that needs to be taken into account, because several of the forest values change so dramatically over time, but the mix of species in the stock also plays an important role," says Micael Jonsson.
The study is based on data from the Swedish National Forest Inventory, and confirms the results of previous studies conducted by the research team, namely, that mixed stocks and stocks that are allowed to become older than 100 years of age are of great importance.