Competition modulates the response of growth to climate in pure and mixed Abies pinsapo subsp. Maroccana forests in northern Morocco

The effects of changes in structure and composition on the dynamics and responses of secondary forests to climate change are understudied. Secondary forests of Abies pinsapo var. maroccana are often a mosaic of patches of various development stages showing different stand structures and successional stages. We hypothesise that the structure, species composition and tree-to-tree competition of A. pinsapo stands in Talassemtane National Park (Riff in northern Morocco) modulate the potential response of these secondary forests to climate change. Nine plots representing three species mixtures were established in A. pinsapo-dominated forests, and dendrometric and dendroecological surveys were conducted. The two-parameter Weibull function was used to investigate the diameter distributions of Abies pinsapo, Pinus nigra subsp. mauritanica. Cedrus atlantica, Acer opalus, Juniperus oxycedrus and Quercus ilex for each site along the compositional gradient. We quantified changes in radial growth, response to climate and drought, and components of growth resilience. Finally, the influence of neighbourhood competition on individual tree growth was evaluated using the a competition index. Diameter distributions had an exponential, reverse-J character for A. pinsapo in pure and C. atlantica mixed stands. By contrast, A. pinsapo presented a positively-skewed diameter distribution dominated by small-sized trees in P. nigra mixed stands. There was a significant correlation between tree growth and climate on all forest types, with differing magnitudes per species. Significant precipitation-growth correlations were found for the previous autumn and winter (October, December) in A. pinsapo. Drought events reduced A. pinsapo's resistance to subsequent droughts when growing in pure and mixed stands with C. atlantica. This decline was statistically significant by the end of the studied period (1999-2005). In contrast, mixed forests showed the highest recovery after drought. Successive drought events consistently reduced A. pinsapo's resilience to drought, regardless of species composition. In pure A. pinsapo and in A. pinsapo-C. atlantica mixed forests, competition reduced A. pinsapo's growth, while in A. pinsapo-P. nigra mixed forests increased competition with P. nigra seemed to have a positive-to-neutral effect on A. pinsapo growth. Here, we showed that the response to climate and resilience of A. pinsapo forests could be severely influenced by structure, species composition, and competition, potentially influencing our expectations of long-term persistence of old-growth coniferous trees in the Riff mountains. Understanding forest resilience and response to changing climate has important implications towards managing and safeguarding the productivity and health of these old growth forests.