Abstract
The global tendency of increasing temperatures and severity and frequency of drought events are considered among the main threats for forests vulnerability and survival. Uncertainties related to forest ecosystems’ adaptive capacity towards such fast changes are promoting an extensive scientific research focused on forest conservation and related ecosystem services. In this context, forest ecosystems responses to global warming and extreme droughts play an important role in the global carbon cycle counteracting atmospheric CO2, an ability that can be strongly reduced and temporarily reversed under climatic extremes as drought. Forest management has a great impact on carbon sequestration in forest ecosystems, especially in the temperate zone. In Central Europe, the wood-oriented silvicultural practices of the last century promoted mono-specific forests, which seem more vulnerable towards extreme events such as pests, fungi and droughts. Mixed forests on the other hand, seem more productive and more resilient in the face of future climatic conditions, but there are several uncertainties and contrasting findings related to the beneficial effects of an increased biodiversity on tree performance. The aim of this research is to assess tree responses to climate change and drought events in different tree admixture, to contribute to the scientific knowledge regarding tree-species interaction in forests and tree responses to drought. To achieve these aims Pinus sylvestris was selected. Its performance was investigated in a setting of pure P. sylvestris and in mixed P. sylvestris - Quercus sp. forests in contrasting climatic conditions (i.e., Continental-Poland, and Mediterranean-Spain). Patterns of carbon sequestration were investigated in P. sylvestris with traditional growth measures and with a new Index called BAIden, which combines Basal Area Increment and Wood density, two proxies of carbon sequestration. An intensive study of variability in xylem traits along the whole chronology of trees was employed to understand the role of tree admixture on tree responses to drought events aggregating per different drought which have different occurrences (i.e., spring, summer and all-year drought), in contrasting climatic conditions. Tree admixture influenced xylem traits responses to climatic variability and droughts, even though the trees were not seriously affected or compromised by the selected droughts. In Poland, the role of tree admixture was prominent, where it varied depending on drought seasonality. The presence of oaks worsened drought conditions for pines during spring drought, but induced benefits to pines during summer drought. In Spain, the effects of tree admixture after analyzing extreme drought periods were not apparent, and are not unequivocally related to the tree admixture but also to the stand density of the forest. In Poland, BAIden
highlighted similar performances between pines growing in the two forest types, and showed that carbon sequestration ability will be advantaged by future spring climatic conditions but they will be likely compromised by future summer climatic conditions. The mixing P. sylvestris - Q. petraea results compared favourably to pure pine forests, in the face of the severe future summer droughts expected for this region.