Retaining Forest Structure (A2)

Retaining Forest Structure (A2)

A2) Retention of structural elements in selectively used forests

Jürgen Bauhus, Patrick Pyttel & Thomas Asbeck

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

Background and state of research

Deadwood and standing deadwood, dead (or dying) trees that have not fallen to the forest floor, provide unique ecological niches for biodiversity. These habitat trees are defined as providing microhabitats in the form of cavities, large dead branches, loose bark, water-filled tree holes (dendrotelmata), epiphytes, bracket fungi, cracks, sap runs, or trunk rot, among others (1).

In managed forests these microhabitat structures are typically rare because they have low economic value. Even in close-to-nature based management, these habitat trees are preferentially removed due to their defining structures and characteristics (2).  Microhabitat abundance and diversity increase with tree diameter and bark thickness, typically aligned with tree age(1).

Owing to the rarity of these habitat trees in managed forests, many of the species that rely on these niche microhabitats are also rare and threatened – or have become locally extinct. The same applies to dead wood and deadwood dependent species (3). In natural forests, the occurrence of habitat trees or dead wood is typically highly clumped and variable(4). This may have an important influence on the structure and viability of communities, and meta-communities, of species reliant on the deadwood microhabitats.

The relevance of this study, for practical forest management, is that the deliberate retention of structural elements may consider this spatial heterogeneity to maintain important within-stand habitat continuity and hotspots of forest biodiversity.  Promoting heterogenous distribution of habitat trees increases forest structure diversity, which within forests stands may lead to a higher probability of survival of dependent organisms.  At the same time, a clustered distribution may offer great ecological stability, e.g. through mutual protection, and reduce mortality of habitat trees, guaranteeing their function for a longer period of time(5).

Study questions and hypotheses

A2 aims to investigate the effects of different levels of structural retention, number of habitat trees and volume of dead wood per ha, its spatial pattern, e.g. evenly distributed or clumped, and the determinants of microhabitat structures in all 135 ConFoBi plots.

Our specific hypotheses are that:

  • Abundance of habitat trees and dead wood and related habitat features (e.g. hollows, dead branches, etc.) depends on management history and landscape context.
  • Spatial distribution of such structural elements is clumped.
  • Frequency and quality of microhabitat structures and thus habitat trees depends on tree attributes (age, dimension, species, etc.) and the neighbourhood situation (e.g. growing space, number of dead neighbours etc.).
  • Individual selection of habitat trees and their spatial retention pattern will influence the provision of microhabitat structures for selected species.
  • Spatial distribution of retained habitat trees affects their longevity.

 

Approaches and methods

A2 conducts a spatially explicit terrestrial inventory of structural diversity comprising elements in one semi-natural forest stand of each of the 135 ConFoBi landscapes. This full stand inventory specifically includes microhabitat features that cannot be readily identified by remote sensing (see above). Recent management intensity is quantified using the recently developed Index of Forest Management Intensity (6). Coordinates of all habitat trees and the spatial patterning of these habitat features are documented, tree and neighbourhood attributes of habitat trees is quantified and analysed, and the data will be provided as explanatory variables for the B projects.

Statistical models to predict the occurrence of microhabitat structures are developed on the basis of landscape, stand, neighbourhood and tree attributes. Using information of species occurrences, abundances, and activities provided by B projects in relation to dead wood and habitat trees and their microhabitat features, we simulate the effects of different retention strategies (number and type of retained trees, amount of dead wood, their spatial arrangement) on habitat provision for different harvesting scenarios. This information can then be used for future population viability assessments.

Outputs

A2 jointly samples habitat tree data with B1, and provide these data to all B and C projects. A2 further provides index data on forest management intensity to the ConFoBi data pool. A1 uses the data from A2 data to validate LiDAR-based habitat maps. A2 uses data from A1 to compare the different approaches for the assessment of habitat trees and microhabitat features. A2 uses data from the B projects to examine the effects of different retention strategies on habitat provision.

Further reading

  • Bütler, R., Lachat, T. et al. 2013. Habitat trees: key elements for forest biodiversity. In: Kraus D., Krumm F. (eds) Integrative approaches as an opportunity for the conservation of forest biodiversity. European Forest Institute, pp.84-91.
  • Gossner, M.M., et al. 2013. Current near-to-nature forest management effects on functional trait composition of saproxylic beetles in beech forests. Conservation Biology 27, 605-614.
  • Lachat, T. et al.  2013. Deadwood: quantitative and qualitative requirements for the conservation ofsaproxylic biodiversity. In: Kraus D., Krumm F. (eds) Integrative approaches as an opportunity for the conservation of forest biodiversity. European Forest Institute, pp.92-102.
  • Gustafsson, L. et al. 2012. Retention Forestry to Maintain Multifunctional Forests: a World Perspective. Bioscience 62:633-645.
  • Aubry, K. B., Halpern, C. B., & Peterson, C. E. 2009. Variable-retention harvests in the Pacific Northwest: A review of short-term findings from the DEMO study. Forest Ecology and Management, 258(4), 398-408.
  • Kahl, T. and Bauhus, J. 2014. An index of forest management intensity based on assessment of harvested tree volume, tree species composition and dead wood origin. Nature Conservation 7, 15-27.