Dimensionamiento de las barras de la estructura Clic.B.Izq en el campo que sirve

potential gradient that affects the plant’s water flow pathway from soil to

atmosphere.

To further support the argument that WU:LA is similar between species, a simple model of canopy conductance proposed by Sziecz and Long (1969) is presented:

gc = //LAI [3.2]

where _{is mean stomatal resistance. For a canopy to regulate water use, or LAI} may be adjusted. When soil moisture levels are low, adjusting the numerator by increasing _{results in higher leaf-air vapour pressure gradients, lower net carbon} assimilation per unit leaf area, and potentially fatal leaf temperature (Pook, 1986). Adjusting the dominator, by decreasing LAI allows for the remaining leaves to maintain stomatal conductance more efficiently and the leaf shedding accelerates the nutrient cycle, which is particularly important in much of Australia’s nutrient

deficient environment. For this reason, gc varies between species due to the variation in LAI, whereas _{is more constant for a given LA, which is determined by site} conditions to result in a site specific WU:LA.

3.3.3. Role of root systems in regulating forest productivity and

water use

Plant-available water-holding capacity of soil may vary from around 50 to 400 mm per metre of soil depth (Morris & Benyon, 2005). The depth of the root zone and water storage properties of soils is therefore crucial in determining water availability. As shown in figure 3.10, water supply (rainfall, groundwater and irrigation)

62 explained 94% of the variation in transpiration for five south-eastern Australian plantations (Morris & Benyon, 2005).

Figure 3.10: Relationship between annual available water and annual transpiration for plantations in south-eastern Australia (from Morris and Benyon, 2005)

Many studies have identified that eucalyptus transpiration demand, particularly in the drier months, may be met from water obtained from the saturated zone in the lower depths of the soil profile (Talsma & Gardner, 1986; Dye, 1996; Knight, 1999; O'Grady et al., 1999; White et al., 2002). The position of the plantation in the landscape has a large influence on the plantation’s T because topography and catchment hydrogeology have an effect on the depth of the watertable and hence water availability. The rate of groundwater extraction by roots depends on the saturated hydraulic conductivity of the soil, depth of the watertable, and density of roots in the groundwater system. In Deniliquin, groundwater use for 3-5 year old

E.grandis varied due to differences in soil properties rather than depth to the water (Polglase et al., 2002). If there are no chemical and physical barriers to limit root penetration to groundwater, during periods when trees do not receive an adequate supply of rainfall and irrigation, groundwater uptake will dominate the tree’s water supply (Morris & Benyon, 2005). Groundwater can be sourced from a very early age of a plantation, as Dye (1997) has demonstrated that 3-year old E.grandis trees use sub-soil water reserves 8 m below the surface.

63 Differences in root architecture between species are important in determining water use as there is a great deal of inter-specific variability in root systems that exploit the soil profile. Falkiner et al. (2006) made comparisons between root systems of

Corymbia maculate and E.grandis and found significant differences, with

C.maculate roots being more developed around the capillary fringe just above the groundwater table. Due to differences in root systems, C. maculata had a

groundwater uptake of 733 mm year-1 (72% of the annual water use) whereas at the same site E.grandis of the same age used only 377 mm year-1 (56% of the annual total water use) (Morris & Benyon, 2005).

In south-eastern Tasmania, Honeysett et al. (1992) analysed plantation water use and growth of two contrasting species E.nitens and E.delegatensis during their fourth and fifth year of growth. Experiments involved measuring stand volumes, soil water deficit ∆W up to the depth of 1 m for soils assumed to be at least 1.5 metres deep, and LA. Soil water deficit (∆W) represented the water content of the root zone defined as difference between water content at field capacity and measured water content. The results found that LA and growth rates for E.nitens were approximately twice that of E.delegatensis over the experiment period. This meant both species had similar stand volume per unit leaf area over the experiment period, supporting figure 3.6, which suggests productivity is proportional to the light absorbed.

In assuming ET was simply gross rainfall (P) minus ∆W over the study period, Honeysett et al. (1992) did not include groundwater uptake when calculating WUE of each species. Using the definition, ET =P - ∆W, Honeysett et al. (1992) calculated

E.nitens to have a much higher WUE, and attributed it to the reduction in stomatal conductance of E.nitens rather than the unaccounted groundwater uptake. Honeysett

et al. (1992) results are contrary to Sinclair (Sinclair, 1980; Florence, 1996), who found that Symphyomyrtus species (E.nitens) are known to actively transpire at a time of water stress by accessing ground water, whereas Monocalyptus

(E.delegatensis) are more sensitive to drought stress and respond with rapid stomatal closure to preserve water at the expense of productivity.

In figure 3.11, a fortnightly break-down of WUE by Honeysett et al. (1992) shows

64 174), and most different when soil moisture dropped towards wilting point (week 232-246). As Honeysett et al (1992) do not account for groundwater use in their model, the WUE estimates are likely to be higher when the plants rely on

groundwater. Considering Sinclair (1980), this was likely to be the case for E.nitens

in figure 3.11 when soil moisture dropped during weeks 188 to 190, and after week 232. Root systems are very opportunistic and are capable of penetrating below fractures in bedrock and rock floaters to access deep groundwater many metres below the assumed and highly uncertain soil depth level. Effective root depths represented by the depth of a hand held augur penetrated into soil are unlikely to correlate well with the maximum amount of water available for deep rooted eucalyptus trees (Dye, 2000).

Figure 3.11: Water use efficiency of stands of E.nitens (shaded) and E.delegatensis (clear) as a function of stand age (in weeks) (from Honeysett et al., 1992)

In follow up experiments undertaken by Honeysett et al. (1996) on a site with a mean soil depth to bedrock or rock floater of 0.6 m, groundwater was measured using an oversized hole mechanically drilled into the rock base to a total depth of 3 m. The experiment involved soil moisture, and groundwater measurements in E.globulus and

65 plantations began to source water from the groundwater near the end of the second year of growth, and this coincided with the time when the soil profile had a moisture deficit close to wilting point conditions. These experiments highlight that inter- specific variation in growth rates was due to differences in the root system’s ability to exploit groundwater and not due to differences in WUE.