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A2) Retaining Forest Structure

Retention of structural elements in selectively used forests

Jürgen Bauhus
Doctoral researcher: Andreea Petronela Spinu (since 2019)

University of Freiburg, Faculty of Environment & Natural Resources, Institute of Forest Sciences,
Chair of Silviculture


Tree-related microhabitats (TreMs) are specific structures that occur on standing trees and are an essential habitat component for many forest-dwelling species. Some examples of TreMs are cavities, large dead branches, loose bark, epiphytes, bracket fungi, cracks, or trunk rot. Live or dead trees that support or have the potential to develop TreMs are called habitat trees. The development of both the habitat trees and their TreMs influence the habitat provision for many forest-dwelling species.


Research Question

In this project we focus on:

  • the determinants of the occurrence of microhabitats on trees; for example tree species and dimension, environmental variables, and spatial distribution of habitat trees;
  • the influence of forest or tree management intensity and time since cessation of management in reserves on TreM development;
  • the temporal dynamics including longevity of habitat trees and TreMs;
  • the indicator value of TreMs for the richness and diversity of different forest-dwelling taxa.
  • the influence of habitat trees on their surroundings, in particular on tree regeneration

 The subproject comprises at this stage three different PhD projects:

  1. "Tree-related microhabitats as selection criteria for habitat trees in close-to-nature forest management" conducted by Thomas Asbeck (completed in 2019);
  2. "Quality and longevity of habitat trees and their tree-related microhabitats" conducted by Andreea Spinu (commenced in 2019);
  3. "Formation and development duration of microhabitats at single tree and forest stand level" conducted by Josef Grossmann (completed in 2021).

In the first phase, we have focused on the determinants of microhabitat abundance and diversity on potential habitat trees in 135 one-hectare plots of the ConFoBi design. For that purpose, a full inventory of tree-related microhabitats (TreMs) was carried out following a detailed catalogue on the largest 15 trees per plot. Based on these data, the influence of tree, stand and landscape attributes on the abundance and richness of TreMs was analyzed.
In the second phase, the focus is on the mortality of habitat trees and the dynamics of TreMs. Information on habitat tree mortality was so far available only for regions such as North-America, Scandinavia, or Australia, where trees have been retained in clearcuts, but not for continuous cover forests in Europe. This information is of critical importance because habitat trees are meant to provide habitat structures for a very long period, ideally until a new generation of mature and over-mature trees have developed. However, if habitat trees already die after a short period of time or if TreMs disappear, habitats for species will be lost or impaired. Yet, when selecting habitat trees for retention, there can be trade-offs between the provision of certain highly-valuable TreMs such as bracket fungi, which can host hundreds of other species, and the longevity of habitat trees that are attacked by these fungi.

Specific hypotheses investigated in the second phase comprise:

  • The overall abundance and diversity of TreMs is significantly higher on dead trees.
  • Longevity of TreMs that are related to wood properties (e.g. crown dead wood, hollows, etc.) is higher in conifers (fir and spruce) than in broadleaves (beech and maple).
  • The mortality of habitat trees is higher than that of average (and smaller) trees, since trees with larger DBH are more likely to be affected by environmental stressors and their vitality is decreased by occurrence of some TreMs (sap runs, occurrence of bracket fungi, mistletoe presence).
  • Habitat tree mortality differs among tree species and is influenced by the landscape context (eg., altitude, local climate).

The associated project conducted by Joseph Großmann focused on drivers for the frequency, distribution and temporal development of tree microhabitats to assess the relative ecological value of tree microhabitats. One focus of his work were habitat tree groups and small forest reserves from the Veteran-Tree and Deadwood-Concept (AuT-Konzept) that is being applied in the state forest of Baden-Württemberg. Another part of his project dealt with structural diversity of urban trees. For this purpose, the influence of different management approaches on TreMs on urban trees was studied in the city of Montréal. This study has been carried out in collaboration with Christian Messier, who was supported by an Alexander von Humboldt prize.


Approaches, methods, and linkages

The occurrence of TreMs on habitat trees is typically carried out through terrestrial surveys, partially in combination with remote sensing approaches (see A1). The assessment of habitat tree mortality is based on a combination of repeated ground-based inventories of all ConFobi plots and remote sensing approaches in the wider landscape to quantify the attrition of trees that have been retained, either individually and scattered or in clumps. Based on this, we will develop models that include habitat tree attributes (e.g. species; dimensions; predisposing factors, such as the presence of bracket fungi, etc.), site attributes (e.g. soil type, exposition), and spatial arrangement of habitat trees to predict their mortality.
The assessment of TreM longevity requires the development of a range of different approaches for the different types of TreMs, which are currently developed. Additionally, new techniques for a more objective TreM assessment will be developed, including detailed measurements of the sizes of TreMs (i.e. terrestrial laser scanning, jointly with A1).



