The effect of global warming on forest productivity has gone from positive to negative over the last 25 years. The increase in temperature leads to greater summer water stress, according to an analysis of the growth of three pine species over eight decades led by the Complutense University of Madrid.

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Mediterranean forests are increasingly vulnerable

Research led by the Complutense University of Madrid (UCM) shows that, in Mediterranean environments, the positive effects of global warming do not offset the negative effects of increased stress and the greater frequency and magnitude of drought events.

Specifically, according to the study published in Ecosphere, the net effect of temperature went from positive to negative in the last 25 years, coinciding with the approximate increase of 1ºC and with the greater frequency of extreme drought events.

“Moreover, we found this effect in all the species studied, despite the fact that they show contrasting differences in their tolerance to water stress,” says Enrique Andivia, a researcher at UCM’s Department of Biodiversity, Ecology and Evolution.

These results can be interpreted as an early warning sign of the vulnerability of our forests to climate change.
Enrique Andivia

To carry out the work, the researchers analysed growth over six decades (1951-2015) in three pine species in Castilla y León with different drought tolerances (Pinus pinaster, P. nigra and P. sylvestris). To do so, they used dendrochronology, the study of growth rings that also allows for estimating the age of the trees.

“These results can be interpreted as an early warning sign of the vulnerability of our forests to climate change. On the one hand, they indicate that the positive effect that global warming could have on forest productivity and therefore on the capacity of forest ecosystems to sequester carbon and mitigate climate change is limited, at least in water-limited ecosystems,” says the UCM expert.

On the other hand, Andivia continues, the fact that different species show similar negative growth responses to rising temperatures suggests that stress conditions could exceed the tolerance capacity of the species, even those most adapted to water stress.

Along with the UCM, the universities of Alcalá and Valladolid, the Institute of Agricultural Sciences and the Pyrenean Institute of Ecology are participating in the study.