Status and development trends of Scots pine plantations outside the species areal in Bulgaria

Abstract Our study attempted to find a spatial analogue of the anticipated climate change in present times by investigating the status and development trends of four half-century-old man-made Scots pine stands outside the species areal in Central and North-Eastern Bulgarian lowlands. Stand composition, competition intensity, stand stability, site quality, tree mortality and ecosystem fit according to nationally established standards, were evaluated and growth correlations with climatic variables were analysed. Two stands experiencing an ongoing dieback were distinguished, regardless of the good productivity potential of the trees (Stand 2) or the low competition pressure and good mechanical stability maintained (Stand 1). The other 2 plantations showed either no mortality (Stand 4) or mortality due to self-thinning (Stand 3), forming a relatively productive and dense pure Scots pine stand (Stand 4) or a mixed, highly productive stand with diverse composition and good structure and stability (Stand 3). Tree growth was positively affected by the summer and the annual precipitation amounts, while negative correlation was found with the summer heat moisture index. Our investigation revealed that pine growth still can be sustained even in a slightly humid climate, if a high productivity level and good stand structure are assured, that indicates the overall capability of the species to adapt. However, the Scots pine trees are not able to adapt to relatively dry environmental conditions, suitable for lowland thermophilic species such as oaks, where the stands collapse when the heat-moisture balance worsens.