In Central Europe, anthropogenic coniferous monocultures have been subject to conversion to more diverse mixed forests since the 1990s, however, they are still abundant across many forest landscapes. Artificial and natural tree regeneration both play a key role during conversion by determining the species composition and structure of the future forests. Many abiotic and biotic factors can potentially influence the regeneration process and its specific combinations or interactions may be different among tree species and its developmental stages. Here, we aimed to identify and quantify the effect of the most important drivers on the density of the most abundant regenerating tree species (i.e., Norway spruce and European beech), as well as on species and structural diversity of the tree regeneration. We studied tree regeneration in four former monospecific coniferous stand types (i.e., Norway spruce, Scots pine, European larch, and Douglas fir) in Southwest Germany that have been under conversion to mixed forests since the 1990s. We sampled tree regeneration in four growth height classes together with a variety of potentially influencing factors on 108 sampling plots and applied multivariate analyses. We identified light availability in the understorey, stand structural attributes, browsing pressure, and diaspore source abundance as the most important factors for the density and diversity of tree regeneration. Particularly, we revealed species-specific differences in drivers of regeneration density. While spruce profited from increasing light availability and decreasing stand basal area, beech benefited either from a minor reduction or more strikingly from an increase in overstorey density. Increasing diaspore source abundance positively and a high browsing pressure negatively affected both species equally. Our results suggest that humus and topsoil properties were modified during conversion, probably due to changes in tree species composition and silvicultural activities. The species and structural diversity of the tree regeneration benefitted from increasing light availability, decreasing stand basal area, and a low to moderate browsing pressure. We conclude that forest managers may carefully equilibrate among the regulation of overstorey cover, stand basal area, and browsing pressure to fulfil the objectives of forest conversion, i.e., establishing and safeguarding a diverse tree regeneration to promote the development of mature mixed forests in the future.

中文翻译：

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中欧皈依混交林过程中针叶林单作树木再生的驱动因素 – 对森林恢复管理的影响


在中欧，自 1990 年代以来，人为针叶林单一栽培已被转变为更多样化的混交林，然而，它们在许多森林景观中仍然很丰富。人工和自然树木再生在转换过程中都发挥着关键作用，它决定了未来森林的物种组成和结构。许多非生物和生物因素可能会影响再生过程，其特定的组合或相互作用可能因树种及其发育阶段而异。在这里，我们旨在识别和量化最重要的驱动因素对最丰富的再生树种（即挪威云杉和欧洲山毛榉）密度的影响，以及对树木再生的物种和结构多样性的影响。我们研究了德国西南部四种以前的单种针叶林类型（即挪威云杉、苏格兰松、欧洲落叶松和花旗松）的树木更新，这些类型自 1990 年代以来一直在转变为混交林。我们在 108 个采样样地上对 4 个生长高度类别的树木再生以及各种潜在影响因素进行了采样，并应用了多变量分析。我们确定了林下层的光可用性、林分结构属性、浏览压力和散孢子源丰度是影响树木更新密度和多样性的最重要因素。特别是，我们揭示了再生密度驱动因素的物种特异性差异。云杉受益于光可用性的增加和林分基底面积的减少，而山毛榉则受益于林分密度的轻微减少或更明显的增加。 正向增加散孢子源丰度和高浏览压力对这两个物种产生了同等的负面影响。我们的结果表明，腐殖质和表土特性在转换过程中发生了变化，这可能是由于树种组成和造林活动的变化。树木再生的物种和结构多样性受益于光可用性的增加、林分基部面积的减少以及低至中等的浏览压力。我们得出的结论是，森林管理者可以在层级覆盖、林分基底面积和浏览压力的调节之间小心地平衡，以实现森林转换的目标，即建立和保护多样化的树木更新，以促进未来成熟混交林的发展。