Enhancing structural complexity: An experiment conducted in the Black Forest National Park, Germany

Abstract

We report on a structural complexity enhancement (SCE) experiment that was designed to test ecological restoration measures in the Black Forest National Park, Germany. The main goal was to understand as to whether the creation of standing and downed deadwood within previously managed, single‐layered Norway spruce ( Picea abies L.) forests accelerates the development of forest structure, richness, and diversity of a range of taxonomic groups. Here we introduce the experimental design and describe the development of stand structure including abundance and richness of tree‐related microhabitats (TreMs) within 5 years after initiation of the experiment in October 2016. To enhance structural complexity in treatment plots, 10 trees per plot were toppled using a skidder winch, and another 10 trees were ring barked at a height of around 60 cm above ground level with a chainsaw. To monitor stand structure, we collected data on common forest attributes such as diameter at breast height (DBH), tree height, and TreMs of all trees in the six experimental and six control plots measuring 0.25 ha in size before the treatments were carried out in 2016 and again in 2020/21. We analyzed the abundance and richness of TreMs using generalized linear mixed models with DBH and treatment vs. control as predictors. The SCE treatment resulted in a significant increase in deadwood volumes (4.2 vs. 439.5 m 3 ) as well as in TreM abundance and richness (increase of 0.74 TreMs per tree). This indicates that the SCE treatment was effective to increase biodiversity‐relevant structures such as deadwood and TreMs, in previously managed Norway spruce‐dominated stands. The ongoing monitoring of a range of taxonomic groups (birds, bats, small mammals, coleoptera, fungi, mosses, and vascular plants) in this experiment will demonstrate to what extent the enhancement in structural complexity will lead to an enrichment in species richness and diversity.

Stand age diversity and climate change affects forests’ resilience and stability, although unevenly

“…However, their interaction surprisingly cancels them out when considering the combination of these two factors. This first evidence proves that there is indeed a dampening effect of forest heterogeneity at the landscape level on the climate sensitivity of forests, which can be related to the fact that multi-aged forests benefit from an increasing structural complexity (Jandl et al, 2019;deWergifosse et al, 2022;Asbeck et al, 2023).…”

“…Indeed, the ANOVA analysis applied to models outputs shows that, independently on species, when considered individually, stand age and climate show equally significant effects in terms both productivity and carbon stocks losses, yet when considering the combination of these two factors, their interaction surprisingly cancels them out. This first evidence proves that there is indeed a dampening effect of forest heterogeneity at landscape level on climate sensitivity of forests which can be related to the fact that multi-aged forest benefit from an increasing structural complexity (Jandl et al 2019;deWergifosse et al 2022;Asbeck et al, 2023). Structurally, a forest with different age groups will have a combination of young, middle-aged, and mature trees, creating vertical stratification that enhances ecosystem complexity and stability.…”

“…The diverse behavior exhibited by forests of varying ages, in terms of productivity, stability, and resilience, underscores the potential of utilizing structural heterogeneity at the landscape level to counter climate change impacts while preserving resilience. Heterogeneous and un-even aged forests profit from increased structural complexity, as evidenced by Jandl et al (2019), de Wergifosse et al (2022), and Asbeck et al (2023). A forest of different age groups integrates young, middle-aged, and mature trees, leading to vertical stratification that augments ecosystem complexity and stability.…”