Maintaining the diversity of forests ensures their productivity and may lead to an increase in the amount of carbon and nitrogen accumulated in the soil, which supports soil fertility and helps to slow down the effects of global warming.
This is the major finding of a recent study that examined the relationship between tree variety and variations in soil carbon and nitrogen in natural forests. Data from hundreds of plots in Canada’s National Forest Inventory were studied for the study.
Higher tree diversity may promote increased carbon and nitrogen sequestration in forest soils, according to a number of biodiversity-manipulation experiments. But the new study, published online April 26 (2023) in the journal Nature, is the first to show a similar outcome in natural forests, according to the authors.
The researchers using a statistical technique known as structural equation modeling evaluated the links between tree diversity and soil carbon and nitrogen buildup. They found that increased tree diversity enhanced soil carbon storage by 30% to 32% and enhanced nitrogen storage by 42% to 50% on a decadal timescale.
“Our study, for the first time, shows the sustained benefits of tree diversity in storing soil carbon and nitrogen in natural forests,” said study lead author Xinli Chen, a postdoctoral exchange fellow at U-M’s Institute for Global Change Biology and a postdoctoral fellow at the University of Alberta.
“Our results highlight that promoting tree diversity not only increases productivity but also mitigates global climate change and reduces soil degradation. And the size of the diversity dividend is large. It reinforces the importance of biodiversity conservation in forests and will guide the growing efforts to use forests for carbon and nitrogen sequestration.”
A greater diversity of species translates into a mixture of different types of trees with different ways of acquiring and storing biomass both in live trunks, roots, branches and leaves and in newly dead and decaying plant detritus on and in the soil.
Peter Reich
The researchers calculated changes in soil carbon and nitrogen storage over time by comparing data from two National Forest Inventory sample-plot censuses, one from 2000-2006 and the other from 2008-2017.
They measured the diversity of trees in terms of species richness, species evenness, and functional diversity based on the functional features of the trees.
In a sample plot, species richness refers to the total number of tree species present, whereas species evenness measures the relative abundances of tree species. Functional diversity refers to the range of functional characteristics present in a community of tree species, such as leaf nitrogen content and mature tree height.
The research team found that increasing species evenness from its minimum to its maximum value enhanced carbon storage in the organic soil layer by 30% and nitrogen storage by 42%. Increasing the functional diversity of trees to its maximum value enhanced carbon storage in the soil mineral layer by 32% and nitrogen storage by 50%.
“We find that greater tree diversity is associated with higher soil carbon and nitrogen accumulation, validating inferences from biodiversity-manipulation experiments,” said study co-author Peter Reich, a forest ecologist and director of the Institute for Global Change Biology, which is part of U-M’s School for Environment and Sustainability.
“A greater diversity of species translates into a mixture of different types of trees with different ways of acquiring and storing biomass both in live trunks, roots, branches and leaves and in newly dead and decaying plant detritus on and in the soil.”
Canada’s National Forest Inventory database is based on a network of plots covering much of the country’s landmass. The new study analyzed organic soil horizon samples from 361 plots and mineral soil horizon samples from 245 plots.
Those plots are home to various species of fir, maple, birch, spruce, pine, poplar, cedar and hemlock, among other tree types.
The carbon that is removed from the planet-warming carbon dioxide gas during photosynthesis is sequestered in large part by forest soils. At least three times as much carbon is stored in such soils than there is in living plants.
Nitrogen is a crucial nutrient that promotes plant growth and carbon uptake in forest ecosystems. Plant diversity is rapidly declining globally, leading to the degradation of ecosystem function, including the function of soils.
The other authors of the Nature study are Anthony Taylor of the University of New Brunswick, Masumi Hisano of the University of Tokyo, Han Chen of Lakehead University, and Scott Chang of the University of Alberta and Zhejiang A&F University.
The Discovery Grants program of the Natural Sciences and Engineering Research Council of Canada, the Canada Foundation supported the research for Innovation, the Ontario Research Fund, the Banting Postdoctoral Fellowship, and a grant from the U.S. National Science Foundation’s Biological Integration Institutes.
