Background and Aims Many recent studies emphasize that mixed species is a promising silvicultural option for sustainable ecosystem management under uncertain and risky future environmental conditions. However, compared with monocultures, knowledge of mixed stands is still rather fragmentary. This comprehensive study analysed the most common Central European tree species combinations to determine the extent to which mono-layered species mixing (1) can increase stand productivity and stem diameter growth, (2) increase stand density or growth efficiency, and (3) reduce competition and attenuate the relationship between stand density and stem diameter growth compared with mono-specific stands. Methods The study was based on 63 long-term experimental plots in Germany with repeated spatially explicit stand inventories. They covered mono-specific and mixed species stands of Norway spruce (Picea abies), silver fir (Abies alba), Scots pine (Pinus sylvestris), European beech (Fagus sylvatica), sessile oak (Quercus petraea), European ash (Fraxinus excelsior) and sycamore maple (Acer pseudoplatanus). Based on spatially explicit measurement, we quantified for each tree the intra- or inter-specific neighbourhood, local stand density and growth. We applied mixed models to analyse how inter-specific neighbourhoods modify stand productivity, stand density, growth efficiency, individual tree growth and the trade-off between individual tree growth and stand productivity. Key Results We found stand productivity gains of 7–53 % of mixed versus mono-specific stands continuing over the entire rotation. All mixtures achieved a 3–36 % higher leaf area index until advanced stand age. Stem diameter growth increased by up to 31 % in mixed stands. The growth efficiency of the leaf area was up to 31 % higher, except in mixtures of sessile oak and European beech. The trade-off between stem diameter growth and stand productivity was attenuated by the mixture. Conclusions The increased productivity was mainly based on a density increase in the case of Norway spruce/silver fir/European beech and sessile oak/European beech and it was based on a more efficient resource use given the same stand density in the case of Scots pine/European beech and European ash/sycamore maple. In the other species assemblages the increased productivity was based on a combination of density and efficiency increase. We hypothesize that the density effect may be site-invariant and mainly depends on the structural species complementarity. The efficiency increase of growth may depend on the growth-limiting factor that is remedied by mixture and thus be co-determined by the site conditions. For forest management, the results indicate increased stand and tree size growth by species mixing. For the common mixtures examined in this study the results show that thinning for the acceleration of stem growth requires less density reduction and causes less stand growth losses than in monocultures. We discuss the consequences of our findings for silvicultural prescriptions for mixed-species stands.

中文翻译：

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树种混合可以提高林分生产力、密度和生长效率，并减弱整个轮作过程中密度和生长之间的权衡

背景和目标 最近的许多研究强调，在不确定和危险的未来环境条件下，混合物种是可持续生态系统管理的一个有前途的造林选择。然而，与单一栽培相比，混合林的知识仍然相当零碎。这项综合研究分析了最常见的中欧树种组合，以确定单层树种混合的程度（1）可以提高林分生产力和茎直径生长，（2）增加林分密度或生长效率，以及（3）减少与单一特定林分相比，竞争并减弱了林分密度和茎直径生长之间的关系。方法 该研究基于德国 63 个长期试验田，并进行了重复的空间明确林分清查。它们涵盖了挪威云杉 (Picea abies)、银冷杉 (Abies alba)、欧洲赤松 (Pinus sylvestris)、欧洲山毛榉 (Fagus sylvatica)、无柄栎 (Quercus petraea)、欧洲白蜡树 (Fraxinus excelsior) 的单一树种和混合树种林）和无花果枫树（Acerpseudoplatanus）。基于空间明确的测量，我们量化了每棵树的种内或种间邻域、当地林分密度和生长。我们应用混合模型来分析特定邻域如何改变林分生产力、林分密度、生长效率、单树生长以及单树生长和林分生产力之间的权衡。主要结果 我们发现，在整个轮作期间，混合林分与单一林分的生产率持续提高 7-53%。直到高龄时，所有混合物的叶面积指数均提高了 3-36%。混合林中的茎直径生长增加高达 31%。除了无柄橡树和欧洲山毛榉的混合物外，叶面积的生长效率提高了 31%。混合物削弱了茎直径生长和林分生产力之间的权衡。结论 挪威云杉/银杉/欧洲山毛榉和无柄橡树/欧洲山毛榉生产力的提高主要基于密度的增加，而欧洲赤松则基于在相同林分密度的情况下更有效的资源利用/欧洲山毛榉和欧洲白蜡木/梧桐枫木。在其他物种组合中，生产力的提高是基于密度和效率提高的结合。我们假设密度效应可能是位点不变的，并且主要取决于结构物种的互补性。生长效率的提高可能取决于通过混合物补救的生长限制因素，因此由场地条件共同决定。对于森林管理，结果表明通过物种混合增加了林分和树木尺寸的增长。对于本研究中检查的常见混合物，结果表明，与单一栽培相比，为了加速茎生长而进行的间伐需要较少的密度降低，并且造成的林分生长损失也较少。