Resumen de Dynamics of p. Pinea and p. Pinaster forests: implications for adaptive management in a climate change context
Over the last decades, climate change has been particularly severe in the Mediterranean basin, where the intensity and frequency of drought events had a significant effect on forest dynamics. As a consequence, tree allometry is modified, reducing tree and stand growth, provoking tree or stand decay and dieback and altering plant recruitment. In this context, differences in structural and physiological strategies of tree species, could mitigate the damage inflicted by climate change and reduce risk. Programs aiming to adapt forests to climate-changing conditions should take profit from the knowledge of these strategies, and propose a set of actions oriented to emulate or enhance the inherent capacity of the forest for adaptation. Between these actions, this thesis highlights the promotion of natural regeneration or the increase of intra and interspecific diversity. A most diverse ecosystem could help to reduce vulnerability and increase resistance against the lack of water availability. In addition, tree species interactions could lead to complementary effects during low-growth years by the optimization of resources.
This thesis provides a variety of methodologies based on analyzing how different factors control the response dynamics of P. pinea and P. pinaster in mixed and monospecific Mediterranean forests throughout stand development, from regeneration to adult stages. The main hypothesis was that species responses, could help to cope with climatic variability and that the composition, mixed or monospecificic, could attenuate these responses. The thesis used data from a network of 1936 plots of 0.02 ha to study natural regeneration, and a network of 75 mixed and monospecific temporal plots on adult stands, both installed in the Northern Plateau of Spain. In this last group of plots, the use of tree-ring width series permitted us to evaluate species-mixing effects on inter-annual and intra-annual tree growth dynamics.
The study of natural regeneration showed that regeneration of P. pinea was more successful than that of P. pinaster over the whole studied period, at both seedling and sapling stages. We found that P. pinaster regeneration was associated with conspecific adult trees, while the relationship between P. pinea regeneration and adult trees varied depending on the regeneration development stage, i.e., seedling or sapling. Furthermore, the presence of mixtures of adult trees enhanced the natural regeneration in both species. On the other hand, we observed that while seedlings occurrence was more conditioned by the annual climate, saplings establishment were more conditioned by site conditions. Here, the maximum summer temperature was the climate variable which had the greatest influence on natural regeneration, especially in P. pinaster.
Different attributes of the adult stands of P. pinea and P. pinaster, such as age, size, composition as well as the stand density seem to act synergistically and compensate drought stress in different ways. Here, larger and/or older trees often appear more prone to drought impacts, especially in the case of P. pinaster. The comparison between the interspecific -synchrony in growth in mixed composition and monospecific composition showed the existence of interrelationship between both species in these mixtures. Thus, we observed that species exhibited an unbalanced competitive relationship for aboveground and belowground resources, and while P. pinea is mainly controlled by intraspecific competition, especially by the size-asymmetric competition, P. pinaster is strongly competing with P. pinea for below resources.
The findings obtained in the present thesis could help to identify problematic areas with regard to natural regeneration. Here, the appropriate management of forest could make a significant contribution, such as maintaining the appropriated stand density along with the different ontogeny states. Similarly, the control of stand structure or the increase in stand heterogeneity (i.e. size, age, and species) could result in increased resistance and resilience to climate change for both P. pinea and P. pinaster, as well as a reduction in the competitive status.
