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Damage Dynamics and Structure of the Pedunculate Oak Tree Crowns in Stands in Microelevations and Microdepressions – Results from the Monitoring on Permanent Experimental Plots

Tomislav Dubravac orcid.org/0000-0003-3133-8322 ; Hrvatski šumarski institut Jastrebarsko
Stjepan Dekanić ; Hrvatski šumarski institut Jastrebarsko
Valentin Roth ; Hrvatski šumarski institut Jastrebarsko

Sažetak

The crown damage of a tree has a very prominent place among the broad spectrum of factors defining the pedunculate oak (Quercus roburL.) decline and dieback, largely due to the fact that it represents the first visible symptom of the decline in tree‘s vitality. Besides, crown damage is a key factor for the selection of trees with high risk of dying during the sanitary and salvage cuttings, in order to salvage high value timber that would be otherwise lost or severely degraded when the tree dies. Aim of this contribution is to investigate dynamics of crown damage of pedunculate oak trees, as well as the effect of the crown damage on tree’s basal area increment during longer time period in two most important types of pedunculate oak forests in Croatia. Research was done on permanent experimental plots situated in the stands of pedunculate oak and common hornbeam (II-G-10) and stands of pedunculate oak and Genista elata (II-G-20). In total, 38 permanent experimental plots were used, out of which 13 were re-measured with intervals of 9 to 17 years (Table 1). In first measurement, all trees on plots were permanently tagged, and map of tree locations and crown projections was made. Trees were measured for diameter at breast height (DBH), total tree height and height to crown base. Crown length was calculated as the difference between the total tree height and height to crown base, while the crown ratio was calculated as the ratio of crown length and total tree height. From the map of horizontal crown projections the average crown width was calculated as a mean of the maximal and minimal crown diameters. Assessment of the crown damage was done according to the methodology of the ICP Forests Programme, whereby the trees were tallied into following five crown damage (CD) classes: 0 (< 10 % CD), 1 (11-25 % CD), 2 (26-60 % CD), 3 (61-99 % CD) and 4 (100 % CD – dead tree). For some analyses trees were further grouped into two classes: healthy trees with crown damage of 25 % and less, and severely damaged trees with crown damage over 25 %. Results from this research are, however, not comparable to the damage assessment reports of the ICP Forests because of the different sampling strategies. On 13 plots the second measurement was made in which DBH of all trees still present on the plot was recorded, and the crown damage was assessed according to the same methodology used in the previous measurement, and furthermore, by the same observer. Average basal area increment per year (cm 2 year-1) per tree was obtained by dividing tree’s total basal area increment by the interval (number of vegetation periods) between the two measurements. Total number of oak trees analyzed was 749 and 540 in first and second measurement, respectively.
Prior to the crown damage analyses, the basic structural features of the stands on re-measured plots, as well as morphological features of pedunculate oak trees in two forest types were analyzed (Table 2 and Figure 1). Stands of the forest type II-G-20 have much larger shares of oak trees in the total number of trees, and total stand basal area compared to the type II-G-10. oak trees in both types exhibit the same relation of tree height (Figure 1A) and crown width (Figure 1D) to DBH, but the crown length (Figure 1B) and crown ratio (Figure 1C) are higher in the forest type II-G-20. Reason for this lies probably in the effect that common hornbeam trees in the lower canopy layers have on the morphological development of oak trees in the stands of II-G-10 type, while the stands in the II-G-20 have almost no sub-canopy layers.
Relationship between the share of oak trees with severe crown damage (> 25 %) and stand age was examined for the 33 plots of forest type II-G-10 spanning almost entire rotation length of pedunculate oak forests in Croatia (II. to VII. age class). Linear regression with logarithmic transformation of independent variable (stand age) was used for this analysis. According to the model, significant increase of severely damaged trees occurs after the stand age of 70 years (Figure 2A). The stands in the type II-G-20 departed significantly from the model in the first measurement, with only 26 % of severely damaged trees compared to 76 % in stands of II-G-10. By the second measurement, however, these plots also conformed to the behavior predicted by the model (Figure 2B), due to the deterioration of the overall crown condition.
Distributions of oak trees by the CD classes in two measurements (Figure 3) reveal the direction of the crown damage dynamics, which is in more detail presented in the Figure 4. In both forest types, the change in crown class of the oak trees was predominantly in the direction of the worsening crown health status. Negative trend is more pronounced in the II-G-20 forest type. According to the CD classes recorded in the first measurement (Figure 5), decline intensity was most pronounced in the CD class 3, from which 70 % and 50 % of trees in forest types II-G-10 and II-G-20, respectively, died off by the second measurement. This result provides further foundation for the use of the CD assessments in the day-to-day forestry as a reliable indicator of the tree’s imminent death.
Effect of crown damage on the basal area increment of oak trees was examined through: (i) the regression analyses of basal area increment per year per tree over DBH (Figure 6) and crown width (Figure 7), comparison of distributions of healthy and severely damaged trees over basal area increment classes (Figure 8), and by comparison of median values of basal area increment of healthy and severely damaged trees by forest types and stand age classes (Figure 9). Crown damage was found to have a significant impact on the basal area increment of pedunculate oak trees, with severely damaged trees having significantly lower basal area increment compared to healthy trees of the similar dimensions. These differences were very similar for both forest types.

Ključne riječi

pedunculate oak; crown damage; stand structure; crown structure; significant crown damage; basal area increment; II-G-10; II-G-20

Hrčak ID:

72316

URI

https://hrcak.srce.hr/72316

Datum izdavanja:

14.10.2011.

Podaci na drugim jezicima: hrvatski

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