The effect of plant–plant interactions as a key biotic process mediating the spatial variation of phenotypes in a Pinus sylvestris forest

KEY MESSAGE: In this work, we highlighted the importance of the phenotypic structure of forest in regulating inter-tree competition with scattered individuals showing larger growth than close neighbours, with lower growth rates. Plant interactions are among the fundamental processes shaping the structure and functioning of ecosystems as they modulate competitive dynamics. However, the connection between the response of individual growth to neighbours and to environmental conditions and the mechanisms determining interactions in monospecific stands remain poorly understood. Here, we followed a phenotypic-based approach to disentangle the effect of plant size, neighbourhood interactions and microhabitat effects on Pinus sylvestris growth and traits, as well as their spatial variation of growth. We mapped all adult trees (1002 pines) in a 2 ha stand and measured their height, DBH and crown projection. For each individual, we assessed its growth and a competition index in relation to the closest neighbours. Soil chemical and physical properties and ground cover were also measured in a grid within the stand. We analysed the effects of tree size, neighbour competition and microhabitat variation on tree growth with a linear model. We also used spatial mark-correlation functions to explore the spatial dependence of tree age, secondary growth and phenotypic traits. Our results showed that trees with close neighbours displayed lower growth rates, whilst individuals with larger growths appeared scattered throughout the stand. Moreover, we found that growth depended on competition, tree height and crown area while tree growth poorly correlated with age or microhabitat conditions. Our findings highlight the importance of forest structure, in regulating inter-tree competition and growth in a Mediterranean pure stand and they provide insight into the causes and consequences of intraspecific variation in this system.