Removal of trees
Recalling the silvicultural instruction given to the participants, the presentation of results first displays the volume to be removed (target: 50 m3), the inclusion of high quality timber (target: 10%), and the number of trees removed (target: low removal rate). In addition, we analysed the habitat value of the removed trees. Whereas conservationist teams (virtually) removed a volume of 60 m3 on average, forester teams only selected a slightly higher average volume of 63 m3. The strongest difference between the two groups was however that the foresters removed less trees (13 on average) than conservationist (19 on average). Related to that, the trees removed by foresters had a significantly larger average diameter and volume (Table 1). Furthermore, trees selected by foresters for removal had an average economic value almost twice as high as the conservationist’s choices. More importantly, though, the mean proportion of high quality timber extracted by the foresters was 11.8% and was thus very close to the set 10%-target. The conservationists extracted only 5.8%. It thus appears that foresters complied better with the given economic targets by focussing on the removal of few high-value trees, while conservationists removed more and smaller trees with a lower economic value.
Regarding the habitat value of the removed trees, we did not find any significant difference regarding species composition, (chi-squared-test, df = 2, p = 0.60; Table 5 in Appendix), the total number of TreMs per tree or the removed trees’ mean habitat value (Table 1) between conservations and foresters. Overall, conservationists removed, on average, trees with a slightly lower habitat value than foresters, but as conservationists selected more trees to be harvested, the average aggregated habitat value removed per team was similar.
Designation of habitat trees
All but one of the habitat trees retained by the conservationists are oaks, whereas foresters included significantly more hornbeam in their habitat tree selection (chi-squared-test, df = 2, p < 0.001; Table 6n Appendix). On average, foresters selected habitat trees with a smaller DBH and volume (Table 2). This difference in DBH is due to foresters selecting hornbeam as habitat trees (smaller diameter trees, see Fig. 3) as well as oak of smaller dimension (t-test: p = 0.009; Table 3). Habitat values of the retained trees do, however, not yield clearly significant differences between foresters and conservationists (Table 2) and the same holds for the number of TreMs per habitat tree (Table 2).
Habitat trees of foresters have a lower economic value as compared to those of conservationists (Table 2). This because, one the one hand, conservationist retained oaks with almost double the economic value of the oaks that were selected by foresters (t-test: p = 0.020; Table 3). On the other hand, forester included hornbeams as habitat trees, which have a much lower economic value than retained oaks (t-test: p < 0.001; Table 3). Also the habitat values of the retained hornbeams (as compared to retained oaks) are significantly lower (t-test: p = 0.002; Table 3), as is the number of TreMs (t-test: p < 0.001; Table 3). In summary, conservationist and foresters selected habitat trees with similar habitat values, but habitat trees retained by foresters have a lower economic value partly due to foresters including more hornbeams, with low economic and habitat value.
The trade-off between economic and ecological goals
Regarding trees selected for removal, foresters obtained a significantly higher economic revenue per habitat point as compared to conservationists (t-test: p = 0.010). Regarding the habitat tree selection, the opportunity cost of a habitat point was significantly lower for foresters compared to conservationists (t-test: p = 0.030, Fig. 4), which means that on average conservationists retained a unit habitat value at a higher economic cost.
Figure 5 plots the aggregated habitat and economic values of the sets of habitat trees selected by the different teams (the information underlying this plot can be found in Table 4). It is important to note that only those teams’ results (i.e. positions in figure) with exactly 10 habitat trees retained (teams C1, C4, C5, F1, F2 and F3) can be fully compared to the circular line of diamond shaped points, which depicts an approximation of the boundary of the set of all possible combinations of 10 trees.Footnote 1 This approximation is sufficient to show that most of the teams have actually selected a combination of trees that is positioned towards the lower right area of the entire set of possible combinations. Solutions lying in that direction combine a low economic with a high ecological aggregated value, which means that they reconcile the trade-off between habitat retention and forgone timber value better. Apart from some exceptions (such as teams C2 and F6), the results achieved by foresters and conservationists are rather distinct. The foresters are located towards the lower-left part of the plot. That is, they have retained less habitat value at lower opportunity costs during the exercise. The conservationists are located more towards the upper-right angle of the plot. That is, they have retained a higher number of habitat points at the cost of waiving a larger amount of potential income.
Results from qualitative data analysis—tree selection strategies
The group discussions following the marteloscope exercises provide deeper insights into the reasons for the observed tree selection. In this section, we present these insights seeking to answer the questions which criteria and strategies the participants apply in tree selection, and how they deal with the trade-off between timber production and forest biodiversity. Insofar these aspects were also covered by the standardized questionnaire participants responded to (see method section), this information is added to the text.
