Results

3.3.1 Leaf- and sapwood-specific hydraulic conductivity and tree height

We found a significant positive correlation of LSC as well as ks with increasing tree height across species (Fig. 3.1). Highest values of LSC and ks were measured in twigs of the tall-growing C. acuminatissima (Fig. 3.1). Lowest values of LSC and ks were not found in the

46

Chapter 3 Tree height and twig hydraulic properties

smallest measured tree; rather, low specific conductivities were found in twigs of various tree species ranging in height between 10 and 20 m. Hydraulic conductivities differed greatly between and within species. V. arborea showed consistently low values of LSC and ks across a comparatively narrow range in tree height (19.5 - 26.9 m). Rates of LSC and ks varied greatly among individuals of different height (6.5 - 29.4 m) in S. apiculata (Fig. 3.1). The steepest increase in LSC and ks with increasing tree heights (27.5 - 44.2 m) was found in C.

acuminatissima. Due to the absence of other tree species > 30 m in our sample, C.

acuminatissima had a large influence on the height-related increase in LSC and ks found in our study.

Fig. 3.1: Empirical leaf-specific (LSC) and sapwood-specific hydraulic conductivity (ks) of exposed upper-canopy twigs of 8 tree species as related to tree height. Regressions are based on each 2 to 11 tree individuals of the 8 species (see Tab. 3.1).

3.3.2 Vessel anatomy and tree height

Across species, we found a significant, positive correlation between tree height and the xylem vessel diameter of stems. Mean trunk vessel density was negatively related to tree height, though not significantly (Fig. 3.2). Plotting mean twig vessel diameter and mean twig vessel density against tree height was significant in both cases. The positive correlation between twig vessel diameter and height was less steep than for the trunk vessel sizes. Nevertheless, the negative relationship between twig vessel density and height was much more pronounced than found for the trunks (Fig. 3.2).

Within species, mean trunk vessel diameter showed an increasing and vessel density a declining trend with increasing tree height (Fig. 3.2). However, these correlations could not be verified statistically which is partly a consequence of low numbers of replicates per species (Tab. 3.2). Nevertheless, a general trend of increasing vessel diameter and decreasing vessel

47

Chapter 3 Tree height and twig hydraulic properties

Fig. 3.2: Mean vessel diameter and vessel density of trunks and twigs as related to tree height. Regressions are based on two to 11 tree individuals per species measured (see Tab. 3.1). For symbol explanation see Fig. 3.1.

density with tree height was observed across and within species and this trend could be found for the trunk as well as for the twig xylem tissues.

The correlation between vessel diameter and height was less rigid for upper canopy twigs than for the trunk xylem. Twigs from the upper crown varied less in diameter (values not shown) than the DBH across species (Tab. 3.1). The narrow range in twig diameter across trees most likely resulted in a more equalized xylem anatomy across species and hence showed a less pronounced increase in vessel size with tree height. However, species with a large (small) mean trunk vessel diameter also showed a large (small) mean twig vessel diameter (Tab. 3.2).

Maximum vessel diameters were observed in C. acuminatissima for twig- and trunk-xylem.

The largest conducting vessels of trunks and twigs in C. acuminatissima were accompanied by the lowest vessel densities. The smallest trunk and twig xylem vessel diameters accompanied by high vessel densities were found in P. excelsa (Tab. 3.2).

48

Chapter 3 Tree height and twig hydraulic properties Tab. 3.2: Mean (SD in parentheses) xylem vessel diameter (µm) and vessel density (n mm^-2) of the trunks (at breast height) and of the exposed, upper-crown twigs of 7 species used for the hydraulic measurements.

Trunk Trunk Twig Twig

Species n

vessel diameter vessel density vessel diameter vessel density S. apiculata 5 100.3 (32.9) 16.3 (3.2) 44.1 (10.1) 52.7 (19.9)

V. arborea -

C. acuminatissima 3 199.2 (91.9) 2.9 (0.7) 60.3 (21.6) 20.1 (5.6)

P. excelsa 2 69.4 (26.1) 15.9 (2.7) 39.8 (7.9) 50.4 (11.2)

C. laevigata 3 80.3 (30.9) 12.6 (0.8)

Myrtaceae sp. 8 2 103.8 (49.8) 7.4 (0.1) 43.5 (10.6) 40.0 (6.2) Myrtaceae sp. 10 1 136.1 (40.0) 9.2 (0.1) 47.1 (12.1) 48.1 (8.0)

P. firma 3 105.4 (57.1) 6.4 (1.6) 40.7 (8.6) 39.1 (10.2)

3.3.3 Huber value and tree height

The calculated Huber value (HV) varied greatly across and within species. We found no clear trend of HV with increasing tree height across or within species (Fig. 3.3). Independent of tree height, HV averaged between 1.57 and 2.80 across species (Tab. 3.1).

0 1 2 3 4 5 6

Hubervalue(10)-4

0 10 20 30 40 50

Tree height (m)

Fig. 3.3: Huber value (HV; i.e., ratio of the cross-sectional area of sapwood to the distal leaf area) as related to tree height. For symbol explanation see Fig. 3.1.

3.3.4 Vertical microclimate

Vertical VPD significantly increased from 5 to 30 m, when analyzed for the 12-days-period (Tab. 3.3) and when analyzed for two single days differing in cloud cover (Fig. 3.4).

However, differences in the diurnal course of VPD were not significant between 5 and 10 m and between 20 and 30 m at either of the analyzed time periods (Tab. 3.3, Fig. 3.4). Saturation deficits at 5 m reached daytime maxima of 0.4 and 0.9 kPa around noon at a cloudy and a

49

Chapter 3 Tree height and twig hydraulic properties

cloudless day, respectively. Maximum daytime values of VPD at 30 m were 0.9 around noon at a cloudy and 1.8 kPa in the early afternoon at a cloudless day.

Tab. 3.3: Vertical microclimate as measured between 5 and 30m height (h) along the stem and within the canopy of one single tree for 12 days in June 2009. Given are mean values of atmospheric vapor pressure deficit (VPDmean, kPa; SD in parentheses) and maximum values (VPDmax) for the 12-days-measurement campaign.

Analyses are restricted to daytime hours (6 am to 6 pm). Different letters indicate significant differences between height positions (p < 0.001, Tukey´s HSD).

h VPDmean VPDmax

5m 0.28 (0.24) a 0.88 10m 0.36 (0.30) a 1.12

20m 0.58 (0.46) b 1.9

30m 0.63 (0.50) b 1.81

Differences in the daytime course of VPD between height positions were less pronounced on cloudy than on cloudless days (Fig. 3.4). As a consequence, differences in VPD between 10 and 30 m were significant at cloudless, but not at cloudy days (Fig. 3.4). Highest daytime maxima were measured at 20m height position during both a cloudy (0.91 kPa) and a cloudless (1.9 kPa) day, probably because the lower canopy maintained a higher over-temperature during sun-light hours than the exposed upper canopy.

Fig. 3.4: Diurnal courses of atmospheric vapor pressure deficit (VPD) at four different height positions along a vertical gradient in one single tree as measured on a cloudy (left) and a cloudless (right) day. Different letters indicate significant differences in VPD among height positions within one day (p < 0.05).