Análisis temporal de la estructura Clic.B.Izq en el campo que sirve

To explain streamflow trends with forest growth models it is necessary to account for the causal plant physiological processes and environmental variables that influence

WUE. Figure 3.21 shows a relationship between annual transpiration and annual stem volume increment for 22 plantations at five sites located in South Australia, southern New South Wales (NSW), and northern NSW (Morris & Benyon, 2005). The plantations had highly variable management strategies and variable age classes consisting of four species; E.grandis (age 5 to 6), E.globulus (age 5 to 9), P.radiata

(ages 5 to 30 years), and C.maculata (age 4). The sites’ soil types are highly variable and include sands, loamy sands, heavy clays, saline soils and sodic massive clays. The WUE of the sites has considerable variation about the mean regression line, which may be attributed to a range of causal plant physiological processes and environmental variables. Below is a concise synthesis of the plant physiological theory addressed in the review that may explain the variable deviation in the mean regression line.

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Figure 3.21: Relationship between current annual stem volume increment and current annual transpiration from a range of plantations in south-eastern Australia. Open circle are plantations with rainfall only. Closed circles represent plantations accessing additional water from the water table (from Morris & Benyon, 2005)

Stomata optimise WUE with gs by responding to diurnal conditions in VPD and soil

moisture (Morris & Benyon, 2005) in order to maintain a relatively constant ratio of water transpired to carbon gain (Farquhar & Sharkey, 1982; Whitehead, 1985; Hubbard et al., 2010). For this reason, regulation of gs is strongly reflected in tree water use and growth (Hellmuth, 1968). Different species regulate gs differently in response to high VPD due to differences in the root system’s interaction with soil moisture and groundwater, as demonstrated between the Symphyomyrtus and

Monocalyptus species (Sinclair, 1980). Literature provides little evidence that inter- specific variation in WUE is innately regulated by gs for Australia’s main timber producing species, as gs variation is a function of environmental variables and the plant’s ability to access soil moisture. Importantly, the uncertainty associated with quantifying and scaling gs for catchment-scale studies means WUE should be quantified at a spatiotemporal scale of a forested catchment (Denmead, 1984; Meinzer, 1993; White et al., 1999).

84 Differences in annual forest production are highly correlated with differences in accumulated annual light intercepted by LA (Cromer & Williams, 1982). Cromer et al. (1982) demonstrated that fertiliser increases LA, and hence increases intercepted light, which linearly increases forest diameter increments. Large seasonal variability in soil moisture conditions coincide with fluctuations in LA (Pook et al., 1997), and irrigation experiments have shown that changes in LA due to soil moisture conditions are also linearly related to forest productivity (Cromer et al., 1984). LA is central to the relationship between forest productivity and water use as it also represents the transpiring surface that largely explains T rates (Watson et al., 1999a). The relationship is not completely explained by LA as eucalypts grown in different climatic regions may have different WUE as a result of different climatic pressures on LA and gs. For this reason, regionalising a forest growth and water use

relationship needs to consider the negative linear relationship between seasonal variability of VPD and WUE (Morris & Benyon, 2005). This may be done by developing a separate forest WUE relationship for broadly uniform climatic conditions represented by temperate, sub-tropical, and arid climatic regions.

Changes in LA with changes to water availability cause WU:LA to be relatively constant (Mahmood et al., 2001). Within relatively uniform climatic regions, WU:LA

of a particular species is similar on contrasting soil types as gs is similar, whereas gc increases as a result of denser LA on soils that store plant available soil moisture more effectively (Benyon et al., 1999). Seasonally, plants regulate gc by adjusting LA rather than gs because with water shortages, decreasing gs results in higher leaf-air vapour pressure gradients, lower net carbon assimilation per unit leaf area, and potentially fatal leaf temperature. Alternatively decreasing LA allows for the

remaining leaves to maintain gs more efficiently and the leaf shedding accelerates the nutrient cycle (Hatton et al., 1998). For this reason, the light intercepting and water transpiring LA strongly influences tree diameter increments through its response to plant available soil moisture.

The rate of groundwater extraction by roots depends on the saturated hydraulic conductivity of the soil (Polglase et al., 2002), depth of the watertable (Dye et al., 1997), and differences in root architecture between species (Noble, 1989). WUE of plants can be variable when comparing sites with contrasting soil hydraulic

85 descriptors and/or plants that have contrasting root architecture (Theivaeyanathan et al., 2001). Soil dryness increases soil hydraulic resistivity, which shifts plant

resource distribution from foliage to fine root systems, reducing WUE of above- ground growth. Impenetrable soils require the plant to invest a lot more energy into soil penetration, root elongation, and fine root development for a less rewarding water resource, which reduces WUE due to water uptake per unit of root length (Falkiner et al., 2006). For these reasons, regionalising a relationship between forest growth and water use needs to consider the effects of contrasting soil types on WUE

as a result of contrasting ratios of root:above-ground biomass development.

