Short-term thinning effect on inter- and intra-annual radial increment of Mediterranean Scots pine-oak mixed forest

Abstract

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.