Habitat trees – one of the key elements of integrative retention approaches in European forests – are increasingly studied regarding their benefits to forest biodiversity. In this regard, treerelated microhabitats (TreMs) and deadwood serve as indicators of forest biodiversity.
Specific recommendations of amounts of deadwood to be retained in managed forest stands to support related taxa are available, but not for TreMs. Retention of habitat trees aims to increase availability of TreMs in managed forests. To be able to refine selection criteria for habitat trees, I focused in this thesis on factors affecting TreMs on living trees. Therefore, I calculated species specific diameter thresholds bases on TreM occurrences, I investigated the effect of tree maintenance on TreMs in urban areas and I evaluated the short-term value of the
retention of habitat tree groups.
Chapter 1. The aim of this chapter was to explain TreM occurrence from a qualitative perspective by considering their diversity. Tree diameter at breast height (dbh), tree species, and canopy class were useful predictors of TreM diversity. TreM diversity on broad-leaved trees was on average higher than in conifers of the same diameter. In contrast to TreM abundance their diversity saturated towards higher dbh levels. Those TreM saturation levels were used to derive diameter thresholds. Habitat trees support not only more, but also more diverse, microhabitats in comparison to crop trees.
Chapter 2. In this chapter I further developed the findings of chapter one and focused on the calculation of species specific diameter thresholds to precise recommendations for selecting habitat trees. Based on the relation between TreMs and tree diameter as well as TreMs and species I derived diameter thresholds for 18 European tree species (13 broad-leaved, 5 coniferous). Those thresholds refer to statistically disproportionate high levels of TreM richness or abundance. Complementing other aspects that need to be considered during habitat tree selection processes in managed forest stands, I recommend to select habitat trees with or close to a dbh of 70 cm for broadleaves and 86 cm for conifers. The differences of dbh thresholds between broadleaves and conifers as well as between species indicate species specific TreM dynamic.
Chapter 3. In the third chapter, I investigated the TreM availability on urban trees along a maintenance gradient in Montréal, Canada. Intensive tree maintenance in urban trees led to levels of certain microhabitats such as cavities and injuries that were comparable to natural, unmanaged forests. Light maintenance of urban trees encouraged more crown deadwood than typical and intensive maintenance levels. My results underline the importance of conserving and maintaining large living trees, especially in urban areas to provide tree microhabitats. These results also demonstrate the important role of intensive tree maintenance in stimulating tree microhabitat development in urban areas.
Chapter 4. Here, I addressed the effectiveness of habitat tree group (HTG) retention in forests of Baden-Württemberg, Germany, 10 years after the introduction of the approach. Large living trees (LLTs), standing deadwood and TreMs were significantly more abundant in HTGs than in reference plots. When retaining 5 % of a forest stand area as HTG, old-growth attributes increased significantly at the stand scale: amount of LLTs doubled and its volume almost tripled, and standing deadwood increased on average by 25 %. However, quantities of both attributes remain below recommended minimum thresholds. Retaining 5 % of stand area in HTG had a significantly positive effect on woodpecker cavities, rot holes and exposed heartwood, whereas 15 to 25 % area in HTGs would be required to increase stand level abundance of concavities, exposed sapwood or crown deadwood significantly. Retention of HTGs enriched managed, multiple-use forests with old growth structural attributes. Yet, the selection of HTGs could be made more efficient by focusing on forest patches with high tree volume or low tree density and by further considering snags, tree species mixture and LLTs as well as less vital trees.
Overall the findings of this thesis suggest, that common tree attributes (species, diameter, vitality, canopy class, life status) can be used to predict the occurrence of TreMs. Species specific diameter thresholds can help to identify trees with higher levels of TreM richness.
The specification of tree attributes in regard to TreMs allow to optimize habitat tree selection procedures. In addition or absence of trees rich in TreMs, tree maintenance could increase structural diversity. Intensively maintained trees providing comparable amounts of TreMs to trees from long-term unmanaged forests emphasize the relevance of artificially induced structures. To consider a holistic view on trees as biodiversity relevant feature throughout the landscape, I propose to expand further TreM research to urban and rural trees. Regardless how TreMs evolved and in which landscape they occur, the relation between TreMs and related taxa needs to be specified to strengthen the biodiversity indicating function of TreMs. Results from current TreM inventories, that followed standardized procedures, allow a approximation of TreM occurrence on a landscape level by linking species and diameter information to observed TreM abundances. To better understand development and persistence of TreMs repeated inventories were needed.
Finally, my results allow to refine selection criteria of habitat trees based on the presence of TreMs and the consideration of species specific diameter thresholds. Furthermore, in absence of TreMs or when minimum diameters were not reached, I propose the possibility of artificial TreM creation to be useful for structural enrichment.