Increasing forest diversity turns out to be insufficient in the face of extreme drought events

  • Research

First publication date: 10/02/2021

Asier Herrero
Asier Herrero. Photo: Nuria González. UPV/EHU.

Trees of different species tend to compete less with each other in the use of forest resources. That is why forest diversity may exert a beneficial effect on their productivity stability when facing climate changes. However, does this solution always work? A research group led by the Complutense University of Madrid in which the UPV/EHU-University of the Basque Country has participated has confirmed this beneficial effect on productivity, although when facing extreme climate events, such as major droughts, this improvement is not observed.

Increasing tree diversity in forests improves productivity in the face of climate variations, although in the case of extreme events, such as severe drought, resilience does not increase, according to a piece of research by the Complutense University of Madrid, the UPV/EHU, the Autonomous University of Madrid and the University of Alcalá.

The results, published in ‘Forest Ecology and Management’, warn that in a context of increased aridity and frequency of extreme events, adaptation measures such as the increase in diversity may not suffice to offset the consequences of climate change.

Individuals of different tree species tend to use resources differently, so they compete less with each other than if they all belonged to the same species. That is why increasing forest diversity would improve their productivity thanks to growth that is more stable in the event of environmental fluctuations. However, it was not known whether the mixture of different species was also as beneficial in forests with water limitations, such as the Mediterranean ones, and in response to extreme events.

 “Our study shows that the biodiversity-productivity relationship in Mediterranean forest ecosystems is linked to an increase in growth stability, but in the response to extreme events the positive effect of diversity appears to end up diluted by the very sensitivity of the species to water stress and competition,” explained Enrique Andivia, researcher in the Department of Biodiversity, Ecology and Evolution at the Complutense University of Madrid (UCM).

Examining pines and oaks in the Madrilenian mountains

To conduct the study, the researchers analysed pine and oak individuals in mixed stands of both species as well as in unmixed stands on the Sierra de Guadarrama (Madrid). To justify the choice, Andivia pointed out that “this sierra is an excellent case because the mountainous Mediterranean areas are hotspots for exploring the consequences of climate change on the dynamics of plant communities".

By applying dendrochronological techniques –the study of tree rings– the growth of 120 trees throughout their lifetimes was reconstructed, focussing above all on the last 60 years in which their growth response to various events of extreme drought was quantified.

So the experts have revealed the complexity of positive and competitive relations between species, which may vary according to climate fluctuations. “These results have significant implications for forestry management, specifically for adapting our forests to climate change,” concluded Asier Herrero of the Department of Plant Biology and Ecology at the UPV/EHU.

The study was carried out within the ADAPTAMIX project, led by Enrique Andivia and funded by the Spanish Ministry of Science, Innovation and Universities.

Bibliographic reference