Data from: Multiple factors modulate tree growth complementarity in central European mixed forests

Mixed species forests can often be more productive and deliver higher levels of ecosystem services and functions than monocultures. However, complementarity effects for any given tree species are difficult to generalize because they can vary greatly along gradients of climatic conditions and resource availability. Identifying the conditions where species diversity can positively influence productivity is crucial. To date, few studies have examined how growth complementarity across species and mixture types is modulated by stand and environmental factors, and fewer have considered more than one or two factors. We investigated how complementarity effects for several major central European tree species change with climatic and edaphic conditions, and with stand structural characteristics, including species composition. We used data from the Swiss National Forest Inventory, which is based on 3231 plots of pure and mixed stands (19 mixture types) across a broad environmental gradient, to test (i) how mixing effects change depending on the identity of the admixed species and (ii) if complementarity consistently increases when environmental conditions become harsher. The magnitude, whether positive or negative, of complementarity increased with increasing stand density and stand developmental stage, but no general pattern could be identified across mixture types. Complementarity for many species increased as drought intensity and temperature increased, but not for all species and mixture types. While soil conditions, nitrogen and site topography influenced complementarity for many species, there was no general pattern (increases and decreases were observed). SYNTHESIS. Our study indicates that complementarity varies strongly with stand density and stand development as well as with topographic, climatic and soil conditions. This emphasizes the need to account for site-dependent conditions when exploring mixture effects in relation to forest productivity. We found that under certain conditions (i.e., increasing drought, higher temperature), mixed forests can promote individual-tree growth in central European temperate forests. However, careful assessments depending on the species composing the stands are required under changing resource availability as well as under different levels of stand density and development.

混交林（mixed species forests）通常可比纯林（monocultures）具备更高的生产力，并能提供更丰富的生态系统服务与生态系统功能。然而，针对特定树种的互补效应（complementarity effects）难以形成通用规律，因为其会随气候条件与资源可获得性的梯度变化产生显著差异。明确物种多样性可正向影响生产力的环境条件，至关重要。
截至目前，鲜有研究探讨林分与环境因子如何调控不同物种及混交类型间的生长互补效应，且同时考虑超过1-2种调控因子的研究更为稀缺。本研究聚焦中欧多种主要乔木树种，探究其互补效应如何随气候、土壤条件（edaphic conditions）以及林分结构特征（含物种组成）发生变化。
本研究依托瑞士国家森林清查（Swiss National Forest Inventory）的数据集，该数据涵盖了横跨广泛环境梯度的3231块纯林与混交林样地，涉及19种混交类型，以此验证两项假设：(i) 混交效应如何随混交树种的种类发生改变；(ii) 当环境条件愈发严苛时，互补效应是否会持续增强。
研究结果显示，互补效应的强弱（无论正负）随林分密度与林分发育阶段的提升而增大，但在不同混交类型间未发现统一的普适模式。多数树种的互补效应随干旱强度与温度的升高而增强，但并非所有树种与混交类型均遵循此规律。尽管土壤条件、氮素水平与立地地形对多数树种的互补效应存在影响，但并未发现普适性的变化趋势——研究中同时观测到效应增强与减弱的情况。
综合分析表明，互补效应随林分密度、林分发育阶段，以及地形、气候与土壤条件发生显著变化。这强调了在探究混交效应对森林生产力的影响时，需纳入立地特异性的环境条件。本研究发现，在特定条件下（即干旱加剧、温度升高时），中欧温带森林的混交林可促进单木生长。然而，在资源可获得性发生变化，以及不同林分密度与发育阶段下，需针对组成林分的树种开展针对性的细致评估。