Donald A. White

Water use by contour-planted belts of trees comprised of four Eucalyptus species

Over a 12-month period, soil water content and water use were measured concurrently in a contour-planted belt of trees comprised of Eucalyptus saligna (Smith), E. camaldulensis (Dehnh), E. leucoxylon (F. Muell) and E. platypus (Hook). These data were used to calculate water uptake by the trees from current rainfall, and from sources other than current rainfall (U 0 ), to describe the temporal pattern of switching between these water sources, and to calculate the number and spacing of tree-belts necessary to reduce groundwater recharge to 5 mm per year. The data were also used to quantify competition for water between trees and adjacent crops or pastures. The tree-belt used 595 mm on a projected crown area basis over a 12-month period. Of this, 440 mm was transpiration, 100 mm interception and 55 mm soil evaporation. Rainfall was 445 mm and was all captured by the upper 2 m of the soil profile. U 0 was 150 mm. Both soil water measurements and piezometric data from the same site indicated that most of U 0 was from groundwater flowing under the trees. This

Photographic exposure affects indirect estimation of leaf area in plantations of Eucalyptus globulus Labill

Calibrations of indirect methods for estimating leaf area are usually based on small data sets and are often species-specific and may even be stand-specific. We used the Licor LAI-2000 plant canopy analyser (PCA) as a reference to calibrate leaf area measurements based on hemispherical photography. Ten stands of 6‐8 year-old, plantation grown Tasmanian bluegum (Eucalyptus globulus Labill.) in Western Australia were used to investigate the effects of variations in sampling position, photographic exposure and image processing on leaf area (or leaf area index, L) estimated using hemispherical photography. We also compared our photographic estimates of L with those obtained via destructive sampling (allometry) in both evenly spaced and highly clumped stands of E. globulus. Varying exposure by one stop affected estimated L by approximately 13% but we confirmed that correct exposure can be approximately predicted by metering exposure outside the canopy. In situations where metering exposure outside the canopy is impractical, we recommend the use of empirically derived relationships between L estimated from photographic images made at a constant exposure and actual L derived from other means. Mean tilt angle obtained for E. globulusfrom the photographic method (68.7 2.5 s.e.) agreed well with estimates for Eucalyptusspecies from other studies. Owing to the non-random arrangement of crowns within plantations, sampling position significantly affected mean tilt angle. In highly clumped stands with closely spaced double rows and wide inter-double row gaps, underestimation of L by 16‐30% with the photographic method was probably the result of greater foliage clumping at the crown level. We concluded that, in stands of E. globulus and probably other broadleaf species with evenly distributed crowns, foliage clumping at the shoot or branch level is unlikely to be a significant source of error in indirect estimates of L. Scattering of blue light may result in large underestimates of L when using the PCA. In stands with ‘extreme’ architecture, indirect, light interception-based methods are likely to greatly underestimate L, although, positioning the sensor so as remove large gaps from view may allow accurate estimates of L even in these stands. ©2000 Published by Elsevier Science B.V. All rights reserved.

Observed and modelled leaf area index in Eucalyptus globulus plantations: tests of optimality and equilibrium hypotheses

This paper reports on variation in leaf area index (L) in five Eucalyptus globulus Labill. plantations in response to application of nitrogen, thinning at age 2 years and variation in climate wetness index (the ratio of rainfall to potential evaporation). Observed L is compared with: (i) L predicted to optimize net primary productivity for a given average annual temperature, annual water use and potential evaporation (L(opt)) and (ii) L calculated as a linear function of climate wetness index (L(eq)). L peaked in fertilized plots at between 4 and 5 years of age or immediately after canopy closure. The value of L from canopy closure to age 8 years was not strongly related to annual rainfall or climate wetness index. At two sites with total soil nitrogen <1.2 mg g(-)(1), L in fertilized plots was about two units greater than in unfertilized plots. This difference persisted until measurements ended in 2004 when the trees were 8 years old. The L of plots thinned to 300 and 600 stems ha(-)(1) at age 2 years recovered quickly and was not significantly different from L in unthinned plots when the trees were 8 years old. L(opt) was a good predictor of the leaf area index of 8-year-old plots of E. globulus when nitrogen and phosphorus were non-limiting (model efficiency (EF) was 0.5). For the same plots, L(eq) underestimated observed L by an average of two units, and the model efficiency was low (-3.25). Data from two nitrogen-limited sites demonstrated that for fertilized plots L(opt) (EF = 0.6) was a much better predictor of L than L(eq) (EF = -3.36). At the same sites, L(eq) (EF = 0.42) was a better model for predicting L of unfertilized plots than L(opt) (-3.59). These results provide evidence that comparing observed L with L(opt) can identify stands limited by factors other than growing climate.

Selecting species for recharge management in Mediterranean south western Australia – some ecophysiological considerations

This paper makes a case for coupling quantitative relationships between plant function and environment with an understanding of local hydrology as a basis for selecting species for recharge control and salinity management in the cropping zone (rainfall < 500 mm per year) of south western Australia, a region with a Mediterranean-type climate. This case is made in two parts: (a) A review of some physiological responses of trees to drought and trends in these responses in relation to the rainfall patterns across the natural distribution of species. In general low-rainfall species use less water during winter and early spring and have greater physiological activity during summer than high-rainfall communities. Trends in leaf water relations and stomatal responses to soil drying are consistent with this observation; species from lower rainfall environments have lower leaf water potential at the turgor loss point and weaker stomatal responses to soil drying than those from high rainfall areas. (b) A physiological growth model for Eucalyptus globulus is used to demonstrate how a quantitative description of key plant-environment interactions may be used to analyse the growth and mortality risk for combinations of site, climate and stand management.

The fate of hydraulically redistributed water in a semi-arid zone eucalyptus species

Although hydraulic redistribution has been observed for a range of tree species, including Eucalyptus kochii subsp. borealis (C. Gardner) D. Nicolle, there is limited direct evidence that water taken up by deep roots in moist soil is in fact exuded by shallow roots in dry soil. This paper reports an experiment designed to test this hypothesis. Water enriched with deuterium was added to the groundwater via a slotted tube at 4.5 m depth below 5-year-old E. kochii subsp. borealis trees. Nocturnal sap flow increased markedly immediately after deep irrigation, indicating that the trees were using water from this depth. Two weeks later, samples of surface soil and xylem water were found to contain levels of deuterium up to 30% higher than soils and xylem water from a control plot upslope of the main treatment plot. This is strong evidence that trees used groundwater and that efflux of important amounts of hydraulically redistributed water occurred via the roots of E. kochii subsp. borealis.

Interaction of N supply and water relations in Eucalyptus globulus Labill

Carbon assimilation, foliar N concentration and instantaneous transpiration efficiency were measured in 3.5 year old plantation Eucalyptus globulus in a N fertiliser trial in the lower south-west of Australia. N application increased foliar N and carbon assimilation. The efficiency of use of transpired water in assimilating carbon also increased with N addition. These results indicate improved performance at the leaf scale. However, the effect of enhanced canopy size on trees well-supplied with N growing in water limited environments needs to be assessed.