The occurrences of eleven different TreMs were related to forest management, forest type, altitude, and mean DBH. Hence, the average abundance and diversity of TreMs may be predicted with readily available forest attributes (Asbeck et al. 2018). We further found that retaining clumped live habitat trees does not provide a greater abundance and richness of most TreMs than if live habitat are being retained in a dispersed pattern. These results show that it is more important to select the right retention trees in appropriate quantities rather than focusing on grouping them, which may be done for other reasons. For the Black Forest region, future forest management intensity will have a more pronounced influence on microhabitat provision than climate change, as was found in an interdisciplinary, collaborative analysis with the projects C1 and B6 (Augustynczik et al. 2018). In a collaborative project with A1, it was found that the abundance and diversity of TreMs cannot be sufficiently well predicted with remotely sensed variables including forest structure (e.g. tree height, stand density) and terrain variables (Frey et al., 2020). There was only a weak significant relationship between several predictors and the TreM abundance and diversity. This shows that we still need to identify and quantify TreMs based on terrestrial inventories. However, laser scanning may also be used in the future for a range of TreMs.

The European TreM typology and catalogue were tested and proved to be suitable also for the forests of North America. A comparitive study showed that drivers of the abundance and richness of TreMs are similar in mountain forests of Europe and North America and their occurrence may be explained by tree functional groups (Asbeck et al., 2020). Another collaboration between A2 and researchers from the CZU University in Prague showed that trees in primary forests of the Carpathians hosted similar but greater richness of all and specific types of TreMs than individuals in managed forests of the Black Forest. Furthermore, a review was published highlighting that TreMs can provide an important tool for forest managers to guide the selection of habitat trees for the conservation of the associated biodiversity (Asbeck et al., 2021). Numerous other collaborative studies were conducted covering the relationship between TreMs and taxa (Basile et al. 2020, Kaufmann et al. 2021), between forest management intensity and remotely sensed data (Asbeck & Frey, 2021); or the biodiversity response to various forest structure variables (Asbeck et al. 2021).

Based on relationships between TreM occurence and tree diameter and species, diameter thresholds for 18 European tree species (13 broad-leaved, 5 coniferous) were derived. Those thresholds refer to statistically disproportionate high levels of TreM richness or abundance. Selecting habitat trees in managed forest stands, broadleaves with a dbh greater than 70 cm and conifers with more than 86 cm dbh will yield trees with a very high probability to carry (several) TreMs.

Intensive tree maintenance in urban trees led to high levels of certain microhabitats such as cavities and injuries that were comparable to natural, unmanaged forests (Großmann et al. 2020). Light maintenance of urban trees encouraged more crown deadwood than typical and intensive maintenance levels. These 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.


Future projects

Next PhD project (starting 1 July 2022).

Initially (years 1-3). The Subproject A2 has previously focussed on a) the relationships between tree and forest stand attributes and the occurrence of microhabitats and the influence of forest management on the abundance and diversity of tree microhabitats, and b) the longevity and mortality of habitat trees and the temporal dynamics of microhabitats on these trees. In addition, in collaboration with other subprojects, A2 has analysed the relationships tree-related microhabitats and occurrence of forest-dwelling species. In the next PhD project, A2 will:

  • continue the monitoring of habitat trees and microhabitats on all ConFoBi plots and participate in synthesis projects attempting to analyse relationships between forest structural attributes and the occurrence, abundance and diversity of forest dwelling species determined in other subprojects.
  • analyse the influence of habitat trees of different species, dimension, and vitality on their surroundings, in particular on the tree regeneration and regrowth stand. This work, which will also be carried out at new sites outside the existing ConFoBi plots, will provide a basis for assessing and modelling of the economic consequences of habitat tree retention.
  • quantify the economic aspects of habitat tree retention on tree regeneration and wood production, and possibly other ecosystem services such as carbon sequestration. 

With this information, we aim to support the development of retention strategies that optimize conservation benefits while reducing negative economic impacts. Thus the results of this project are expected to be highly relevant for forest management praxis.

Skills required for PhD3 applicants:

The successful applicant should have competences and practical experience in forest mensuration and inventories, ideally also with assessment of tree microhabitats. She/he should have a very good background in forest ecology and be familiar with concepts of forest structure, diversity and function. A basic understanding of forest economic concepts is desirable. She/he should be able to communicate in English and German.


ConFoBi publications by A2

Asbeck, Thomas; Basile, Marco; Stitt, Jessica M.; Bauhus, Jürgen; Storch, Ilse & Vierling, Kerri T. (2020). Tree-related microhabitats are similar in mountain forests of Europe and North America and their occurrence may be explained by tree functional groups. Trees, 34, 1453–1466.

