Old-growth forests feature trees of varying sizes and ages, as well as large pieces of deadwood that create a highly complex forest structure composed of multiple vegetation layers.
Aigüestortes National Park boasts the highest quantity and quality of old-growth forests in Spain
Old-growth forests serve as a benchmark for what forests would look like had they not been altered by human impact or affected by extreme events—such as massive wildfires—that fall outside their natural dynamics. These conditions create an optimal environment that fosters highly diverse and unique flora and fauna, featuring centuries-old trees and an abundance of large-sized deadwood. Such forests are rare in the Mediterranean region, estimated to cover less than 1% of the landscape. A new study covering the entire Aigüestortes i Estany de Sant Maurici National Park and its peripheral zone concludes that this area holds the greatest quantity and highest quality of old-growth forests identified to date in both Catalonia and Spain. To appreciate the value of these Pyrenean forests, consider this: while the 2020 State of Nature Report for Catalonia assigned the region's forests an average maturity score of 2.3 out of 10, some of the forests studied in Aigüestortes reached a score as high as 8.7. When compared to similar silver fir, mountain pine, and Scots pine forests across Spain, the National Park also ranks among the areas with the highest forest maturity values.
As part of the project, the 4th Ecological and Forest Inventory of the National Park was also conducted; this involved sampling and analyzing 192 plots—located both within the National Park and in its peripheral protection zone—to obtain a detailed snapshot of the forests' conservation status, degree of maturity, and composition. Among many other findings, the study reveals that the average maturity scores for the National Park's forests range from 4.2 to 4.7 out of 10. Regarding the 20 most mature stands—which cover a total area of 432 hectares—the majority consist of silver fir forests, while four are Scots pine forests and three are mountain pine forests. The maturity scores for these stands range from 5.3 to 8.7 out of 10; they are concentrated in the Sant Nicolau (Alta Ribagorça) and Espot (Pallars Sobirà) valleys—within the National Park's strict protection zone—as well as in the Colomèrs valley (Val d'Aran), within the peripheral protection zone.
Jordi Vayreda, CREAF researcher and co-leader of the inventory
This diversity generates a multitude of microclimatic conditions that help retain moisture more effectively, enhancing resilience against disturbances such as fires or droughts, while providing shelter and food for a wide range of unique, rare, and often threatened species, such as the capercaillie. Furthermore, they serve as important carbon reservoirs. "That is why locating and preserving them must be a priority", Vayreda emphasizes.
In mature stands, trees and shrubs of different ages, heights and diameters coexist, the result of the regeneration, growth, aging and mortality of all species. Image: Galdric Mossoll
There are some very large and old trees, with wide crowns, dead branches, thick bark and roots, and very often with cavities and other microhabitats. Image: Galdric Mossoll
Thick deadwood is created when trees die naturally and remain in the forest, either standing or on the ground. The thicker it is, the more biodiversity it can support. The abundance of deadwood must be proportional to the living wood. Image: Galdric Mossoll
Regarding the composition of the National Park's forests, the inventory reveals that mountain pine (Pinus uncinata) is the dominant species, followed by silver fir (Abies alba) and Scots pine (Pinus sylvestris), among others. As for shrubs, rusty-leaved alpenrose (Rhododendron ferrugineum) is the most common species, followed by bilberry (Vaccinium myrtillus). To determine the degree of forest maturity, researchers analyzed factors such as the number of large trees, the amount of deadwood, and the diversity of dendromicrohabitats—small morphological features found on the trunks and crowns of living trees (such as cavities, wounds, exposed wood, dead branches, loose bark, or other irregularities) that provide shelter for numerous organisms.
The results are part of the BHAVEC project (Assessment of the conservation status of natural habitats and measures to enhance forest maturity in Aigüestortes i Estany de Sant Maurici National Park), coordinated by CREAF and the CTFC in collaboration with the National Park and funded by the "Recovery, Transformation and Resilience Plan" (Next Generation funds). The initiative includes the 2024 Ecological Forest Inventory and a specific study on old-growth forests. Furthermore, a roadmap has been created to accelerate the aging process in certain plots, and various measures have already been piloted. Another outcome is the first-ever analysis of how the Park's forests have evolved over the last three decades.
Infographic "What mature forests are like". Click on the image to enlarge. Design: Alba Mas, image: Galdric Mossoll
Greater maturity and more stored carbon than 30 years ago
Greater maturity and more stored carbon than 30 years ago
Additionally, for the first time, the current inventory has been compared with those from 1993, 2004, and 2014. The comparison reveals a notable increase in stored carbon within the Park's forests over the past thirty years. For instance, in silver fir-dominated plots, the carbon stored in living tree biomass rose from 69.8 to 148.5 tonnes per hectare—more than doubling; in Scots pine plots, it increased from 57 to nearly 100 tonnes per hectare; and in mountain pine plots, it went from 47 to 65 tonnes per hectare. This means that over the last 30 years, the park's forests have continued to sequester CO2 and will continue to do so in the future.
Overall, we also observe that the forests are becoming increasingly mature, with more large trees, more deadwood, and greater structural and species diversity, this demonstrates the importance of using the National Park designation as a form of maximum protection to achieve such mature forests.
Lluís Comas, CREAF researcher
Dead wood, clearings, and other proposals to age a forest
Dead wood, clearings, and other proposals to age a forest
According to researchers, the processes that lead a forest toward maturity can take decades or even centuries. Consequently, they also propose actions to accelerate this aging process. "Some of the proposals aim to conserve and reinforce existing elements of forest maturity while simultaneously creating conditions for new ones to develop. Planned measures include selectively reducing competition around large trees and associated species, as well as creating small clearings to benefit these trees, stimulate regeneration, and diversify the forest's structure and composition," explains Pol Guardis of the CTFC.
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Another important measure involves conserving and generating large-scale dead wood—leaving dead trees standing or on the ground—"as this serves as a key habitat for a wide range of species, including fungi, bryophytes, lichens, beetles and other invertebrates, birds, and small mammals," highlights Xènia Jaumejoan of the CTFC.
These measures have already been tested in a pilot project within one of the forest stands in the Park's peripheral zone, with the aim of analyzing their evolution and assessing the extent to which specific interventions can accelerate processes that would occur much more slowly under natural conditions. "These studies should enable us to better understand the nature and evolution of the Park's forests, but above all, they provide us with tools to decide how to conserve them," highlights Lluís Florit Molina, Director of Aigüestortes i Estany de Sant Maurici National Park.
The research team also agrees that these initiatives could serve to accelerate the maturation of other forests in Spain and Catalonia in the future.
