New Insights into the Consequences of Post-Windthrow Salvage Logging Revealed by Functional Structure of Saproxylic Beetles Assemblages

Windstorms, bark beetle outbreaks and fires are important natural disturbances in coniferous forests worldwide. Wind-thrown trees promote biodiversity and restoration within production forests, but also cause large economic losses due to bark beetle infestation and accelerated fungal decomposition. Such damaged trees are often removed by salvage logging, which leads to decreased biodiversity and thus increasingly evokes discussions between economists and ecologists about appropriate strategies. To reveal the reasons behind species loss after salvage logging, we used a functional approach based on four habitat-related ecological traits and focused on saproxylic beetles. We predicted that salvage logging would decrease functional diversity (measured as effect sizes of mean pairwise distances using null models) as well as mean values of beetle body size, wood diameter niche and canopy cover niche, but would increase decay stage niche. As expected, salvage logging caused a decrease in species richness, but led to an increase in functional diversity by altering the species composition from habitat-filtered assemblages toward random assemblages. Even though salvage logging removes tree trunks, the most negative effects were found for small and heliophilous species and for species specialized on wood of small diameter. Our results suggested that salvage logging disrupts the natural assembly process on windthrown trees and that negative ecological impacts are caused more by microclimate alteration of the dead-wood objects than by loss of resource amount. These insights underline the power of functional approaches to detect ecosystem responses to anthropogenic disturbance and form a basis for management decisions in conservation. To mitigate negative effects on saproxylic beetle diversity after windthrows, we recommend preserving single windthrown trees or at least their tops with exposed branches during salvage logging. Such an extension of the green-tree retention approach to windthrown trees will preserve natural succession and associated communities of disturbed spruce forests.

风暴、树皮甲虫虫害与林火是全球针叶林内重要的自然干扰因子。风倒木可提升用材林内的生物多样性并促进生态修复，但同时会因树皮甲虫侵染与真菌分解加速而造成巨额经济损失。此类受损林木通常会通过灾后伐木（salvage logging）进行清理，该操作会导致生物多样性下降，由此愈发引发经济学家与生态学家就合理应对策略展开的讨论。为揭示灾后伐木后物种丧失的内在机制，本研究采用基于4种生境相关生态性状的功能性状分析法，研究对象为蛀木甲虫（saproxylic beetles）。我们曾提出如下假说：灾后伐木会降低功能多样性（通过零模型（null models）计算的平均成对距离效应量进行衡量），同时降低甲虫体型大小、木材直径生态位与冠层覆盖生态位的均值，但会提升腐朽等级生态位。正如预期，灾后伐木导致了物种丰富度下降，但通过将物种组成从生境过滤类群转变为随机类群，反而提升了功能多样性。尽管灾后伐木移除了树干，但对小型喜光物种以及特化栖息于小径级木材的物种造成了最为显著的负面影响。本研究结果表明，灾后伐木破坏了风倒木上的自然群落组装过程，其负面生态影响更多源于枯木对象的微气候改变，而非资源数量的流失。这些研究结果凸显了功能性状分析法在解析生态系统对人为干扰响应方面的优势，并可为保护管理决策提供理论依据。为缓解风倒灾害后对蛀木甲虫多样性的负面影响，我们建议在灾后伐木作业中保留单株风倒木，或至少保留其带有外露枝条的顶部。将保留木保留法（green-tree retention）推广应用至风倒木场景，将有助于维持受扰云杉林的自然演替过程及其伴生物种群落。