Stand structure

Of the total density of 350 trees ha^-1 in the Steinkreuz stand, beech held 76 % and was present in all DBH-classes from 7 to 70 cm (Fig. 5.1.2.1, upper panel). Its abun-dance peaked around DBH-class 20. Oak was present only in classes 25–50, reach-ing a maximum in class 35. The basal area Ab of beech was 19.9 m² ha^-1 or 69 % of total stand Ab (29.0 m² ha^-1, Tab. 3.3.2) with the highest values in the DBH-class 60 (Fig. 5.1.2.1, middle panel); for oak, Ab peaked in the class 45. Beech dominated the total sapwood area of the stand As of 18.6 m² ha^-1 (Tab. 3.3.2) by 89 % and was maximum in DBH-class 60 (Fig. 5.1.2.1, lower panel). Beech trees in classes 55 to 70 (14 % of the beech trees or 10 % of beech and oak combined) represented almost 50 % of the total stand’s As.

In the Großebene stand, which was only marginally younger but almost twice as dense, 67 % (424 trees ha^-1) of the trees were beech (Tab. 3.3.2), most of which (> 80 % of all beech trees) were concentrated in the DBH-classes 10 to 25 (Fig.

5.1.2.1). The DBH of beech ranged from 7 to 47 cm. Oak in contrast was absent in the lower DBH-classes, like in Steinkreuz, and the DBH varied between 26 and 46 cm, complementary to the distribution of the abundance of beech across DBH-classes. Oak dominated the Ab of the stand, with the DBH-classes 40 and 45 holding more than 40 %. The Ab of Großebene (37.9 m² ha^-1) was 1.3 times that of Stein-kreuz, but the As (17.2 m² ha^-1) was slightly lower than that of Steinkreuz, due to the large number of small beech trees and the low fraction of sapwood in the many large oak trees at Großebene. Thus beech contributed > 70 % to the As of this stand (Tab. 3.3.2, Fig 5.1.2.1).

The diameter of beech trees at the sloped stand Farrenleite in the Fichtelgebirge spanned from 14 to 47 cm at a stand density of almost 900 trees ha^-1 (Tab. 3.3.2),

the DBH-classes 25 and 30 being the largest (approx. 60 % of all trees, Fig. 5.1.2.1).

The DBH-class 35 contributed most (about 30 %) to Ab and As of the stand (in total 52.7 m² ha^-1 and 42.7 m² ha^-1, respectively), the former being 1.4 times, the latter 2.5 times higher than that of the stand Großebene (Tab. 3.3.2).

Figure 5.1.2.1: Distribution of stand structural characteristics across DBH-classes for the two mixed sites in the Steigerwald, Steinkreuz (left) and Großebene (centre), and for the pure beech stand Farrenleite in the Fichtelgebirge (right). Columns are filled for F. sylvatica and shaded for Q. petraea. DBH is the stem diameter 1.3 m above the ground. The upper panel shows the relative abundance of trees, refer-enced to the total number trees of each species per plot. The middle panel repre-sents the basal area (Ab), the lower panel the sapwood area (As, as calculated from regression equations given in Tab. 5.1.1.1); note the different scales of the y-axes for Farrenleite. DBH-classes are in steps of 5 cm, the numbers indicating the upper limit of a class, e.g. DBH-class “70 cm” includes all trees with a DBH of 65.1–70.0 cm.

The DBH-class “10 cm” spans from 7.0–10.0 cm only. For the DBH-class "35 cm" of Farrenleite, the basal area is 15.3 m² ha^-1 and the sapwood area 12.3 m² ha^-1. The data are from an inventory in spring 1998, except at Farrenleite, where the inventory was in spring 2000. The absolute stem densities etc. are given in Table 3.3.2.

Farrenleite Großebene

0 0.1 0.2 0.3 0.4

relative abundance F. sylvatica Q. petraea Steinkreuz

0 2 4 6 8 10

Ab [m2 ha-1 ]

0 5 10 15

Ab [m2 ha-1 ]

10 20 30 40 50 60 70

DBH-class [cm]

0 1 2 3 4

10 20 30 40 50 60 70

As [m2 ha-1 ]

0 4 8 12

10 20 30 40 50 60 70

As [m2 ha-1 ]

The Steinkreuz-pure beech plot within the mixed stand was less dense than both the mixed Steinkreuz stand and the Farrenleite beech stand (Tab. 3.3.2). The Ab and As

of the Steinkreuz-pure beech plot reached 73 % and 75 % of the values of Farren-leite, respectively. The largest and second largest tree of the Steinkreuz-pure beech plot contributed 40 % and 30 % to Ab and As, respectively (Tab. 3.3.2, Fig. 5.1.2.2).

