Thin leaves and strong roots gain ground in response to climate change

The study, led by Josep Padullés, researcher at the Department of Animal Biology, Plant Biology and Ecology of the UAB and CREAF, explores how the combination of rural abandonment and climate change has affected the trees where they are at the limit of the heat and dryness and the cold and humidity they can withstand.

According to the results, which have taken into account almost seventy species, there is more forest density throughout the Peninsula due to the agricultural and silvicultural abandonment of recent decades, allowing forests to recover, but with a new distribution of species due to the impact of climate change.

In general, the data suggest that species that share traits such as strong roots or leaves that are easy to produce are the ones that are expanding the most. For example, in areas that are increasingly hotter and drier, Aleppo pines (Pinus halepensis) and cork oaks (Quercus suber) are gaining ground because they are more tolerant of higher temperatures and dryness. In contrast, those associated with riparian forests are decreasing, such as the common alder (Alnus glutinosa), which was already close to its limit and with the rise in temperature can no longer live in those areas. In colder and wetter regions, some species such as beech (Fagus sylvatica) and broadleaf oak (Quercus petraea) are expanding, while conifers such as maritime pine (Pinus pinaster) have declined, among other reasons, because colder ecosystems are becoming warmer and trees that previously could not survive because it was too cold, can now do so. Another reason could be that there is much less logging than before.

“Each species of tree, like any animal, can live in a certain temperature and dryness gradient and, if this threshold is exceeded above or below, it can no longer grow in those conditions. To get an idea, in the south of Catalonia we will not find beech trees because it is too dry, nor in the higher areas of the Pyrenees where it is too cold. With the increase in temperature these conditions may change and our research provides new data on how this may affect the distribution of trees,” explains Josep Padullés, lead author of the article.

Among other things, these results help to understand how the forests of the future could change and, therefore, to better manage the territory. For example, areas that are becoming increasingly arid could lose the biodiversity of animals and vegetation associated with more humid environments, such as riparian ecosystems, or increase the risk of fires if the species that “reconquer” these areas are more flammable, such as Aleppo pines.

Better adapted roots and leaves 

The research also identified characteristics that make some trees adapt better than others to more arid or humid environments. In the case of dry conditions, species that have larger and thicker roots in relation to the total size of the tree stand out, since they can access deeper layers of the soil to get water and, therefore, survive when there is lack of water. “This trait also gives an advantage in cold environments, although in dry regions it is especially useful,” Padullés clarifies. In the case of cold areas, a factor that favours survival are thin leaves, “cheap” to produce and with a large surface area to capture light, carry out photosynthesis and, therefore, obtain nutrients; a characteristic that gives planifoliate and deciduous trees, such as oaks and ashes, greater tolerance to shade and is an advantage in leafy forests. In contrast, conifers, such as pines, have thicker, smaller, tougher leaves, which are useful in arid areas because they lose less water through transpiration, but cost more to produce and do not have as much surface area to capture light.

More trees in cold environments 

The analysis also highlights that the areas that are becoming hotter and drier are making it more difficult for some species to survive, since they were already close to their climatic heat limit. In this sense, those associated with riparian forests are especially affected, such as common alder (Alnus glutinosa) and ashy willow (Salix cinerea), and others such as maritime pine (Pinus pinaster), which do not tolerate water stress well. In contrast, species such as Aleppo pine (Pinus halepensis), Aleppo oak (Quercus faginea) and cork oak (Quercus suber) are increasing thanks to their resistance to drought and ability to colonise arid soils.

As for the wetter and colder areas, another of the conclusions of the research is that there is a greater increase in trees than would be expected under normal conditions. Some species that are expanding their territory are planifolias such as beech (Fagus sylvatica), broadleaf oak (Quercus petraea), white fir (Abies alba) and pubescent oak (Quercus pubescens). Conifers such as the stone pine (Pinus pinaster), however, have slightly decreased, because they are not good competitors to the planifolias, as they are not well adapted to living in dense shaded forests. Other species that are generally increasing in all regions are those linked to human activity such as cherry (Prunus avium) and acacia (Acacia melanoxylon), an invasive species, which can now grow without control.

To carry out the study, the team analysed data spanning two different periods of the Spanish forest inventories (1986-1996 and 2008-2019), which have a wide variety of data from 21,717 plots of mature and young forests throughout Spain. Specifically, it focused on quantifying the increase or decrease of the abundance of tree species that are at the limit of their climatic distributions, areas where conditions are near the maximum or minimum temperature, and precipitation needed for a given species to survive. “It has been a way to prevent the increase in forest mass due to rural abandonment from masking the effect of climate change,” Padullés points out.

The study is also signed by CREAF researchers Javier Retana, also from the UAB, and Albert Vilà-Cabrera, an associate at the University of Stirling in the UK. Regarding the next step of the research, Padullés says that they will continue to study the interaction between forest abandonment, climate change and species characteristics to understand what the forests of the future will be like.

Original 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. https://doi.org/10.1111/1365-2745.14419