Data from: Tree-to-tree competition in mixed European beech-Scots pine forests has different impacts on growth and water-use efficiency depending on site conditions

1. Mixed conifer-hardwood forests can be more productive than pure forests and they are increasingly considered as ecosystems that could provide adaptation strategies in the face of global change. However, the combined effects of tree-to-tree competition, rising atmospheric CO2 concentrations and climate on such mixtures remain poorly characterized and understood. 2. To fill this research gap, we reconstructed 34-year series (1980-2013) of growth (basal area increment, BAI) and intrinsic water-use efficiency (iWUE) of Scots pine (Pinus sylvestris L.) – European beech (Fagus sylvatica L.) mixed stands at two climatically contrasting sites located in the southwestern Pyrenees. We also gathered data on tree-to-tree competition and climate variables in order to test the hypotheses that (i) radial growth will be greater when inter-specific competition exceeds intra-specific competition, i.e. when species complementarity occurs, and (ii) enhanced iWUE could be linked to improved stem radial growth. 3. Growth of both species was reduced when intra-specific competition increased. Species complementarity was linked to improved growth of Scots pine at the continental site, whilst competition overrode any complementarity advantage at the drought-prone Mediterranean site. Beech growth did not show any significant response to pine admixture likely due to shade tolerance and the highly competitive nature of this species. Increasing inter-specific competition drove recent iWUE changes, which increased in Scots pine but decreased in European beech. The iWUE enhancement did not involve any growth improvement in Scots pine. However, the positive BAI-iWUE relationship found for beech suggests an enhanced beech growth in drought-prone sites due to improved water use. 4. Synthesis. Complementarity may enhance growth in mixed forests. However, water scarcity can constrict light-related complementarity for shade intolerant species (Scots pine) in drought-prone sites. BAI-iWUE relationships were negative for Scots pine and positive for European beech. These contrasting behaviours have got implications for coping with the expected increasing drought events in Scots pine-European beech mixtures located near the ecological limit of the two species. Complementarity effects between tree species should be considered to avoid overestimating the degree of future carbon uptake by mixed conifer-broadleaf forests.

1. 针叶阔叶混交林（mixed conifer-hardwood forests）的生产力高于纯林，且其作为可应对全球变化的适应性生态系统愈发受到学界重视。然而，林木个体间竞争（tree-to-tree competition）、大气CO₂浓度升高与气候三者对这类混交林的综合效应，目前仍未得到充分阐明与理解。2. 为填补这一研究空白，我们重建了位于比利牛斯山脉西南部两个气候差异显著样地的欧洲赤松（Scots pine, Pinus sylvestris L.）-欧洲山毛榉（European beech, Fagus sylvatica L.）混交林的34年（1980-2013）生长序列，包括断面积增量（basal area increment, BAI）与内在水分利用效率（intrinsic water-use efficiency, iWUE）。同时，我们还收集了林木个体间竞争与气候变量数据，以验证两项假说：（i）当种间竞争（inter-specific competition）超过种内竞争（intra-specific competition）（即发生物种生态位互补（species complementarity）时，径向生长将更为旺盛；（ii）内在水分利用效率提升与茎干径向生长改善存在关联。3. 当种内竞争加剧时，两个树种的生长均受到抑制。在大陆性气候样地，物种生态位互补与欧洲赤松生长改善相关；而在干旱频发的地中海气候样地，竞争抵消了生态位互补带来的生长优势。欧洲山毛榉的生长未对欧洲赤松的混交呈现显著响应，这可能与该树种的耐荫性与极强的竞争特性有关。种间竞争加剧驱动了近期的iWUE变化：欧洲赤松的iWUE上升，而欧洲山毛榉的iWUE下降。欧洲赤松的iWUE提升并未伴随生长改善。然而，欧洲山毛榉的BAI与iWUE呈正相关关系，这表明在干旱频发的样地中，欧洲山毛榉可通过提升水分利用效率实现生长增强。4. 综合分析。物种生态位互补效应可提升混交林的生长表现。然而，在干旱频发的生境中，水分短缺会抑制耐荫性较弱的欧洲赤松的光照相关生态位互补效应。BAI与iWUE的相关性在欧洲赤松中呈负相关，而在欧洲山毛榉中呈正相关。这种截然相反的响应模式，对于位于两个物种生态极限附近的欧洲赤松-欧洲山毛榉混交林应对未来预期加剧的干旱事件具有重要参考价值。应考虑树种间的生态位互补效应，以避免高估针阔混交林未来的碳吸收潜力。