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.

混交林通常较纯林具备更高的生产力，且可提供更为丰富的生态系统服务与生态系统功能。然而，针对任一特定树种的互补效应（complementarity effect）难以被泛化推广，因其会随气候条件与资源可利用性的梯度变化产生显著差异。明确物种多样性对生产力产生正向影响的环境条件，至关重要。
迄今为止，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匮乏。本研究旨在探究中欧主要乔木树种的互补效应如何随气候、土壤条件以及林分结构特征（包括物种组成）发生变化。
本研究依托瑞士国家森林清查（Swiss National Forest Inventory）数据集展开分析，该数据集涵盖横跨广泛环境梯度的3231块纯林与混交林样地（共19种混交类型），旨在验证两项科学假设：（1）混交效应如何随伴生树种的种类发生变化；（2）当环境条件愈发严苛时，互补效应是否会持续增强。
无论互补效应为正向还是负向，其效应强度均随林分密度与林分发育阶段的提升而增大，但不同混交类型间并未显现统一的变化规律。多数树种的互补效应随干旱强度与气温升高而增强，但并非所有物种及混交类型均呈现该趋势。
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调控作用，但并未发现普适性的变化规律——研究观测到效应强度兼具升、降两种趋势。
综合分析：本研究表明，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。这一结果强调，在探究混交效应与森林生产力的关联时，必须纳入立地特异性条件进行考量。
本研究发现，在特定条件下（即干旱加剧、气温升高时），中欧温带森林的混交林可促进单木生长。然而，在资源可利用性发生变化，以及林分密度与发育阶段存在差异的情境下，需依据林分组成树种开展针对性的细致评估。