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Tree regeneration is a key demographic process influencing long-term forest dynamics. It is driven by many biotic and abiotic factors. Thus, predictions of tree regeneration are challenging because of complex feedbacks along climatic gradients. The stress gradient hypothesis (SGH) and life-history strategies (LHS) provide a framework for assessing such feedbacks across different species ranges. To address these topics, we analyzed regeneration for 24 tree species in 6,540 plots from 299 unmanaged European forests. Negative interactions predominated, with their intensity decreasing under stressful conditions for most species, as predicted by the SGH. However, positive interactions were only evident for a few species. Our study indicates that SGH and LHS can be combined to partially explain within- and between-species differences in tree recruitment. Moreover, our findings imply that projections of forest dynamics along wide climatic gradients must accommodate both negative and positive biotic interactions, as they strongly affect rates of community turnover.

Harmonized forest area information provides an important basis for environmental modelling and policy-making at both national and international levels. Traditionally, this information has been provided by national forest inventory statistics but is now increasingly complemented with remote sensing tools. Reliability and harmonization of both sources are important aspects to ensure comparability and to enable the development of international forest scenarios. Initiatives with the purpose of harmonization of forest area for both sources are currently on going. Nevertheless, all forest area estimates contain uncertainties, which must be quantifi ed and included in the error budget. This is a prerequisite for combining and comparing data. The purpose of this study is to compare, taking into account uncertainties, forest area estimates for year 2000 derived from four different harmonized satellite-based maps , covering Europe with recognized offi cial forest statistics. It was found that the major cause of disagreements between offi cial statistics and map-derived forest area originates from the general issue of accounting for land cover instead of land use. Consequently, CORINE land cover results had the best accordance with offi cial statistics due to its focus on land use. The other maps overestimated the forest area in mountainous countries and showed underestimation in countries with large forest area or open forest formations.