Just like a sunrise, the emergence of leaves in spring is one of nature’s poetic, awe-inspiring moments. It reminds us of the phases of nature, inexorable and punctual. However, that punctuality is being influenced by climate change, which is causing plants to come into leaf earlier and earlier each year. That, in turn, has consequences for trees, as well as global effects on the carbon cycle and climate change itself. 

Against that backdrop, an international team of scientists has recently published an article, in the journal Nature Climate Change, which explains that biodiversity protects forests from premature leafing because the more diverse a forest is, the less sensitive it will be to rising temperatures. According to data collected in a study to which CREAF and the Spanish National Research Council (CSIC) have contributed, forests with a greater variety of tree species (and, thus, a lower level of the aforementioned sensitivity) produce leaves when they ought to. This is attributable to such forests being better able to generate fertile soil and to their trees having deeper roots and being well equipped to take up water and nutrients, making them more productive.

Biodiversity protects forests from premature leafing because the more diverse a forest is, the less sensitive it will be to rising temperatures. 

To undertake the study, researchers from CREAF, the CSIC, the Chinese Academy of Sciences, ETH Zurich and other institutions from the USA and China compiled data on species richness from the Global Forest Biodiversity Initiative (GFBI) for 393,139 forest inventory plots, covering various forest types and species found in the Northern Hemisphere. They obtained leaf-emergence data for the 2001–2022 period from MODIS (Moderate Resolution Imaging Spectroradiometer) instruments carried aboard satellites. They also collected climate and soil data spanning 2000 and 2022, in addition to data on vegetation growth.

“Trees that grow vigorously do not experience as much stress when the temperature rises and do not produce leaves before they should. That protects them from expending a significant amount of energy in a way that exhausts trees around the world”.

JOSEP PEÑUELAS, CREAF-based CSIC researcher and co-author of the article.

The study’s results are so striking that the authors say biodiversity has a greater effect on a forest’s sensitivity to temperature than its local climate variables do. On that basis, they have stressed the need to incorporate biodiversity’s buffering effects into climate change models, so as to better understand global warming’s impact on leafing in spring and on carbon uptake.

Deeper roots and increased microbial activity

The study indicates that when there is a high degree of species diversity in a forest, the physicochemical properties (e.g. pH) of the soil improve, accelerating the activity of plants and of microorganisms that carry out decomposition. As a result, the amount of stable organic matter increases. This, in turn, enhances soil fertility and structure, making it more porous and allowing roots to grow much more. The study found that trees in biodiverse forests have deeper roots, which facilitate access to nutrients and moisture lower down in the ground. Furthermore, a more biodiverse forest that generates more organic matter will be able to absorb more carbon from the atmosphere. The carbon-to-nitrogen ratio increases in such forests, where plants respond to the lack of nitrogen by delaying leafing and allocating more carbon to root growth.

A more biodiverse forest that generates more organic matter will be able to absorb more carbon from the atmosphere.

“Greater soil fertility also improves the soil microbiome, which is beneficial for plants in the long term because it makes them more resilient and better able to absorb nutrients,” says Peñuelas.

Models that predict when plants will leaf are very important for understanding the way in which ecosystems work and the nature of their relationship with the climate. Such models help scientists see how Earth’s carbon and water cycles change each year and how soil, vegetation and the atmosphere interact. It is hoped that the study discussed here will contribute to making the models in question more precise, through the inclusion of biodiversity’s buffering effects, which had never been factored into predictions before.

Reference article: Shen, P., Wang, X., Zohner, C.M. et al. Biodiversity buffers the response of spring leaf unfolding to climate warming. Nat. Clim. Chang. (2024). https://doi.org/10.1038/s41558-024-02035-w