The Observatory of Natural Heritage and Biodiversity presents an assessment of the effects of extreme droughts on the biodiversity of Catalonia. The document, prepared with the participation of around thirty experts from twelve institutions, including CREAF, collects most of the scientific and expert knowledge available on this field of research and provides context for the three years of extreme drought that Catalonia experienced between the summer of 2021 and the spring of 2024.
The assessment concludes that extreme droughts can alter the landscape on a large scale in forests and scrublands, eliminating the most vulnerable species and favoring the most resistant ones, can cause local extinctions of animals and plants in aquatic ecosystems and can particularly affect riparian forests, which are already scarce and vulnerable in Catalonia. The report also notes that some of these changes can have lasting and irreversible consequences on the country's landscape and biodiversity.
At the end of the document, a series of nature management and monitoring actions are proposed that could help to better understand nature's response to future droughts and mitigate their impacts . Among other actions, it is proposed to promote more diverse and interconnected forests, guarantee minimum ecological flows in river courses, favor the dimensions and connection of aquatic biodiversity refuges during dry periods, and promote the restoration of riparian forests and the long-term monitoring of their biodiversity.
Visual synthesis of the effects of extreme droughts on biodiversity, design: Jose Luís Ordoñez
This evaluation not only aims to provide data, but also to help interpret what these transformations mean in our territory and how they can better guide collective decisions in these areas.
The report was written by CREAF authors Jofre Carnicer, Mireia Banqué, Guille Peguero and Josep Maria Espelta, and was also reviewed by Paco Lloret. The coordination, editing and layout were also led by CREAF by Lluís Brotons, Dani Villero and Alba Gimbert as the coordinating team, José Luís Ordóñez and Sergi Herrando as editors and Nora Soler as designer.
This first evaluation has been, above all, a collective learning process. I would like to highlight the great willingness and involvement of all the experts who participated and who made it possible to build a shared narrative based on very diverse data. I believe that we have learned a lot that will allow us to continue improving and offer, in future evaluations, increasingly refined and useful answers.
Alba Gimbert, coordinator of the Prismatic and member of the evaluation coordination team.
Forests are changing with droughts
The response of Catalonia's forests to the intense and continuous lack of water shows a gradient between fragility and adaptability. According to the report, in the current climate scenario, forest recovery after a drought is not always complete: drought episodes are increasingly frequent and this does not allow forests and scrublands to recover before the next drought and, therefore, they do not easily return to their initial state. When droughts are prolonged, the most sensitive species suffer higher mortality rates and give way to those that can better withstand water stress, often causing drastic changes in the landscape. The loss of vegetation due to drought affects the entire food web (the set of relationships that are established between all organisms depending on who eats whom): insects, frugivores and herbivores are the first to suffer the consequences, and from there, the effect reaches their predators.
Experts point out, however, that tree mortality generates dead wood and leaves more open spaces in the forest, a fact that generates less competition for light and water, and that allows the surviving trees to improve their situation. In addition, dead wood and the creation of open spaces can promote forest maturity . Decaying dead wood helps retain soil moisture, and this usually favors both resistance to future droughts and the diversity of plants and other organisms in the understory. This wood could initially and temporarily increase the risk of fire, but within a few years, this risk clearly decreases and dead wood is a key element in the recycling of nutrients and the supply of nesting sites for many species of fauna.
Forest affected by drought in Montseny during the year 2022. Image: Gerard Gaya
The most diverse forests are the most resilient
Knowing, therefore, the response of forests to droughts, the assessment recommends promoting more diverse and connected forests, which are those that are more resilient to most disturbances. There is no doubt that this can be done with well-planned forest management. However, on the other hand, there is no consensus when the objective of forest management is to increase our water reserves. Some models suggest that removing trees from the headwaters of rivers to reduce the forest's water consumption could significantly increase the water available in rivers and swamps, while others indicate that this would only be effective in the short term: after a few years, vegetation occupies the space opened by the cuts with new leaves and the forest's water consumption returns to its initial level. This implies that, to obtain more water, very intense, extensive and constant actions would be required, with the risk of losing other ecosystem services that trees also provide, such as erosion control, CO₂ capture or the maintenance of biodiversity.
Refuges and minimum flows to recover aquatic life
In Mediterranean river systems, temporality is the key: many rivers naturally alternate between wet and dry periods, and fauna and flora have adapted to coexist with this alternation. However, the assessment indicates that when periods are very dry or last too long, this balance is broken, because rivers can fragment into very isolated stretches or even dry up completely, making it difficult for species to recolonize when the rains return. In addition, in these situations, some species do not complete their life cycle and local species losses or extinctions can occur. Amphibians usually recover well after less intense droughts, but if they are prolonged or combined with other stress factors, the viability of some species in the territory can be put at risk. It should be noted that more than 50% of amphibian breeding sites in Catalonia were permanently dry between 2021 and 2023, especially on the coast.
The environments most sensitive to long and intense droughts are small rivers that carry water all year round and high mountain lakes, especially at the headwaters, where fauna and flora are not adapted to reducing their reproductive times so much. To improve the resilience of these ecosystems, the assessment recommends identifying and preserving refuges , such as isolated ponds with high biological value. It also underlines the importance of ensuring the connectivity of these refuges and maintaining minimum river flows that allow species to overcome dry periods and recover more quickly when the water returns.
Riparian forests suffer from the terrestrial and aquatic effects of droughts
Riparian forests play a key role in the transition between water bodies and the terrestrial ecosystem. They provide sufficient shade to significantly regulate water temperature, filter sediments and pollutants, connect habitats, and feed aquatic and terrestrial food webs through the exchange of leaves and invertebrates. The assessment highlights that when droughts are prolonged, the water available to riparian fauna and vegetation is reduced, plant cover and diversity (especially at headwaters) decrease, and less shade, fewer leaves, and fewer invertebrates enter the river, on which fish and amphibians often depend. In addition, the lack of flow causes aquatic fauna and flora to be concentrated in a smaller volume of water, which favors more predation, more competition, and more disease transmission.
Despite this key role of riparian forests, there is still much unknown about how they respond to these contexts, and for this reason the assessment recommends long-term monitoring of the different biological groups that live in these spaces, both in natural and agricultural environments. It also focuses on the desirability of restoring riparians and preventing and controlling invasive species.
Pilot evaluation
After the State of Nature, this was the first thematic pilot assessment promoted by the Observatory of Natural Heritage and Biodiversity, conceived as a starting point to test a new methodology, assess its results and establish the bases for future thematic and global assessments, such as the next State of Nature. The document collects evidence from monitoring programs, official reports and scientific studies, as well as the expertise and knowledge of professionals who study biodiversity in Catalonia from different fields. The process has been carried out with an open call to experts in scientific research and management, inspired by international methodologies such as those promoted by the Intergovernmental Platform on Biodiversity and Ecosystem Services (IPBES).
The report has had the institutional participation of the Santa Coloma de Gramenet City Council, the Calafell Environmental Research and Education Center (CREAC), CREAF, the Forest Science and Technology Center of Catalonia (CTFC), the Generalitat of Catalonia (GENCAT), the Biodiversity Research Institute of the University of Barcelona (IRBio-UB), the Institute of Agro-Food Research and Technology (IRTA), the Museum of Natural Sciences of Barcelona (MCNB), the Catalan Society of Herpetology (SCH), the Autonomous University of Barcelona (UAB), the University of Girona (UdG) and the University of Lleida (UdL).
