Temporal interactions among throughfall, type of canopy and thinning drive radial growth in an Iberian mixed pine-beech forest

Many factors can influence tree growth over time such as different forest management practices, climate or tree-to-tree interactions, especially in mixed forests. We show in this work how the temporal growth patterns for Scots pine and European beech depend on thinning intensity (0%, 20% and 40% extraction of basal area), canopy type (pine-beech vs. pine patches), throughfall and their interactions. To fulfill this objective we monitored radial growth of both species using band dendrometers during a 6-year long period including two very dry years. Temporal growth patterns differed between both species. Whereas Scots pine showed two main peaks of growth in May-June and October, European beech mainly grew from May to early September even when throughfall was very limited. Effects of thinning on growth generally increased for both species during dry periods both at the seasonal and annual scales. The treatment with 20% of thinning intensity was the most effective at the annual scale for enhancing growth of both species. However, increases in growth due to thinning were much higher in beech than in pine and lasted longer. Thinning effects on pine were higher in mixed canopy than in pure canopy and appeared to be modulated by throughfall. Global differences in pine growth between canopy types as a function of throughfall increased during the main growing season as beech canopy developed. Growth of Scots pine, but not that of European beech, generally increased with throughfall which suggests that pine might be more dependent for its growth on water from the soil surface layer while beech would depend more on water from deeper soil layers. Our findings have implications to select the most convenient thinning treatments and canopy type under a potential climate change scenario characterized by warmer conditions, more severe droughts and less throughfall.

We are thankful to the Public University of Navarre for supporting this study through a grant to fund Cardil's Postdoctoral studies. This study has been carried out within the framework of the projects AGL2006-08288 and AGL2009-11287 financed by Ministerio de Economía y Competividad (Spain). Camarero JJ acknowledges the support of the Fundiver Spanish project (Spanish Ministry of Economy and Competitiveness, project CGL2015-69186-C2-1-R).