Asbeck, Thomas; Kozák, Daniel; Spînu, Andreea P.; Mikoláš, Martin; Zemlerová, Veronika & Svoboda, Miroslav (2021). Tree-Related Microhabitats Follow Similar Patterns but are More Diverse in Primary Compared to Managed Temperate Mountain Forests. Ecosystems.

Asbeck, Thomas; Messier, Christian & Bauhus, Jürgen (2020). Retention of tree-related microhabitats is more dependent on selection of habitat trees than their spatial distribution. Eur J Forest Res, 139, 1015–1028.

Asbeck, Thomas; Pyttel, Patrick; Frey, Julian & Bauhus, Jürgen (2019). Predicting abundance and diversity of tree-related microhabitats in Central European montane forests from common forest attributes. Forest Ecology and Management, 432, 400–408.

Asbeck, Thomas; Sabatini, Francesco; Augustynczik, Andrey L. D.; Basile, Marco; Helbach, Jan & Jonker, Marlotte et al. (2021). Biodiversity response to forest management intensity, carbon stocks and net primary production in temperate montane forests. Scientific reports, 11, 1625.

Augustynczik, Andrey L. D.; Asbeck, Thomas; Basile, Marco; Jonker, Marlotte; Knuff, Anna & Yousefpour, Rasoul et al. (2020). Reconciling forest profitability and biodiversity conservation under disturbance risk: the role of forest management and salvage logging. Environ. Res. Lett., 15, 0940a3.

Augustynczik, Andrey Lessa Derci; Asbeck, Thomas; Basile, Marco; Bauhus, Jürgen; Storch, Ilse & Mikusiński, Grzegorz et al. (2019). Diversification of forest management regimes secures tree microhabitats and bird abundance under climate change. The Science of the total environment, 650, 2717–2730.

Basile, Marco; Asbeck, Thomas; Cordeiro Pereira, João M.; Mikusiński, Grzegorz & Storch, Ilse (2021). Species co-occurrence and management intensity modulate habitat preferences of forest birds. BMC biology, 19, 210.

Basile, Marco; Asbeck, Thomas; Jonker Marlotte; Knuff, Anna K.; Bauhus, Jürgen & Braunisch, Veronika et al. (2020). What do tree-related microhabitats tell us about the abundance of forest-dwelling bats, birds, and insects? Journal of environmental management, 264, 110401.

Basile, Marco; Asbeck, Thomas; Pacioni, Cesare; Mikusiński, Grzegorz & Storch, Ilse (2020). Woodpecker cavity establishment in managed forests: relative rather than absolute tree size matters. Wildlife Biology, 2020.

Frey, Julian; Asbeck, Thomas & Bauhus, Jürgen (2020). Predicting Tree-Related Microhabitats by Multisensor Close-Range Remote Sensing Structural Parameters for the Selection of Retention Elements. Remote Sensing, 12, 867.

Großmann, Josef; Pyttel, Patrick; Bauhus, Jürgen; Lecigne, Bastien & Messier, Christian (2020). The benefits of tree wounds: Microhabitat development in urban trees as affected by intensive tree maintenance. Urban Forestry & Urban Greening, 55, 126817.

Gustafsson, Lena; Bauhus, Jürgen; Asbeck, Thomas; Augustynczik, Andrey Lessa Derci; Basile, Marco & Frey, Julian et al. (2020). Retention as an integrated biodiversity conservation approach for continuous-cover forestry in Europe. Ambio, 49, 85–97.

Kaufmann, Stefan; Funck, Sarah-Katharina; Paintner, Franziska; Asbeck, Thomas & Hauck, Markus (2021). The efficiency of retention measures in continuous-cover forestry for conserving epiphytic cryptogams: A case study on Abies alba. Forest Ecology and Management, 502, 119698.

Kirsch, Jennifer-Justine; Sermon, Jana; Jonker, Marlotte; Asbeck, Thomas; Gossner, Martin M. & Petermann, Jana S. et al. (2021). The use of water-filled tree holes by vertebrates in temperate forests. Wildlife Biology, 2021.

Knuff, Anna Katharina; Staab, Michael; Frey, Julian; Dormann, Carsten F.; Asbeck, Thomas & Klein, Alexandra-Maria (2020). Insect abundance in managed forests benefits from multi-layered vegetation. Basic and Applied Ecology, 48, 124–135.

Storch, Ilse; Penner, Johannes; Asbeck, Thomas; Basile, Marco; Bauhus, Jürgen & Braunisch, Veronika et al. (2020). Evaluating the effectiveness of retention forestry to enhance biodiversity in production forests of Central Europe using an interdisciplinary, multi-scale approach. Ecology and evolution, 10, 1489–1509.