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Competition between plantation Eucalyptus nitens (Deane and Maiden) Maiden and naturally regenerating Acacia dealbata Link.


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Hunt, Mark A. (Andrew) 1998 , 'Competition between plantation Eucalyptus nitens (Deane and Maiden) Maiden and naturally regenerating Acacia dealbata Link.', PhD thesis, University of Tasmania.

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Competition between planted Eucalyptus nitens and naturally regenerated Acacia dealbata
weeds was investigated in an industrial plantation in north-east Tasmania. The effects of
this competition on stand growth, canopy architecture, tree water use and leaf gas
exchange were considered in 1988- and 1992-planted stands during the period 1994-1997.
Under favourable moisture conditions, A. dealbata grew at a rate close to (and sometimes
exceeding) that of E. nitens. Competition peaked at age 2-6 years before declining as the A.
dealbata canopy was suppressed, in part by intraspecific Acacia competition and
consequent self-thinning. At ages 2-4 years, E. nitens basal area was highly correlated
inversely with both A. dealbata stem frequency and basal area. However, by age 6 years,
E. nitens basal area was not a significant correlate of A. dealbata basal area.
The effects of competition on the productivity of E. nitens were substantial, reducing
standing volume compared to Acacia-free plots by almost 25% by age 8 years. Soil
nitrogen status was positively correlated with A. dealbata presence and negatively
correlated with E. nitens basal area. A. dealbata was therefore considered to be a net
contributor of nitrogen but competition for other resources negated any positive effects of
this contribution. Silviculture during site preparation and establishment was identified as
an important contributor to the distribution and abundance OA. dealbata throughout the
Highly significant regression relationships were developed among leaf area and a number
of stem variables for A. dealbata and E. nitens. Whilst basal area was found to be a suitable
predictor of leaf area for both species, the closest relationships were developed between
leaf area and sapwood area or cross-sectional area at the crown base. The relationships
between basal area and leaf area were used to scale basal area measurements to stand leaf area index which was partitioned between the two species. In 1995, for nine 7-year-old
plots with Acacia competition, E. nitens had an average stand LAI of 2.6 compared to an
average LA! of 4.3 for five plots of the same age without competition. In 1996, for six 8-
year-old plots with Acacia competition, A. dealbata contributed an average 20% of a stand
LAI of 3.4. In a single 4-year-old plot, A. dealbata contributed 50% of a total LA! of 2.9.
In an adjacent Acacia-free plot, the LA! of E. nitens was 2.5, 86% of canopy LAI in the
Acacia infested plot (above) but nearly twice that of the eucalypt component of that plot.
Estimates of vertical leaf area distribution within an 8-year-old Acacia-infested plot
indicated that leaf area of E. nitens was concentrated between 13 m and 22 m above the
ground, whereas leaf area of A. dealbata was concentrated between 8 m and 16 m above
the ground. Data from transects of vertical irradiance indicated that transmittance was
reduced most markedly between 14 m and 20 m above ground. Computer image analysis was used to investigate the accuracy of measurement of area of
the pinnate leaf of A. dealbata. Standard planimetry techniques were found to have
inadequate resolution for this application. Furthermore, the habit of Acacia pinnae closing
about the rachis precluded accurate area measurement of the intact leaf. Consequently,
reduction of leaves to component primary pinnae was found to be necessary for accurate
area determination. The results suggested that, due to these combined errors, stand leaf
area of A. dealbata may have been underestimated by as much as a factor of 2 in this study.
Whilst the magnitude of the underestimation was marked, the direction and consequences
of the relationships derived in this study were not affected by application of a correction
factor. Hence leaf area data were analysed as measured in the first instance but an
alternative measurement protocol recommended for future experiments.
Whole tree water use was estimated using the heat pulse velocity technique during a six
week summer period. 4-year-old and 8-year-old trees were considered in Acacia-infested and Acacia-free stands as well as stands cleared of Acacia competition. Maximum sap
velocities were recorded between 5 mm and 15 mm under the cambium for all trees and
marked radial and axial variations in sap velocity were observed. The latter source of
variation was most pronounced in mixed stands where crowns were asymmetrical. Mean
daily sapflux ranged from 1.4 - 103.6 1 day -1for eucalypts and from <0.1 - 8.4 1 day-1for
acacias. Stem diameter explained 98% of the variability in sapwood area for E. nitens and
89% for A. dealbata and was determined to be a suitable variable for scaling water use
from tree to stand level. Transpiration varied from 1.4 - 2.8 mm day -Iin Acacia-infested 8-
year-old plots and was 0.85 nun day-1in an Acacia-infested 4-year-old plot. Values of 1.7
and 1.8 mm day-1were returned for two 8-year-old plots recently cleared of Acacia
competition. The level of A. dealbata infestation was associated with absolute plot water
use and regression models predicted that in the absence of Acacia competition plot water
use for the 8-year-old stand would approach 5-6 mm day during the growing season.

Item Type: Thesis - PhD
Authors/Creators:Hunt, Mark A. (Andrew)
Keywords: Acacia, Reforestation, Eucalyptus
Copyright Holders: The Author
Additional Information:

Thesis (Ph.D.)--University of Tasmania, 1999. Includes bibliographical references

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