Trees with thin leaves and powerful root systems are increasing in abundance in Spain due to climate change

A new study, published in Journal of Ecology and led by CREAF and the Autonomous University of Barcelona (UAB), has analysed 25 years of changes in more than 445,000 trees in the Iberian Peninsula to find out how a combination of climate change and rural-to-urban migration has affected them in places where they are exposed to as much heat and dryness or cold and humidity as they can withstand. Based on almost 70 species, its results show there to be greater forest density throughout the peninsula, as the abandonment of agricultural and forest management activities over recent decades has allowed woodland to recover, but with a new distribution of species as a consequence of climate change. In general, the study’s data suggest that the species making the greatest gains in abundance are those that share traits such as powerful root systems or leaves that require little energy to produce. In areas that are getting hotter and drier, for instance, the Aleppo pine (Pinus halepensis) and the cork oak (Quercus suber) are growing in number because they can tolerate such conditions, whereas species associated with riparian forests, such as the common alder (Alnus glutinosa), are in decline, as they were already close to their limit and can no longer endure the rising temperature. In colder, wetter regions, species such as the common beech (Fagus sylvatica) and the broad-leaved oak (Quercus pyrenaica) are expanding, while conifers like the maritime pine (Pinus pinaster) have decreased in number. That is partly because colder ecosystems are getting warmer and trees that were previously unable to grow there due to the cold can now do so. Another reason is that much less logging is taking place than before. 

“Like any animal, each tree species can withstand temperatures and levels of dryness within a specific range; above or below that range, it can no longer grow,” says CREAF and UAB researcher Josep Padullés, the study’s lead author. “For example, you won’t find any beech forests in the south of Catalonia because conditions there are too dry, nor high up in the Pyrenees because it’s too cold,” he explains. “But warming could change those conditions and our research provides new data on how that could affect the distribution of trees,” he adds.

Among other things, the study’s results offer insight into how the forests of the future might change and, thus, how land management can be improved. For example, increasingly arid areas could lose the biodiversity associated with wetter environments, like riparian ecosystems, or could be at greater risk of fires if more flammable species, such as the Aleppo pine, become more dominant.

The study has identified particular traits that make some trees better suited than others to the most arid or humid environments. In dry conditions, species with long, thick roots that make up a greater proportion of a tree’s total mass can access deeper layers of the soil to obtain water and, therefore, survive when it is in short supply. “That trait is an advantage in cold environments too, but it’s especially useful in dry regions,” Padullés remarks. In cold areas, a factor conducive to survival is having thin leaves, which can be produced with little energy expenditure and have a large surface area for capturing light so that photosynthesis can be carried out and nutrients obtained. The characteristic in question is an advantage in leafy forests and makes deciduous and broad-leaved trees, such as ash trees and certain oaks, more tolerant to shade. Pines and other conifers, in contrast, have thicker, smaller, harder leaves, which are useful in arid areas because they lose less water through transpiration, but require more energy to produce and have less surface area for capturing light.

The analysis also highlights that in areas where conditions are getting hotter and drier, tree species that were already close to their limit in terms of the heat they can endure are finding survival increasingly difficult. That particularly applies to species associated with riparian forests, such as the common alder, the grey willow (Salix cinerea) and the maritime pine, which struggle to cope with water stress. On the other hand, species like the Aleppo pine, the Portuguese oak (Quercus faginea) and the cork oak are becoming more abundant in such areas, thanks to their ability to deal with drought and colonize arid soils. As for colder, wetter areas, the research concludes that they are gaining more trees than would be expected under normal conditions. Some of the species expanding their territory are broad-leaved trees, such as the common beech, the sessile oak (Quercus petraea), the European silver fir (Abies alba) and the downy oak (Quercus pubescens). However, there has been a slight fall in the population of conifers like the maritime pine in such areas, because they are not ideally suited to dense, shady forests and find it difficult to compete with broad-leaved trees. Other species, meanwhile, are generally increasing in all regions: they are species linked to human activity, such as the wild cherry (Prunus avium) and the invasive Australian blackwood (Acacia melanoxylon), which now grow unchecked. 

To carry out the study, its authors analysed data on 21.717 plots of mature and young forest, spread across the peninsula, from Spain’s second and fourth National Forest Inventories, which cover the 1986-1996 and 2008-2019 periods respectively. The researchers focused on determining whether there had been a rise or a fall in the abundance of tree species found at the edge of their climatic distributions — areas where temperature and rainfall levels are close to the maximum or minimum a given species can endure — between the two periods. “It was a way of preventing the increase in forest mass caused by rural-to-urban migration from masking the effect of warming,” says Padullés.

The study’s other authors, besides Padullés, are CREAF researchers Javier Retana (also affiliated with the UAB) and Albert Vilà-Cabrera (affiliated with the University of Stirling in the UK). Regarding the next step in their research, Padullés states that they will continue studying the interaction between the abandonment of forest management activities, climate change and species traits to learn more about how the forests of the future will be. 

Referenced article: Padullés Cubino, J., Vilà‐Cabrera, A., & Retana, J. (2024). Tree species abundance changes at the edges of their climatic distribution: An interplay between climate change, plant traits and forest management. Journal of Ecology, 112 (12), 2785-2797. DOI: 10.1111/1365-2745.14419