Figure 5.1.2.2: Distribution of stand structural characteristics across DBH-classes for a small pure beech plot within the mixed Steinkreuz stand, Steigerwald, for the year 1998. The upper graph shows the relative and absolute abundance of beech trees, the lower graph the distribution of basal area Ab (shaded bars) and sapwood area As (filled bars). See also caption to Figure 5.1.2.1.

The height of the Steinkreuz stand (the average height of the 100 strongest trees per hectare, see Chap. 4.2) was 32 m (Tab. 3.3.2). The strongest beech trees at this site reached heights of almost 40 m whereas the highest oaks were about 5 m shorter (Tab. 3.3.2). Oaks in general tended to be less tall than beech trees with the same DBH (Fig. 5.1.2.3 top left), and the trend of tree height to increase with DBH was weaker (R² = 0.273, p < 0.001, cf. Tab. A11.3, Appendix) compared to beech. In the latter species, the height of the crown base increased with the DBH, but not as strongly as the total tree height, so that the crown length increased with the DBH as well. In oaks, the height of the crown base did not change with the DBH and the crown length was much shorter than that of beech trees. The ground-projected crown area Acp of oak was usually also smaller than that of beech with similar DBH (Fig.

5.1.2.3 bottom left) and the beech trees with the largest DBH had by far the largest Acp of up to 110 m².

The maximum height of the trees at Großebene was almost 10 m less than at Stein-kreuz, and both the oaks and beech were shorter than trees with the same DBH at Steinkreuz (Fig. 5.1.2.3 top middle, see also Fig. A11.3, Appendix). The oaks at

hardly changed with the DBH in oak, and in beech the increase of the height of the crown base was not as pronounced as at Steinkreuz. Oak crowns were again shorter than those of beech. Acp was, as at Steinkreuz, much smaller in oak than in beech with the same DBH (Fig. 5.1.2.3 bottom middle), and neither beech nor oak from Steinkreuz and Großebene showed significant differences in their respective rela-tionships of Acp to DBH (cf. caption of Fig. 5.1.2.3 and Fig. A11.3, Appendix).

Figure 5.1.2.3: Tree height and ground-projected crown area Acp of DBH-classes in the two mixed sites in the Steigerwald, Steinkreuz (left) and Großebene (centre), and in the pure beech stand Farrenleite in the Fichtelgebirge (right). Upper panel:

vertical bars represent the average crown length of all trees of a DBH-class, a bar’s lower end indicating the (average) point of insertion of the lowest living branch on the trunk, its upper end the average tree height. Grey bars are for F. sylvatica, dark bars for Q. petraea. Curves shown are original (hyperbolic) regression equations based on individual trees of each species, and 95 %-confidence intervals (dashed). Lower panel: average ground-projected crown area Acp of DBH-classes (open diamonds and filled squares) and regression lines and 95 %-confidence intervals from original data. Both beech and oak trees from Steinkreuz and Großebene were not signifi-cantly different in their relationships of Acp to DBH, so a combined regression is plottet here for each species. Equations and graphs with original data are shown in Table A11.2 and Figure A11.2 (Appendix).

In the densest stand, Farrenleite, the tree tops and crown lengths of beech were low-est while crown bases were highlow-est of all sites (Fig. 5.1.2.3 top right). The stand height was only 20 m, the maximum height 26 m (Tab. 3.3.2b). The tree height varied

Steinkreuz

0 10 20 30 40

height [m]

F. sylvatica Q. petraea

0 40 80 120

0 20 40 60

Acp [m²]

F. sylvatica Q. petraea

Großebene

0 20 40 60

DBH-class [cm]

Farrenleite

0 20 40 60

little with DBH, and the respective correlation coefficient was low (R² = 0.257, p <

0.001, cf. Tab. A11.3, Appendix). The Acp was also considerably smaller than for beech trees from the other sites (Fig. 5.2.3 bottom), its correlation with the DBH rather high (R² = 0.846, p < 0.0001, cf. Tab. A11.3, Appendix). The stand and maximum height of the Steinkreuz-pure beech plot was 34 m (Tab. 3.3.2) and the Acp

was almost three times that of trees with the same DBH at Farrenleite (cf. Fig. 5.1.2.3 bottom left and right).

Both the large and small trees of the same stand selected for sap flow measurements were of the same age, as revealed by ring count on increment cores.

The stands differed very little in ring counts: Farrenleite 108–119 rings, Steinkreuz and Großebene 111–133 rings, in approx. 2 m height of stem, respectively (cf. Tab.

3.3.2, Tab. 4.1).