Criteria applied in tree selection
While monetary criteria were important for all participants when selecting trees to be thinned, their relevance for habitat tree selection varied widely. The group of conservationists barely addressed the economic costs of retaining a habitat tree in the concluding group discussion. In contrast, opportunity cost was mentioned repeatedly as a relevant criterion for foresters’ habitat tree selection. They argued that structures apparently diminishing timber value were considered a key criterion for retaining a tree. This difference is reflected in a quote by a nature conservationist: “Usually I do not care much if a tree is [economically] valuable […] Yet, the comparison is interesting, in order to see in which conflict foresters are when trying to earn money with the forest”. Data from the questionnaires confirm these findings, showing that foresters put more emphasis on the economic than on the ecological dimension in dealing with the ecology-economy trade-off (Table 4) and that conservationists considered the economic values much less during habitat tree selection (Table 7 in appendix). In line with this, argumentation based on structural criteria (namely DBH and tree vitality) was used very differently by the participants. Foresters referred to large DBHs as a reason to harvest trees in order to avoid the risk of diminishing timber value through wood defects in the future, thus emphasising economic considerations. In contrast, conservationists argued that larger DBHs increase the probability of a tree to develop microhabitat structures—especially those that take long time to develop — thus arguing for the retention of these trees. Similarly, the criteria vitality and stability were used for contrasting argumentations. Foresters argued to harvest trees as long as they are healthy, which means that “trees are actually still young when we cut them”. Economically speaking, a tree’s vitality is important to guarantee both its timber quality and the stability of the stand. However, a tree’s vitality also increases the chances that TreMs will be provided for a very long time. Interestingly, both groups were referring to vitality also when justifying the retention of what they called “future habitat trees”.
As far as ecological criteria are concerned, conservationists stated that they focus primarily on ecologically valuable structures when selecting habitat trees. In contrast, several foresters emphasized that they did not search for individual structures, but rather focused on the whole stand, claiming to take a more holistic view. The descriptions of the foresters’ selection criteria remained rather vague, though: their choice of habitat trees seems to result from a mixture of experiential knowledge and related intuition not made explicit. Criteria related to everyday working experience such as traffic and working security are assumed to be part of the consideration process. This is also reflected in the survey data as this aspect was given much more weight by the foresters (Table 7).
As described in the previous section, conservationists almost exclusively selected oaks as habitat trees, while foresters often included hornbeams, whose average economic value was 13 times lower compared with oak. One forester argued, for example, that “hornbeams, which are only considered as a decorative accessory from a forestry point of view, […] can have an incredibly high ecological value here.” This perspective was not confirmed by the quantitative data at least as far as the current ecological value is concerned: on average the amount of TreMs, and habitat value of the selected hornbeams was three times lower compared to the oaks (see Sect. Designation of habitat trees and Table 3).
Summing up, foresters tend to evaluate TreMs somewhat differently than the conservationists. Although conservationists considered themselves somewhat more competent concerning TreMs and tend to inform themselves from more sources, the foresters still consider their own knowledge to be medium to high and they also indicated to consult at least two and even more sources on that topic (Table 4).
Strategies and approaches in tree selection
One obvious strategy applied was prioritization. In the group discussions, some participants reported to search first and foremost for specific criteria, such as economically valuable trees, wood defects or microhabitat structures, while subordinating other criteria. Interestingly, these participants still underlined the chances of integrating various goals within their approach, without perceiving this as contradictory to their rather clear prioritization.
Other participants (of both groups) placed a strong emphasis on balancing goals. Since economic and ecological objectives cannot be reconciled on one and the same tree, their strategy focused on achieving a balance at stand level. One conservationist described a continuous weighing process that often sounded like bartering: “Considering what other high-quality structures can be found in the stand, we can sacrifice this tree […] and have thereby already achieved a relatively high economic added value allowing us to retain more other trees in the stand, which are ecologically more valuable.”
Finally, the observed strategies differ according to their temporal perspective. Assuming that “each tree will develop habitat structures if you leave it long enough”, conservationists argued to retain trees whose ecological value is not high yet, but will be in the future when microhabitat structures develop. In contrast, the foresters argued to harvest trees before the appearance of those structures, which they perceive as wood defects. They themselves mentioned the argument to give emphasis to the development of future microhabitat structures on those trees whose economic value was already considered low now and in the future. Correspondingly, the foresters had indicated in the questionnaire that they put much more weight on the future economic value of a tree during habitat tree selection (Table 7). This became obvious, for instance, when participants discussed about a big, spirally grown oak. One of the foresters described his assessment of the tree as follows: “This [oak] has a real strong spiral grain. That means the [economic] value is very limited […] this strong, steep branch might break off sometime, thinking a bit into the future. This would be such a tree that may become a methuselah tree.”
Factors influencing strategies and approaches in tree selection
Both groups discussed the importance of being familiar with the stand characteristics as an important factor influencing tree retention and harvesting strategies. One forester emphasized that local experience is crucial to assess the interrelation between oak DBHs and the risk of decreasing vitality, which, from an economic perspective, is essential for deciding about the time of harvesting. One of the conservationists recognized that the DBH of the trees he had selected for harvesting were too small, which he attributed to his lack of knowledge about size class distribution within the stand.
Although both groups underlined the importance of visual aspects in tree selection, foresters were hardly equipped with binoculars, while conservationists made intensive use of them. When asked about this different approach, the foresters made clear that time restrictions in their real work life make it impossible to look at all structures in detail, so that some are inevitably overlooked. Yet, they assumed that their holistic view and experience will compensate for this.
In addition to these rather individual factors, the stand context in general has a strong influence on the tree selection strategy. Trade-offs especially occur if a stand’s TreMs are concentrated on only a few trees with high economic value. Obviously, in such a situation a decision can be facilitated by adapting the definition of a habitat tree in a way that allows designating trees with lower habitat or higher economic value. In that respect the following quote of a forester is telling: “You have no problem to find a sufficient number of habitat trees here in this stand. This is a luxury problem”.