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ŠUMARSKI LIST 13/2011 str. 91     <-- 91 -->        PDF

T. Dubravac, S. Dekanić, V. Roth: DINAMIKAOŠTEĆENOSTI I STRUKTURAKROŠANJA STABALA HRASTA... Šumarski list – Posebni broj (2011), 74-89


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.


Key words:pedunculate oak, crown damage, stand structure, crown structure, significant
crown damage, basal area increment, II-G-10, II-G-20