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. 针阔混交林（mixed conifer-hardwood forests）的生产力往往高于纯林，且正日益被视为可应对全球变化的适应性生态系统。然而，树木间竞争、大气CO₂浓度上升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，本研究针对比利牛斯山脉西南部两处气候差异显著的样地，重建了欧洲赤松（Scots pine, Pinus sylvestris L.）与欧洲山毛榉（European beech, Fagus sylvatica L.）混交林的34年（1980-2013年）生长（断面积增量（basal area increment, BAI））与内在水分利用效率（intrinsic water-use efficiency, iWUE）序列。同时收集了树木间竞争与气候变量数据，以验证两项假说：（i）当种间竞争强于种内竞争（即发生物种互补效应）时，径向生长量将更高；（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. 两种树木的生长均随种内竞争加剧而受到抑制。在大陆性气候样地，物种互补效应与欧洲赤松的生长改善显著相关；但在易发生干旱的地中海气候样地，竞争作用抵消了所有互补优势。欧洲山毛榉的生长对混交赤松无显著响应，这可能源于其耐荫性与极强的竞争特性。近年来种间竞争的加剧驱动了内在水分利用效率的变化：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。但针对欧洲山毛榉观测到的BAI与iWUE正相关关系表明，在易干旱样地中，其水分利用效率提升可促进生长。4. 综合结论：物种互补效应可提升混交林的林木生长。但在易干旱样地中，水分短缺会限制耐荫性较弱物种（欧洲赤松）依赖光照的互补优势。欧洲赤松的BAI与iWUE呈负相关关系，而欧洲山毛榉则呈正相关关系。这种截然相反的响应模式，对于地处两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愈发频发的干旱事件具有重要启示。在评估针阔混交林未来碳吸收能力时，应纳入树种间的互补效应，以避免高估其碳汇潜力。