3. Eucalyptus Growth and Physiology in response to 18 months drought stress
3.1 Non-destructive morphological measurements: Height and Diameter
GC eucalypt clones were significantly taller than GUA and GUW after 18 months growth (mean
= 2.5 m; p < 0.0001; Fig. 3.2; 3.3 (a)). Figure 3.1 shows that the GC clones overtook the GU clones, in terms of height growth, after 70 days. The height of the GUA and GUW clones did not differ by more than 5% (mean = 2.07 m and 1.96 m respectively), but GC clones are up to 30%
taller than GU. After approximately 200 days growth, height of the GC clones in the control treatment surpassed the drought stress treatments (Fig. 3.2). The control treatments had the greatest height in all three clones, but the differences were not significant (Figure 3.3 (b); p = 0.127). Drought stress imposition did not significantly affect the height growth of GUA and GUW clones. Drought stress did reduce the growth in terms of height of GC, where the most severe drought stress (acute) showed the least growth in height (Fig. 3.1). Variability of height growth within treatments increased with time, as displayed in Figure 3.2. These results show that height growth was primarily determined by the clonal hybrid (GC > GU) rather than the imposition of chronic or acute drought stress.
For the first 190 days (winter and spring seasons; juvenile phase), growth rates were less than 0.002 m day-1. After 220 days growth, at the start of the summer season, growth rates of the Eucalyptus clones increased to between ± 0.007 – 0.010 m day-1 for all three clones. The final growth rate measurements (during summer at the early adult phase) showed an average of 0.003 m day-1 in all treatments. GC trees had a significantly higher mean growth rate per day (mm height growth) in comparison with GUA and GUW (Fig. 3.4 (a); p = 0.002). Mean growth rate (in height per day) of GUA and GUW were almost identical (GR = 3.20 and 3.14 mm day-1, respectively). The control treatment had a greater growth rate than the chronic and acute treatments, although this difference was not significant (< 10% difference between treatments;
Fig. 3.4 (b)).
Figure 3.2: Mean height of plants of three Eucalyptus clones grown in response to drought stress for 18 months (n=7 / 12; bars represent ± 1.0 standard deviation).
Figure 3.3: Mean height of plants of Eucalyptus (a) clones and (b) response to drought stress treatment after 520 days growth (n = 7; bars represent ± 1.0 standard deviation ; different letters denote significance difference).
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Figure 3.4: Mean growth rate (mm height day-1) of plants of Eucalyptus (a) clones and (b) response to drought stress treatment after 520 days growth (n = 7; bars represent ± 1.0 standard deviation; different letters denote significance difference).
GUW trees have a greater diameter than GUA and GC, although it was never more than 10%
greater (Fig. 3.5; 3.6 (a)). Figure 3.5 shows that there was no distinct difference among clones or in terms of diameter, in contrast to height (Figure 3.2). There was no significant difference in diameter between eucalypt clones after 520 days (p = 0.890). The diameter of Eucalyptus trees was significantly greater in the control treatment (mean = 40.5 mm; p < 0.0001; Fig. 3.6 (b)).
Diameter growth was shown to decrease with the imposition of drought stress, and Figure 3.6 (b) shows that the smallest tree diameter was measured in the acute drought stress treatment (mean = 33.2 mm). Variability of diameter growth increased with time, but variability within treatments was considerably less than that of height growth (Fig. 3.5). Diameter appeared to be influenced primarily by water stress (Control > Chronic > Acute). The acute water stress treatment (drought stress watering cycles of complete wilting and then recovery for a period of time) had a more negative impact on eucalypt diameter than the chronic water treatment (watered daily to 2% soil water content) by decreasing diameter by up to 8% in comparison with the control.
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Figure 3.5: Mean monthly diameter of plants of three Eucalyptus clones grown in response to drought stress for 18 months (n=7 / 12; bars represent ± 1.0 standard deviation).
Figure 3.6: Mean diameter of plants of Eucalyptus (a) clones and (b) response to drought stress treatment after 520 days growth (n = 7; bars represent ± 1.0 standard deviation; different letters denote significant difference).
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Growth rate, in terms of wood area (mm2 / day) was greatest in GUW Eucalyptus clones (Fig.
3.7 (a); p = 0.005). Figure 3.7 (b) shows that the control treatment had the highest growth rate, of wood area per day, and was significantly greater than the acute treatment (p = 0.001). When growth was expressed as a function of diameter*height, the GC clones grew significantly more than GUA and GUW (Fig. 3.8 (a); p = 0.002). This was prodominantly because GC was significantly taller than the other clones. As was evident in growth rate (height or area per day), growth as diameter*height was greatest in the control treatment (Fig. 3.8 (b); p = 0.003). The acute stress treatment had the lowest growth, thereby showing that that periodic drought stress cycles affected growth more negatively than a chronic (constant, low water availability) water treatment.
Figure 3.7: Mean growth rate (mm2 diameter day-1) of plants of Eucalyptus (a) clones and (b) response to drought stress treatment after 520 days growth (n = 7; bars represent ± 1.0standard deviation; different letters denote significant difference).
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Figure 3.8: Mean diameter*height of Eucalyptus (a) clones and (b) in response to drought stress treatment after 520 days growth (n = 7; bars represent standard deviation ± 1.0; different letters denote statistical significance).
Eucalypt tree volume was calculated with reference to Bredenkamp, 1982 (vander Willigen and Pammenter, 1997). Diameter and height were used to determine volume (m3) in an euqation specific for eucalyptus volume growth. Volume (m3) was significantly greater in GC clones (3.46 ± 0.16 10-3 m3) compared with GUA clones (2.71 ± 0.17 10-3 m3; p = 0.019; Fig. 3.9(a)).
Figure 3.9 (b) shows that eucalypt tree volume was signifcantly greater in the control treatment compared with chronic and acute treatments (p = 0.039).
Growth efficiency was calculated as the amount of wood volume per total leaf area (m2) per year (vander Willigen and Pammenter, 1997). The GC clone had a significantly higher growth efficiency than GU clones (GC = 5.1 10-3 m3/m2/yr; p = 0.022; Fig. 3.10 (a)). GC clones produce more wood volume by a smaller leaf area than GU clones. There was no signifcant difference between water treatments in terms of growth efficiency, however the acute treatment was slightly higher, primarily because trees treated with acute water stress had significantly less total leaf area i.e. see also Fig. 3. 28 (p = 0.175; Fig. 3.10 (b)).
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Figure 3.9: Mean volume (m3) of plants of Eucalyptus (a) clones and (b) in response to drought stress treatment after 520 days growth (n = 7; bars represent ± 1.0 standard deviation; different letters denote significant difference).
Figure 3.10: Mean growth efficiency (m3 wood / m2 leaf area / year) of Eucalyptus (a) clones and (b) in response to drought stress treatment after 520 days growth (n = 7; bars represent ± 1.0 standard deviation;
different letters denote significant difference).
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b Volume (10-3 m3 )
Growth Efficiency (10-3 m3 wood / m2 leaf area / year)
3.2 Non-destructive physiological measurements: Photosynthetic characteristics