Mixed forests make up the majority of natural forests, and they are conducive to improving the resilience and resistance of forest ecosystems. Moreover, it is in the crown of the trees where the effect of inter- and intra-specific interaction between them is evident. However, our knowledge of changes in crown morphology caused by density, competition, and mixture of specific species is still limited. Here, we provide insight on stand structural complexity based on the study of four response crown variables (Maximum Crown Width Height, MCWH; Crown Base Height, CBH; Crown Volume, CV; and Crown Projection Area, CPA) derived from multiple terrestrial laser scans. Data were obtained from six permanent plots in Northern Spain comprising of two widespread species across Europe; Scots pine (Pinus sylvestris L.) and sessile oak (Quercus petraea (Matt.) Liebl.). A total of 193 pines and 256 oaks were extracted from the point cloud. Correlation test were conducted (ρ ≥ 0.9) and finally eleven independent variables for each target tree were calculated and categorized into size, density, competition and mixture, which was included as a continuous variable. Linear and non-linear multiple regressions were used to fit models to the four crown variables and the best models were selected according to the lowest AIC Index and biological sense. Our results provide evidence for species plasticity to diverse neighborhoods and show complementarity between pines and oaks in mixtures, where pines have higher MCWH and CBH than oaks but lower CV and CPA, contrary to oaks. The species complementarity in crown variables confirm that mixtures can be used to increase above ground structural diversity.

Thinning treatment and mixed forest stands have been suggested as possible adaptation strategies to cope to climate change but there is still scarce knowledge about the combination of both subjects. In this study, we aim to better understand the thinning effect and the growth differences between two coexisting species on inter- and intra-annual cumulative radial increment patterns. We studied radial increment of a Scots pine-oak (Pinus sylvestris-Quercus pyrenaica) Mediterranean mixed forest during two climatically contrasted years (2016-2017) in north-western Spain. Data came from a thinning trial consisting in a randomized block experimental design with a control and two thinning treatments from below: a moderate and heavy thinning removing 25% and 50% of initial basal area respectively focused on both species. Tree radial increment was analyzed based on bi-weekly readings from band dendrometers installed in 90 oak and pine trees. Non-linear mixed model based on double-Richards curve was fitted to show thinning and species differences in intra-annual cumulative radial increment patterns. Inter-annual basal area increment at species and stand levels were estimated using the model previously fitted at tree level and aggregating the results for exploring thinning effects at these levels. Scots pine leaded the tree and stand growth, and had also a better respond to early spring drought compared to oak. Heavy thinning increased tree radial increment for both species at the expense of decreased stand basal area. At species level, basal area increment decreased for Scots pine, however, heavy thinning generated the same oak basal area increment than control. Thus, heavy thinning may be good strategy towards a conversion from overaged coppice stands into high forests to conform a stable mixed forest stand.