Linking biodiversity to ecosystem functioning in forest ecosystems: biotic drivers of wood decomposition

由于土地利用压力的增加，分类多样性的大幅下降可能会损害生态系统的功能。枯木的分解是森林中碳和养分循环等方面的一个关键过程，但对土地和森林产品的需求日益增加，导致世界各地森林物种的数量不断减少。然而，整个过程是由几个分解者分类群之间的复杂相互作用驱动的，主要是木材腐烂真菌和腐生昆虫，但仍然知之甚少，特别是更具体的问题，如分解者物种丰富度对木材分解的作用迄今尚未得到回答。即使对于真菌，物种丰富度和腐烂之间也没有明确的总体关系，因为已经观察到了正和负的关系。另一个复杂的因素是，某些分解者群可直接促进木材分解，例如腐植酸甲虫消耗木材，但也间接通过其他分解者群，例如腐植酸甲虫传播木材腐烂真菌。为了弄清腐胺甲虫物种丰富度和功能多样性的作用，2015年在巴伐利亚森林国家公园成功地建立了一个中游实验，操纵190个腐胺甲虫群落，跨越物种丰富度和功能多样性(身体大小和挖掘功能)的独立梯度。拟议的项目旨在通过一种创新的方法，即X射线计算机断层扫描(CT)来衡量被操纵的甲虫动物群对木材分解的贡献。与传统的基于样本的方法不同，一整块木材可以在几秒钟内进行非破坏性扫描，并且分辨率足够高，可以对木材密度进行定量分析，从而检测出木材分解的微小差异。此外，我们的目标是测量木材腐烂真菌的分解(麦角甾醇和酶含量)，并应用下一代腐烂甲虫和实验原木上真菌群落的DNA测序，以准确记录真菌对木材的定居。通过将分解和木材腐烂真菌与单一甲虫物种和甲虫群落的特征联系起来，我们将能够解开腐胺甲虫对木材分解的直接和间接影响。具体地说，我们想检验以下假设：(H1)腐植酸甲虫群落的功能多样性对木材分解的速度比甲虫物种丰富更重要；(H2)甲虫物种越大，其廊道越深，其对分解的直接和间接贡献越强；(H3)腐氧甲虫携带的真菌群落的组成是甲虫物种特有的；(H4)真菌对木材的分解始于甲壳虫廊道。

Massive declines in taxonomic diversity due to increasing pressure by land use can impair ecosystem functions. Decomposition of dead wood is a key process in forests with regard to, e.g., carbon and nutrient cycling, but the increasing need for land and forest products has led to decreasing populations of forest species worldwide. The overall process, driven by complex interactions between several decomposer taxa, mainly wood-decaying fungi and saproxylic insects, however, remains still poorly understood and particularly more specific questions such as the role of species richness of decomposers for wood decomposition has not been answered so far. Even for fungi, no clear overall relation between species richness and decomposition could be derived as both positive and negative relations have been observed. A further complicating factor is certain decomposer groups can contribute to wood decomposition directly, e.g. by consumption of wood by saproxylic beetles, but also indirectly via other decomposer groups, as e.g. saproxylic beetles vectoring wood-decaying fungi. To disentangle the role of species richness and functional diversity of saproxylic beetles, a mesocosm experiment has been set up successfully in the Bavarian Forest National Park in 2015, manipulating 190 saproxylic beetle communities spanning independent gradients of species richness and functional diversity (body size and excavating function). The proposed project aims at measuring the contribution of the manipulated beetle fauna to wood decomposition by an innovative method, namely X-ray computed tomography (CT). In contrast to traditional sample-based methods, a complete piece of wood can be scanned non-destructively within seconds and with a resolution sufficiently high to conduct quantitative analyses of wood density that allow detecting even small differences in wood decomposition. In addition, we aim to measure decomposition by wood-decaying fungi (ergosterol and enzyme contents) and to apply next-generation DNA sequencing of fungal communities on saproxylic beetles and inside experimental logs to record precisely the colonization of wood by fungi. By linking decomposition and wood-decaying fungi to characteristics of single beetle species and beetle communities, we will be able to disentangle direct and indirect effects of saproxylic beetles on wood decomposition. In specific, we want to test the following hypothesis: (H1) Functional diversity of saproxylic beetle communities is more important for the speed of wood decomposition than beetle species richness; (H2) The larger the beetle species and the deeper their galleries, the stronger their direct and indirect contribution to decomposition; (H3) The composition of fungal communities vectored by saproxylic beetles is beetle species specific, and (H4) wood decomposition by fungi starts from beetle galleries.