Following 10 years of forest growth shows the value of functional diversity

What kind of trees should we plant to make reforestation a success and mitigate climate change?

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During the coming Decade of Ecosystem Restoration (UN), various nations and world organizations increase their endeavour to plant trees and restore forest area to improve ecosystem functioning. So far most reforestation projects are becoming monoculture plantations with commercial tree species mainly for the use of timber and carbon sequestration and do not sufficiently consider mixture forests that have many additional benefits such as increased biodiversity and ecosystem services. However, to make reforestation a success, we need detailed information about what kind of trees to plant where and for what purpose.

Fortunately back in 2008-2009, Prof. Keping Ma together with Profs. Helge Bruelheide and Bernhard Schmid set up the BEF-China experiment with contribution from many other scientists, students and local helpers. It was a hard time, since more than 300,000 trees and shrubs had to be planted across over 30 hectares in a mountainous area. This experiment was setup to enable scientists, to study different questions related to Biodiversity-Ecosystem Functioning (BEF). Over the last 10 years this experiment grows into a forest. Each plot is clearly distinct due to the different species compositions and the growth dynamics over the years. It is impressive to see with your bare eyes that species composition and richness have such strong effects on forest growth and productivity.

left: Site B view in 2021; right: the first tree planted by Helge in 2009
left: Site B view in 2021; right: the first tree planted by Helge in 2009

After 8 years of growth, colleagues already showed that 16-species mixtures took up on average twice as much carbon as monocultures (Huang et al., 2018, Science). However, not all communities or ecosystems with high species richness will achieve high functioning due to the specific functions and interactions of the species in the community. Therefore we decided to not only look at species richness but also the functional characteristics of the plant community and their role in explaining and driving community productivity. In our opinion, understanding how functional characteristics of communities and their species influences productivity could provide useful information for forest managers to guide restoration and reforestation projects.

Before starting to work on this specific paper, we studied how functional traits influence the BEF relationship at the inter and intra-specific levels (Bongers et al 2020 J. Ecol.; Bongers et al 2020 New Phytol.). These papers show how trait variation at tree-level influence growth and may scale up to community performance. This new study emphasizes that it is the specifically differences in species’ functional characteristics that is important when the goal is to plant forest with higher productivity. We showed that when planted forests grow older than seven years, forests with high functional diversity reached higher productivity than forests with low functional diversity or with specifically high average functional characteristics. In addition, our test across 10 different species pools shows that focussing on the functional differences between species was not dependent on the particular species within the community, making it a reliable tool to consider for forest managers. Both the effect and the reliability to predict forest functions and services are of utmost importance to connect restoration actions to goals.

Franca back from the field in Site A
Franca back from the field in Site A

With this story we would like to thank all the workers who measured all the trees year in year out, and all the scientists that have collected data and contributed to the BEF-China platform. We specifically thank our co-authors. Without their brilliant ideas and continuous contribution we couldn’t have managed this big dataset and the corresponding analyses to find the answer to the very important question: What kind of trees should we plant to make reforestation a success and mitigate climate change.

The tree height after planted for 10 years; from left to right, Berhard, Xiaojuan and Keping

The tree height after planted for 10 years; from left to right, Bernhard, Xiaojuan and Keping

Trees grow, forest change, we look forward to see what will happen in the next 10 to 30 years.

Xiaojuan Liu

Associate Professor, Institute of Botany, Chinese Academy of Sciences

My current research focus is understanding the mechanisms of biodiversity maintenance and biodiversity and ecosystem functioning at multiple scales based on functional traits. I am particularly interested in 1) tree demographic rate2) biodiversity and ecosystem functioning3) functional traits distribution at large scale.