Leaf area index

Estimates of the leaf area index of the tree canopy, LAI, from all methods used (Chap. 4.4) are summarised in Table 5.1.3.2, as are the interannual averages, the averages across different methods and the interannual variation.

Averaged over the observed growing seasons and the three different methods used, the LAI was 6.7 for the stand Steinkreuz, 6.5 for Großebene, 7.2 for Farrenleite and 6.9 for the pure beech plot within the mixed stand Steinkreuz (Tab. 5.1.3.2). Aver-aging over litter collection and PCA-based estimates only, the LAI for Steinkreuz was 6.8, 6.5 for Großebene, and 6.8 for Farrenleite (Tab. 5.1.3.2); no litter traps had been placed in the pure beech plot of Steinkreuz. Allometric estimates of LAI were 7 % and 13 % smaller than the average of indirect and semi-direct estimates in Steinkreuz and Großebene, respectively (in the year of leaf harvest), 8 % larger for Steinkreuz-pure beech plot and almost 20 % larger in Farrenleite (Tab. 5.1.3.2). At the latter site, however, the harvest had been carried out in the year before the optical measure-ments and litter collection started. In the two mixed stands the proportion of beech on stand-LAI varied ca. 15–20 % between the direct and semi-direct methods (Tab.

5.1.3.2). Maximum seasonal values of the PCA-based LAI were linearly correlated with litter-based values of LAI (R² = 0.74, p = 0.013, n = 7; not shown).

The LAI was reduced at Großebene in 1999 compared to 1998 by 19 % as found by both litter traps and PCA (Tab. 5.1.3.2). At Steinkreuz, LAI was reduced by 9 % based on PCA-estimates but hardly changed based on results from litter collection. In the year 2000, LAI recovered partly and increased by 10 % (litter traps) and 13 % (PCA) at Großebene and rose by 10 % (PCA) at Steinkreuz; litter trap-based esti-mates at Steinkreuz remained at the same value as the previous years (Tab. 5.1.3.2, Tab. 5.1.3.3, below). Interannual variability was found to be up to 21 % in annual leaf litter production at Steinkreuz from 1996 to 2001 (average biomass of leaf litter 3847.9 kg dry mass ha^-1 yr^-1, SD 274.5 kg ha^-1 yr^-1, range 3655.5–4314.8 kg ha^-1 yr^-1, P. Gerstberger, Dept. of Plant Ecology, Univ. of Bayreuth, pers. comm.).

At species level, the proportion of beech leaves in the litter collections showed a small increase in Steinkreuz (2 % per year, cf. Tab. 5.1.3.2). The LAI of beech increased by less than 4 % from 1998 to 1999 and less than 1 % from 1999 to 2000 as shown in Table 5.1.3.3. The contribution of oak leaves to stand level-LAI decreased by 10 % from 1998 to 1999 and another 12 % from 1999 to 2000 (Tab.

5.1.3.3), resulting in a total loss of the LAI in oak of 0.24 in two consecutive years.

These opposing trends in the two species balanced out at the stand level. At Großebene, the contribution of beech decreased by 5 % from 1998 to 1999 and did not change in 2000 compared to 1999 (Tab. 5.1.3.2). The absolute LAI of beech decreased by 27 % from 1998 to 1999 and increased by 11 % in the following year.

Oak-LAI decreased ca. 9 % from 1998 to 1999 and returned to almost the value of 1998 in the year 2000 (Tab. 5.1.3.3). PCA-based estimates of LAI for the pure beech plot within Steinkreuz decreased only marginally from 1998 to 1999 and increased thereafter in 2000 by less than 4 % (Tab. 5.1.3.2).

Table 5.1.3.2: Leaf area index (LAI) of the canopy of the stands investigated and interannual variation (change relative to previous year); average values from litter traps, optical measurements (PCA, LAI-2000 Plant Canopy Analyser;

corrected for stem light interception and foliage element non-randomness ac-cording to Kucharik et al. 1998, 1999; maximum seasonal values) and from species and site-specific allometric relationships between stem diameter and leaf area (Fleck 2002, Fleck et al. 2004; Chap. 4.4.1). In brackets, percentage contribution of beech to stand LAI where applicable. Means of all the years of investigation and across methods are also given (in brackets 1 standard devia-tion, SD). No litter traps were set up in the pure beech plot of Steinkreuz (“n.d.”).

Method PCA litter

(% beech)

allometric (% beech)

LAI change LAI change LAI

Steinkreuz

1998 6.92 6.86 (83) 6.4 (66)

1999 6.29 -9.1 % 6.95 (85) +1.3 %

2000 6.92 +10 % 6.85 (87) -1.4 %

mean (SD) 6.71 (0.30) 6.89 (0.04)

mean (SD) 6.80 (0.13)

total mean (SD) 6.67 (0.20)

Großebene

1998 7.04 7.42 (54) 6.3 (39)

1999 5.68 -19 % 6.04 (49) -19 %

2000 6.40 +13 % 6.61 (49) +10 %

mean (SD) 6.37 (0.55) 6.69 (0.57)

mean (SD) 6.53 (0.23)

total mean (SD) 6.45 (0.17)

Farrenleite

1998 6.70 6.85 8.1^a

mean (SD) 6.78 (0.11)

total mean (SD) 7.22 (0.63)

Steinkreuz-pure beech plot

1998 6.61 n.d. 7.17

1999 6.54 -1 % n.d.

