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Fig. 1: Harvest-induced bark damage.

When trees are harvested, it is virtually impossible to avoid that at least a certain number of the remaining trees will suffer some harvest related damage. However, there is much evidence that in forest practice the amount of bark damage inflicted during a harvest operation often exceeds tolerable levels. For this reason, a series of research projects were carried out at the FVA in order better understand this situation. Several departments were involved in investigating factors governing the level of bark damage inflicted on remaining trees and the consequences resulting from these factors.

For many years, foresters have been well aware that harvest operations are necessarily associated with – unwarranted – bark damages to remaining trees. Likewise, they have known that such damages may result in serious economic loss triggered by wound rot and other pathogens. In fact, a comprehensive project had already been carried out at the FVA targeting the factors governing the level of harvest-related bark damages in the 1970s.

However, since then, technical, silvicultural, and organizational framework conditions have changed considerably. For this reason, a reinvestigation seemed timely. New research topics covered many aspects. They ranged from analyzing the frequency of bark damages and modeling the influence of factors related to harvest techniques to investigating the response of trees to bark wounds and resulting consequences for the quality (grade) of lumber affected ­ by wound-induced decay.

The diversity of targeted topics clearly suggested the involvement of several FVA departments according to their specific area of competence: forest utilisation, biometrics, forest health, and forest growth. Since a major concern was that research results be applied to improve practical forest management, the focus was on analyzing databases in accordance with actual practice in order to generate generalized recommendations. On no account did we intend to produce just another case study whose application was restricted to specific situations within a rather narrow frame of conditions.

Analysis of data from large-scale forest inventories or extensive sampling in a multitude of practical harvest operations ensured that the databases used in the analysis were in step with actual practice. In addition, we included expert knowledge from forest practitioners.

In order to provide the basis for broader generalization of results, the latest methods for statistical analysis and modelling were used, which were capable of matching adequately on one hand the requirements of extensive but heterogeneous databases and on the other hand the requirements of complex research questions.

An important objective of this research was investigating possible species-specific differences in the trees' response to bark wounds, fungus infection, and timber degradation. In addition to using "classic" mycological, microscopic, or annual ring analysis methods we made extensive use of the potential of computer tomography, a technology that recently became available at FVA.

Fig. 2: On the left: undamaged Norway spruce; on the right: Norway spruce with harvest-induced damage: upper boundary damage.

Fig 3: On the left: Norway spruce with harvest-induced damage: half distance upper boundary - breast height; on the right: breast height.

Fig. 4: European fir, traumatic resin channels.

Fig. 5: European beech disk sample: RS = skidding damage, FS = felling damage, BHD = disk at the breast height (undamaged).

We believe that we achieved noteworthy results. Our belief holds true with respect to scientific publications (currently seven papers already published in peer-reviewed journals and three submitted) as well as the relevance of the achieved results to practical management. The FVA research team succeeded in publishing eight different papers in a special issue of AFZ-Der Wald at the end of 2014, which emphasized the topic of harvest related bark damages. The journal AFZ-Der Wald is widely read among forest practitioners. Major results were:

  • Norway spruce and European beech are obviously a major problem because both species display rather excessive rates of harvest induced bark damages.
  • With regard to harvest technique, the most important factors governing the frequency of harvest-induced bark damages are (listed according to decreasing rank of importance): intensity of thinnings, distance to skid trails, harvest system (in combination with length of logs produced), skidding distance, tree species, and height of trees.

Furthermore, our analyses strongly indicate that in addition to directly measurable factors related to technical or natural aspects (tree-, or stand-specific) other "soft" factors appear to strongly impact the level of harvest related bark damage. The challenge was that such "soft" factors are very difficult to measure, if at all. However, the statistical methods that we used allowed for indirect quantification, e.g., through so-called "random effects" in mixed modelling approaches, and demonstrated that these factors were mostly related to the level of the forest enterprise. Thus, we concluded that they are predominantly organizational or staff related in nature.

Response patterns of trees to bark damages and timber degradation triggered mostly by fungus infection proved to differ notably between Norway spruce, European fir, and European beech:

  • In spruce, extraction wounds inflicted at the lower portions of the stem result in extensive disturbance zones with reduced water content in the affected sapwood portions. These disturbance zones are most probably the major reason for the species’ striking vulnerability to fungus infection and wood decay.
  • The compartmentalization of superficial extraction wounds is much more effective in fir compared to spruce. Wound related sapwood disturbance zones are of minor size, and the risk for timber degradation triggered by wound rot is rather low.
  • The same appears true for extraction wounds in beech. However, felling wounds inflicted along higher portions of the stem in this species result in considerable sapwood disturbances and are associated with considerable economic degradation of the logs.

Interestingly, a statewide poll clearly indicated that, as a rule, the expert knowledge of forest practitioners and research results achieved through quantified analyses appear to be rather congruent for a wide range of aspects concerning the factors actually governing the level of harvest related bark damage and timber degradation.

However, there are two conspicuous exceptions. On one hand, the majority of forest practitioners are not (sufficiently) aware of the fact that harvest related bark damages occur at a rather high level and continue to display an increasing trend. On the other hand, "soft" factors, not directly related to technical or natural aspects (e.g. commitment of the management with respect to compliance with quality standards) are only rated among minor factors. This is a pity, as the modeling results strongly suggest "soft" factors like skills, problem awareness, and motivation/commitment might offer a potential for improvement of the same magnitude as the optimization of technical aspects.