Equilibrium between all hydrological components of a forest system provides the most compelling evidence of a strong relationship between forest productivity and water use. As high VPD increases and soil dryness decreases a plant’s water potential gradient, the plant physiology responds to the water potential gradient to make

forested systems balanced in relation to the finite available resource (Jarvis, 1975). To avoid catastrophic xylem cavitation when a tree’s

ψ

LA:SA ratio regulates the tree’s hydraulic flow pathway. A strong relationship between T and growth is reflected in how LA and SA interact, as increasing LA

increases T and growth (i.e. SA production).

Over the period of pre-canopy closure, soil moisture and light conditions become successively more limited, which results in a decrease in LA:SA ratio (Medhurst et al., 1999). Competition increases after canopy closure and LA:SA ratio becomes more constant and non-linear, with larger trees having higher LA:SA ratio and k, and hence higher WU:LA (Medhurst & Beadle, 2002). Increases in LA:SA and k means higher T rates per SA, which may suggest reduced WUE as there is less new growth (SA) for the given T. For the same reasons, a positive correlation between site quality and LA:SA, as well as site quality and k, also suggests reduced WUE at sites with less water limitations (Binkley, 1984). Finally, a negative relationship between

LA:SA and VPD exists, which suggests that eucalypts grown in different climatic regions may have different WUE as a result of different climatic pressures on LA and

gs (Magnani et al., 2002).

86 Thinning in effect increases site quality for retained trees, which may reduce WUE

by increasing k and WU:LA. Over time there is a restoration in pre-thinned LA:SAto restore the stand’s optimal WUE, which is determined by site conditions when water resource is limited by competition (Morikawa et al., 1986). Thinning delays peak in

CAI as net basal area growth increments may be same before and immediately after thinning due to higher growth rates and resource capture of retained trees (Goodwin, 1990). Prior to recovery of pre-thinned LA:SA,WUE is reduced as a result of

reduced competition increasing water availability (Fife et al., 2002). For this reason, the regional model needs to recognise that understocked forests with unlimited water resource may be less WUE.

Replacing natural vegetation with forest management regimes not reflective of the natural environment will result in changes to the landscape’s hydrology. Intensive forest management of plantations has been coined precision forestry (Battaglia et al., 2004) for its exhaustive use of the limited water resource with; soil cultivation (Falkiner et al., 2006), weed control (Florence, 1996), optimised tree spacing (Florence, 1996), fertilisation (Raison et al., 1982), thinning (Goodwin, 1990), pruning, and planting of genetically selected vigorous strains (McRae, 2004). Contrast to this management system, water use by native forests consists of many mechanisms that make certain that available water is not used excessively. Within the natural environment, site factors exert strong control on the vegetation’s water up-take through species composition, natural stocking densities, natural stand

structure, and natural disturbance periods. Considering the plant physiological theory presented, a regional model needs to recognise that plantations optimise competition for resource capture, which effectively increases water use per unit area greatly but imposes pressures on the system to become more WUE.

3.6. Conclusion

Forest growth models of tall eucalypt forested catchments may be used to quantify catchment level forest water use once considerations are made for the causal plant physiological processes and environmental variables that influence WUE. Presently, forest hydrology models in Australia underutilise existing forest inventory and forest mensuration databases for managing the forested water resource. Detailed forest

87 inventory data exists for most forested catchments in south eastern Australia and this information should be used to generate hydrologically relevant forest growth models that are capable of explaining streamflow trends. This chapter has demonstrated that forest inventory data, which is commonly used to model forest growth, may provide significantly more information to explain streamflow trends than the forest

hydrology modelling methodologies described in chapter two.

The hydrological equilibrium in forested systems is driven by the response of LA:SA

and sap flux density to environmental conditions and stand competition for the limited water resource. To improve on the already strong relationship between forest productivity and water use in figure 3.21, considerations need to be made for the:

• negative linear relationship between VPD and WUE, as inter-regional variability in VPD affect LA and gs,

• effects of contrasting soil types, as increases in ease of soil root penetration and soil moisture holding capacity increase WUE by reducing root:above- ground biomass ratio,

• differences in inter-specific root system architecture between Symphyomyrtus

and Monocalyptus species, as effective subsurface water exploitation results in an increase in WUE by reducing root:above-ground biomass ratio, and,

• extent of limitations of water resource due to competition and environmental pressures as; pre-canopy closure forests, understocked forests without water limitations, and forest sites with less water limitations are less WUE, whereas intensive systems such as plantations and water limited forests are more

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Chapter 4: Overview of model structure