2 May 2023

An increase in the number of tree species stimulates greater carbon and nitrogen sequestration, Canadian research explains. But biodiversity is in decline. Promoting it means mitigating climate change and reducing land degradation

by Matteo Cavallito

 

It is no secret, of course, that forest soil makes a crucial contribution to removing carbon from the atmosphere through the process of photosynthesis by plants. Far less known, however, is the weight of a factor capable of proving crucial in increasing the effectiveness of this process: the variety of tree species. This was revealed in a research published these days in the journal Nature.

As plant diversity increases, the researchers claim, sequestration of the element can increase by more than 30 percent. The problem, however, stresses a statement released by the authors, is that tree biodiversity is declining rapidly globally, “leading to the degradation of ecosystem function, including the function of soils.” In contrast, “Preserving the diversity of forests assures their productivity and can increase the accumulation of carbon and nitrogen in the soil, which helps to sustain soil fertility and mitigate global climate change.”

Carbon and nitrogen under scrutiny

The research team, which includes scientists from the universities of Alberta, New Brunswick and Ontario, Canada, Michigan (U.S.) and Tokyo, analyzed data from Canada’s National Forest Inventory. “Looking at organic soil horizon samples from 361 plots and mineral soil horizon samples from 245 plots,” the statement says, “the researchers investigate the relationship between tree diversity and changes in soil carbon and nitrogen in natural forests.”

The investigation, based on a statistical method called structural equation modeling, was carried out on two different censuses conducted at different times that allowed them to examine biodiversity richness, that is, the number of species present, the uniformity of specimens, and the variety of functional traits, such as nitrogen content in leaves and the height of adult trees.

Carbon and nitrogen grow by 32-50%

The results were unequivocal. “Overall,” the authors write, “increased tree diversity enhanced soil carbon storage by 30% to 32% and enhanced nitrogen storage by 42% to 50% on a decadal timescale.”

The results, in summary, “highlight that conserving and promoting functionally diverse forests could promote soil carbon and nitrogen storage, enhancing both carbon sink capacity and soil nitrogen fertility.”

In the past, some researchers have focused on the different storage efficiency demonstrated by different varieties of trees that operate symbiotically with soil microbes. But this is the first time a study has quantified the effect of promoting carbon and nitrogen sequestration associated with the degree of species diversity within an ecosystem.

Plants as allies of climate and soil

“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,” explained Peter Reich, co-author of the study and director of the Institute for Global Change Biology at the University of Michigan.

The implications are clear. Indeed, the results “highlight that promoting tree diversity not only increases productivity but also mitigates global climate change and reduces soil degradation,” explained Xinli Chen, a researcher in the Department of Renewable Resources at the University of Alberta who was involved in the study. The research findings also highlight “the importance of biodiversity conservation in forests and will guide the growing efforts to use forests for carbon and nitrogen sequestration.”