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Allgemeine Forst- und Jagdzeitung, 184. Jg. (5/6), 97-112 (2013)
Bark damages during harvesting operations: Analysis, modelling and evaluation on the basis of two large-scale studies
Abstract
During timber harvest trees of the residual stand may suffer damages through bark lesions. This may happen associated either with felling or transportation processes. Although qualitative information on the causal factors is available, quantitative interdependences between the major factors and the resulting degree of damage are largely unknown so far. To address this gap, two different quantitative models on the occurrence probability of bark damages were developed exploiting a substantial database of 183 harvest operations through GLMM approaches (generalized linear mixed effects models). The answering variables in the two models were either the occurrence of bark damages at the lower stem section (below 1.3 m height) or of bark damages along the whole stem. Among the multitude of potential causal predictors, the following major factors were found to impact on the occurrence of bark damages (sorted in descending order): mean skidding distance, distance of trees to skid trails, intensity of removals, tree species, tree resp. stand height, harvesting system (table 2, table 3). The models were subjected to a rigorous external validation based on the independent dataset that had been gathered previously by Meng (1978a). The validation clearly corroborated the models' capability to predict the occurrence probability of bark damages during harvesting operations (table 4, table 5 and table 6).
Key messageBeech is subjected to devaluating bark damage by logging operations. It is shown experimentally that damage in the upper part of the trunk entails much higher risk of wood discoloration and decay by fungi than damage in the lower part. Cellular defense mechanisms in xylem are elucidated.ContextDuring harvesting operations and more specifically processes of both felling and extraction, remaining trees suffer specific bark damages, leaving the exposed xylem vulnerable to subsequent discoloration and fungal decay. As a consequence, the future value of the produced timber is reduced.AimsThe objective of this study is to test for the consequences of artificial bark damage to the base vs. the upper parts of the trunk in terms of discoloration, fungal infection, wood decay, and wound closure.Methods
An artificial basal “extraction wound” was inflicted along with an elongated “felling wound” at 5-m stem height in 40 beech trees (Fagus sylvatica L.) during either June or October. The trees were felled 3 years later and the impact of the wounds was evaluated with microscopic imaging and microbiological methods.ResultsThe compartmentalization observed in the basal extraction wounds was highly efficient, and rate and intensity of discoloration and susceptibility to decay were significantly lower than in wounds inflicted to the upper segments of the trunk. The latter displayed deeply penetrating discoloration and decay.Conclusion
Bark damage in the higher portions of the stem inflicted by felling is followed by much higher risk of wood discoloration and decay than damage at the base of the stem (extraction damage). Harvesting management has to be optimized in order to avoid felling damage in remaining neighboring trees particularly.
Bark damages caused by logging activities occur at unacceptably high levels in forests of southwest Germany (28% of the trees). Furthermore, the level has increased in the last two decades. Spruce and beech as the major tree species are shown to display the highest level of bark damages (both 28%). Although foresters obviously tend to favour cutting of previously damaged trees in thinnings, this trend is more than counterbalanced by the amount of newly caused damages resulting in an overall increase of damage levels over time. Taking into account the position of the damages along the axis of the stems, the following development becomes apparent: damages occurring at ground-level (< 1 m) didn't increase much, as trees carrying such damages are favoured for cutting in thinnings. In contrast, the level of damages occurring at heights above 1 m doubled approximately, as trees displaying this damage characteristic were relatively seldom selected for cutting. In order to explain these differences with the second possible factor, the amount of newly damaged trees is negligible: in the observed two decades both ground-level and damages occurring at heights above 1 m increased in approximately the same way. Among the tree species investigated, spruce and beech are shown to be significantly more vulnerable to sustain harvesting induced bark damages than the other tree species. Possible operational consequences are discussed by the authors.
Inspection of Norway spruce and Silver fir on experimental plots in south-western Germany showed that Silver fir had suffered
significantly less bark injuries than Norway spruce. Data from both federal forest inventories (1987, 2002) showed a similar
species-specific vulnerability. Additional visual inspections of the basal cross-sections of trees removed from the experimental
plots showed rather high proportions of butt rot in uninjured Norway spruce (51%). The proportion further increased to 93%
in trees, which had sustained bark injuries. In contrast, decay symptoms were almost absent in uninjured Silver fir and less
enhanced in trees with bark injuries (27%). Management implications for risk rating of tree species, as well as the necessity
of implementing low-damage harvesting regimes, are discussed.
The damage caused to temperate forests by forestry operations is examined by a review of the scientific literature. A significant proportion of the remaining trees, especially in older stands, can be damaged during mechanized selective logging in forests, when operations are carried out in summer. Damage is most often caused during transport of timber. Most of the resulting wounds occur near the base of the tree and are up to 200 cm(2) in size. Damage to roots has negative effects on tree growth. Wounds on trees are attacked by fungi, causing stain and decay. In most tree species, the spread of wound decay is extensive and devalues several metres of the butt log. Wound closure is usually too slow to have any significant effect on the incidence of wound infections, but in several tree species it may restrict the spread of decay. The financial losses in wood value at the final harvest, due to previous logging damage, are reported to be significant. Strategies are discussed for controlling the damage and wound decay-in a forest, emphasizing silvicultural options for care of a stand during selective harvesting and wound treatment with appropriate dressings.