Data_Sheet_5_Benefits of Mixtures on Growth Performance of Silver Fir (Abies alba) and European Beech (Fagus sylvatica) Increase With Tree Size Without Reducing Drought Tolerance.docx

To mitigate negative impacts of drought stress in the face of climate change, mixtures of tree species such as those between European beech (Fagus sylvatica) and silver fir (Abies alba) are assumed to lower risks in forest management. This study investigates the influence of mixing beech and fir on tree growth in general and in particular on tree species responses to the extreme drought event of 2003. For this purpose, we analyzed basal area increment series and carbon isotope composition (δ13C) in wood of ~160 trees from three mixed-species sites in Germany and one site in Croatia. Overall growth performance for both fir and beech increased with proportions of the admixed species when accounting for the interactions with tree size and competition intensity. Mixing improved growth of large trees for both species irrespective of neighborhood density, whereas smaller trees benefitted only in denser neighborhoods. Positive mixing effects on radial growth were more pronounced in fir compared to beech, yet the latter benefitted by admixture of fir with regard to growth recovery following drought. Both the resistance of radial growth against reduction during drought as well as the variation of isotopic composition throughout the drought period were not affected by mixing, indicating that water-use in these two species was not complementary under drought stress. Although trees from both species exhibited growth reductions during the drought, fir maintained higher absolute growth levels than beech during the drought. Both species benefited from growing in mixed neighborhoods but complementary effects depended on tree size and neighborhood density. Mixing fir and beech leads to positive or neutral effects on growth performance of trees, also in response to an extreme drought event. Since increasing tree species richness also spreads the risks associated with extreme events, mixtures of beech and fir can be recommended as a possible alternative for more drought-sensitive stands such as spruce monocultures.

为减轻气候变化背景下干旱胁迫的负面影响，将诸如欧洲栎（Fagus sylvatica）与银冷杉（Abies alba）等树种混合被认为是降低森林管理风险的有效手段。本研究旨在探究栎与冷杉混合对树木生长的影响，特别是对2003年极端干旱事件树木物种响应的影响。为此，我们分析了德国三个混合物种地点和克罗地亚一个地点约160棵树木的胸高断面积增量序列和木材碳同位素组成（δ13C）。在考虑树木大小和竞争强度交互作用的情况下，冷杉和栎的整体生长性能随混合物种比例的增加而提高。混合物种的引入，无论在邻域密度如何，均促进了两种树种大型树木的生长，而小型树木仅在密集的邻域中受益。相较于栎，冷杉的径向生长混合效应更为显著，但栎在干旱后恢复生长方面得益于冷杉的混合。在干旱期间，径向生长对减小的抵抗力和同位素组成在整个干旱期间的变异均未因混合而受到影响，这表明这两种树种在干旱胁迫下的水分利用并不具有互补性。尽管两种树种在干旱期间均出现了生长减少，但冷杉在干旱期间的绝对生长水平高于栎。两种树种在混合邻域中均受益，但互补效应取决于树木大小和邻域密度。栎与冷杉的混合对树木的生长性能产生了积极或中性的影响，尤其是在对极端干旱事件作出反应时。鉴于增加树种多样性也能分散与极端事件相关的风险，因此，将栎与冷杉的混合推荐为一种可能的替代方案，适用于更易受干旱影响的云杉纯林等林分。