2000 6.77 +3.5 % n.d.

mean (SD) 6.64 (0.1)

total mean (SD) 6.91 (0.27)

a Harvest carried out in 1997.

Table 5.1.3.3: LAI of the individual species and its change from one year to the next (in brackets) as estimated from litter collection in 10 traps per stand for the two mixed beech-oak sites in the Steigerwald.

LAI Steinkreuz Großebene

F. sylvatica Q. petraea F. sylvatica Q. petraea

1998 5.71 1.15 4.00 3.41

1999 5.92

(+3.7 %)

1.03 (-10 %)

2.94 (-27 %)

3.10 (-9 %)

2000 5.95

(+0.5 %)

0.90 (-12 %)

3.25 (+11 %)

3.37 (+9 %)

At Steinkreuz, the spatial coefficient of variation (CV) for the litter-based estimates of LAI across the traps was 14 % for beech, 28 % for oak and 8 % for both combined in 1998. No CV could be calculated for 1999 and 2000 since litter from all 10 traps in Steinkreuz had been pooled during these years. At Großebene, the spatial CV in litter-derived LAI among the 10 individual litter traps was 14 % for beech, 16 % for oak and 8 % for both combined in 1998, 13 %, 30 % and 15 % in 1999, respectively, and 13 %, 22 % and 12 % in 2000, respectively. At the pure beech stand Farrenleite, CV amounted to 4 % (1998), yet only three traps were set up there. The larger spatial variability in the litter-LAI of oak in both mixed stands compared to beech probably originated from the stand composition, which could be described as a matrix of beech with single to small groups of oak trees mixed in (cf. Tab. 3.3.2a). Yet the effects of differences in the leaf size of beech and oak could also have contributed to the larger CV for oak litter. A methodological limitation in this respect is the use of smaller litter traps with a larger edge effect at Großebene, which may obscure a comparison of the two sites.

PCA-based estimates exhibited values of CV of ca. 10 % in Steinkreuz, 12 % in Großebene, and 3 % in Farrenleite. A direct comparison with coefficients of variation from litter-based LAI-estimates was not appropriate because of the different number of sampling points. But the ranking of coefficients of variation between the stands was similar for both methods: Farrenleite as a monospecific stand showed the lowest CV and Großebene had a slightly higher CV than Steinkreuz.

Figure 5.1.3.1 shows the seasonal change of LAI, as a percentage of maximum sea-sonal LAI observed with the respective method, during the years 1998–2000 for the three sites. Sampling with the PCA was not as frequent as might have been desir-able, but it was not always possible to meet obligatory preconditions (see Chap.

4.4.3). From August 1999 supplemental information on canopy development was de-rived from continuous below-canopy measurement of global radiation Rg at Stein-kreuz (Fig. 5.1.3.1; see Chap. 4.5), which agreed well with the pattern derived from PCA and litter traps. LAI (PCA) peaked around mid July to mid August in the three consecutive years (Fig. 5.1.3.1). Shedding of leaves always started earlier in beech than oak and first began at Farrenleite in the Fichtelgebirge (for the latter stand data was available only for 1998). Leafing out followed the same pattern, beginning with beech in the Steigerwald, followed by oak, finally at Farrenleite. Beech and oak from the Steigerwald stands did not differ significantly between the two sites regarding

these phenological stages. Litter fall ended somewhat earlier in 1998 and 2000 than in 1999, when considerable litter was found in traps until mid December, whereas not later than the beginning of December during the other years (Fig. 5.1.3.1).

Figure 5.1.3.1: Seasonal change of leaf area index (LAI) of the tree canopy during the years 1998–2000 in the three stands studied as estimated from leaf litter col-lected in traps (“Fagus”, “Quercus”, “total”), and optical measurements (“PCA”, LAI-2000 Plant Canopy Analyser, Li-Cor Inc., Lincoln, Nebraska, USA) and normalised to maximum seasonal LAI observed with the respective method. ST, GR, and GR sig-nify the stands Steinkreuz, Großebene and Farrenleite, respectively. “Rg ST” is the inverted transmission of global radiation Rg as estimated from the comparison of below-canopy measurements inside the stand Steinkreuz with above-canopy meas-urements (in the nearby forest opening). Extreme values of inverted Rg during peri-ods of 5 days are shown (the maxima during the vegetation period, minima during winter); measurements started 27.07.1999 and ended 30.10.2000. The beginning of leaf unfolding of Fagus sylvatica is indicated (“leafing”, filled and shaded triangles), as well as the approximate beginning of leaf colour change at Steinkreuz/Großebene (dark arrows) and at Farrenleite (grey arrows). For Farrenleite, litter and PCA-data are available only for the year 1998. In autumn and winter 2000, litter from Groß-ebene was collected only once after all leaves had fallen (thus not indicated in graph).