Mixed stands growth dynamics of Scots pine and Maritime pine: species complementarity relationships and growth effects

Mixed-species forests are becoming more prominent in forest management due to increasing evidence of their greater potential to supply ecosystem functions and services. Given the recognized multifunctionality of mixed forests, it is essential to generate knowledge that provides forest managers with evidence-based silvicultural guidelines. However, species interactions and their impacts on forest dynamics are anything but simple. The functioning of mixed-species stands is modulated mainly by potential niche differentiation, the ability to share available resources and competition dynamics among species. Moreover, species interactions can also change according to environmental spatial gradients and temporal scales. Consequently, analysis of species interactions must include different levels and scales to form a solid working knowledge of mixed-species stand dynamics, which cannot be understood by looking at isolated sub-processes. This thesis provides a multilevel analysis, from tree to stand-level and along temporal and spatial scales, of how species interaction affects the growth and yield processes of Scots pine (Pinus sylvestris L.) and Maritime pine (Pinus pinaster Ait.) mixed stands in a Mediterranean ecosystem. The leading hypothesis was that differences in structural and functional traits between the two species could induce interactions capable of altering stand productivity, above-ground species allometries, species crown dominance, stand structural traits and forest stand dynamics. The study used data from Spain’s National Forest Inventory (NFI) and temporal plots grouped in triplets as primary input in the analysis of species-mixing effects at the three organizational levels: individual tree, tree size distribution, and stand. Additionally, tree-ring width series and band dendrometer measurements from pure and mixed plots made it possible to evaluate species-mixing effects on inter-annual and intra-annual tree growth dynamics. NFI data showed that species-mixing effects at stand level differ from the effects of intra- and inter-specific competition at tree level. Growth efficiency increased in both species according to admixed species proportions at stand level. Intra-specific competition at tree level was higher than inter-specific competition in Scots pine tree growth, indicating a benefit from the mixture. In contrast, Maritime pine showed greater adverse effects of size-asymmetric inter-specific competition. In this species, tree growth was more strongly influenced by the competition structure (size-symmetric and size-asymmetric) than by the species of the competitors. Stand composition influenced intra-specific variation in tree crown allometry. Species-specific traits and species interactions altered vertical structure and size-distribution in mixed stands. Greater vertical structural heterogeneity was related to the relative productivity increment of mixed stands. Crown complementarity and vertical stratification in the canopy space were proposed as crucial mechanisms related to light interception and light-use that enhance ecosystem productivity in mixed stands composed of light-demanding species. Both Scots pine and Maritime pine exhibited species-specific growth fluctuations in response to drought severity as well as different resilience values for drought events. While Maritime pine had a stronger temporal growth-drought correlation and lower resistance to extreme drought than Scots pine, it showed greater resilience and recovered better from drought. The long-term fluctuations and species-specific responses during the drought spells analyzed indicated that species interaction in mixed stands did not affect drought-growth relationships. The two species presented distinct cumulative radial increment patterns in pure stands and species-mixing accentuated these differences. While species mixing did not affect maritime pine patterns, changes in intra-annual radial increment patterns for Scots pine in mixed versus pure stands increased temporal niche complementarity in mixed stands. The intra-annual growing period for Scots pine became longer in mixed stands, but the radial increment rate decreased compared to monospecific stands. Finally, combining mixed-effect models and including parameters that express species-mixing enhanced the estimation performance for tree height and basal area growth, compared to the available models for pure stands. Models for mixed-species stands can be integrated into tree-level models as a planning and evaluation tool for assessing the impact of different management options and determining mixing proportions that maximize performance.