Positive surprise: if the trees grow back after the rainforest has been deforested or slashed by fire, this secondary forest recovers faster than expected. After just 20 years, these secondary forests correspond to a primeval forest in some aspects, such as soil fertility and tree density, to almost 80 percent. However, biodiversity and biomass need around 120 years for regeneration, as researchers have determined.
Forests are important habitats and hotspots for biodiversity. At the same time, they also make a significant contribution to binding carbon dioxide, cleaning the air and producing oxygen. They thus form important buffers in our planet’s climate system. But these buffers are dwindling: In many regions, forests are being cut down to make space for fields, pastures or plantations. In addition, climate change is affecting many forests more and more.
How quickly does a secondary forest regenerate?
But there is also hope: in some areas, the forest area is slowly increasing again. Trees are reclaiming their old territory because fields have been given up or because areas have been deliberately left alone for reforestation. However, this raises the question of how quickly and how well such renewable forests can regenerate. Are they getting closer to pristine, old forests ecologically? To find out, Lourens Porter from Wageningen University and his colleagues took a closer look at the regeneration of tropical forests at 77 locations and over 2,200 test areas on three continents. In doing so, they recorded patches of forest from the first year of their regrowth up to a good 120-year-old secondary forest.
In order to assess the ecological status of these forests, the research team examined twelve parameters from four functional groups for each forest area: soil fertility, plant biomass, forest structure and biodiversity. “These four groups are key components of ecosystem function. Knowing how they regenerate is therefore a prerequisite for global measures to preserve biodiversity, climate protection and reforestation, ”explain the scientists.
Soils are the fastest, biodiversity is the slowest
As the evaluations showed, the secondary forests are developing differently in some aspects and, above all, at a different pace than expected. “We expected that the soil would recover more slowly than the regeneration of the vegetation,” the researchers report. But it is precisely the soils that regenerated the fastest. They only needed around ten years to almost match those of primeval forests in terms of their properties. The plant density also developed faster than expected: On average, it only took around 25 years for the tree density and leaf area to regain 82 to 100 percent of the primeval forest values.
Overall, after around 20 years, the secondary tropical forests reached around 78 percent of the state that characterizes primeval forests, as the scientists found. However, this does not apply to all the properties that characterize the intact ecosystem of an old forest: the forest structure and biodiversity, for example, only approach the primeval forest values after an average of 25 to 60 years. It takes even longer for biomass and species composition to regenerate. This takes at least 120 years, as Porter and his colleagues found.
Important for climate protection and ecology
According to the researchers, these results nevertheless show that the natural regeneration of forests is a cost-effective and nature-based solution for climate protection, the preservation of biological diversity and the restoration of ecosystems. “The local and global importance of secondary forests makes them very worth protecting,” says co-author Florian Oberleitner from the University of Innsbruck. “However, there is no magic bullet for their recovery, and it may be necessary to actively support regeneration. The optimal solution depends on the local conditions, the population and their needs. “
Nevertheless, it is still urgently necessary to protect old forests, emphasizes the scientist. Because they are the habitat of many unique plant and animal species and a kind of germination aid for the regrowing forest.
Source: University of Innsbruck; Technical article: Science, doi: 10.1126 / science.abh3629