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

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 plantation. 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\\(^{-1}\\) for eucalypts and from <0.1 - 8.4 1 day\\(^{-1}\\) for 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\\(^{-1}\\) in Acacia-infested 8- year-old plots and was 0.85 nun day\\(^{-1}\\) in an Acacia-infested 4-year-old plot. Values of 1.7 and 1.8 mm day\\(^{-1}\\) were 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\\(^{-1}\\) during the growing season. The maximum photosynthetic rate (\\(A\\)\\(_{max}\\)) for both species was approximately 25 ˜í¬¿mol CO\\(_2\\) m\\(^{-1}\\) s\\(^{-1}\\) photosynthetic light response curves were similar between species. Amax was rarely observed for A. dealbata, primarily because of stomatal closure in response to declining leaf water status. The two species responded similarly to soil and atmospheric demand as measured by diurnal variation in stomatal conductance and leaf water potential. However, for A. dealbata, reduction of both variables occurred in response to conditions of lower demand than for E. nitens. A. dealbata was concluded to be an important competitor in the system studied, with the potential to markedly reduce the productivity of E. nitens over the course of a pulpwood rotation. A number of management prescriptions was proposed that may alleviate competition to the benefit of stand productivity. A. dealbata was also found to possess an unusual life history, occupying a brief temporal niche in disturbance-prone communities where it can persist in the long term as propagules in the soil between disturbance cycles.