Data_Sheet_4_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.

为减轻气候变化带来的干旱胁迫负面影响，假定由欧洲栎（Fagus sylvatica）与银冷杉（Abies alba）等树种混合组成的混交林能够降低森林管理中的风险。本研究旨在探究栎与冷杉混交对树木生长的影响，特别是对2003年极端干旱事件的响应。为此，我们分析了德国三个混交树种样地以及克罗地亚一个样地中约160棵树木的树干面积增量系列和碳同位素组成（δ13C）。综合考虑树体大小和竞争强度的影响，冷杉与栎的总体生长性能随着混交树种比例的增加而提高。混交促进了两种树种大型树木的生长，无论其邻域密度如何，而小树木仅在密集的邻域中受益。与栎相比，混交对冷杉径向生长的正面影响更为显著，然而，栎通过冷杉的混交在干旱后的生长恢复中获益。径向生长对干旱期间减少的抵抗力以及同位素组成的变异性并未因混交而受到影响，这表明在这两种树种中，干旱胁迫下水利用并非互补。尽管两种树种在干旱期间均表现出生长减少，但冷杉在干旱期间保持了比栎更高的绝对生长水平。两种树种在混交邻域中均受益，但互补效应取决于树体大小和邻域密度。混交冷杉与栎对树木的生长性能产生积极或中性的影响，甚至在响应极端干旱事件时亦然。由于增加树种多样性也能分散与极端事件相关的风险，因此，对于更易受干旱影响的树种如云杉纯林，推荐使用栎与冷杉的混交作为可能的替代方案。