Preserving the diversity of forests ensures their productivity and potentially increases the accumulation of carbon and nitrogen in the soil, which helps maintain soil fertility and mitigate global climate change.
That’s the main finding of a new study that analyzed data from hundreds of plots in Canada’s National Forest Inventory to investigate the relationship between tree diversity and changes in soil carbon and nitrogen in natural forests.
Numerous biodiversity manipulation experiments have collectively suggested that greater tree diversity may lead to greater carbon and nitrogen accumulation in forest soils. But the new study, published online April 26 in the journal Nature, is the first to show a similar result in natural forests, according to the authors.
The researchers used a statistical method called structural equation modeling to assess relationships between tree diversity and soil carbon and nitrogen accumulation. They found that greater tree diversity improved soil carbon storage by 30-32% and improved nitrogen storage by 42-50% on a decadal time scale.
“Our study, for the first time, shows the sustained benefits of tree diversity on soil carbon and nitrogen storage in natural forests,” said study lead author Xinli Chen, a postdoctoral exchange fellow at the Institute for UM Global Change Biology and 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 conservation of biodiversity in forests and will guide increased efforts to use forests for carbon and nitrogen sequestration.
The researchers calculated changes in soil carbon and nitrogen storage over time by comparing data from two censuses of National Forest Inventory sample plots, one from 2000-2006 and the other from 2008-2017.
They quantified tree diversity as species richness, species uniformity, and, based on functional tree traits, functional diversity.
Species richness is the number of tree species in a sample plot, while species evenness is a measure of the relative abundance of tree species. Functional diversity is the variety of functional traits, such as leaf nitrogen content and adult tree height, of tree species within a community.
The research team found that increasing species uniformity from its minimum value to its maximum value improved carbon storage in the soil organic layer by 30% and nitrogen storage by 42%. Increasing the functional diversity of trees to its maximum value improved carbon storage in the soil mineral layer by 32% and nitrogen storage by 50%.
“We found that greater tree diversity is associated with greater accumulation of carbon and nitrogen in the soil, validating the inferences from biodiversity manipulation experiments,” said study co-author Peter Reich, a forest ecologist and director of the Institute for the Biology of Global Change, part of the UM School of Environment and Sustainability.
“Greater species diversity translates into a mix of different types of trees with different ways of acquiring and storing biomass, both in living trunks, roots, branches, and leaves, as well as freshly dead and decaying plant debris on and in the soil. “.
The National Forest Inventory of Canada database is based on a network of plots that cover much of the country’s land mass. The new study analyzed organic soil horizon samples from 361 plots and mineral soil horizon samples from 245 plots.
Those parcels are home to various species of fir, maple, birch, spruce, pine, cottonwood, cedar, and fir, among other types of trees.
Forest soils play an important role in sequestering the carbon extracted from the carbon dioxide gas that warms the planet during photosynthesis. Those soils store at least three times more carbon than living plants.
Nitrogen is an essential nutrient that drives carbon assimilation and plant growth in forest ecosystems. Plant diversity is rapidly declining globally, leading to 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 research was supported by the Natural Sciences and Engineering Research Council of Canada’s Discovery Grants program, the Canadian Foundation for Innovation, the Ontario Research Fund, the Banting Postdoctoral Fellowship, and a grant from the Institutes for Biological Integration of the US National Science Foundation
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