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Uptake - Issue 1 - Towards water-use-efficient plantations

The growth of well-managed plantations in southern Australia is largely determined by available water. The National Water Initiative compels state water planners to account for water use by new plantations. For plantation managers and water planners alike, it is crucial that we develop strategies for improving the water use efficiency of wood production systems. Across the CRC for Forestry we are tackling this issue on a number of fronts ...

Can we improve the water use efficiency of plantations using standard silvicultural techniques?

Gary Ogden downloading sapflow sensors

Gary Ogden downloading sapflow sensors

water use efficiency of wood production graph

Annual Water Use Efficiency of Wood Production (Ww, m3 ha-1) for E. globulus at Scott River (annual rainfall approx 1000mm) grown with two annual rates of nitrogen application (0 and 250 kg ha-1)

In Research Programme One, Paul Drake is quantifying the growth and water balance of coppiced and replanted second rotation Eucalyptus globulus and the effects of spacing and application of nitrogen. This continues the work of Don White, Stuart Crombie and John McGrath of FPC on the effect of thinning, application of nitrogen and climate on the growth and water use efficiency of first rotation E. globulus.

On a nitrogen-responsive site application of urea more than doubled the amount of wood produced per unit water used from 1.4 to 2.8 m3 mL-1 (see figure below). Improving the nitrogen status of E. globulus increased water use by about 150 mm a year (about 10%) but nearly doubled the volume growth. This was because the additional water use occurred in spring when the air saturation deficit was moderate. The most efficient way to grow wood is to use all the available water early in the growing season. Of course this is a risky approach and our research has shown that even on relatively high rainfall sites (approx 1000 mm per year) it is possible to lose yield due to increases drought-induced mortality if you get the balance wrong (the dip in the water use effiicency of the high N treatment in the figure was due to mortality in a very dry year).

At the same time, we found that reducing stocking density from 1200 to 600 stems ha-1 significantly reduced water stress and risk without reducing growth to age 10 years. Compared to the unthinned control, the thinned stand had significantly greater stand scale light use efficiency due to a reduction in water stress and an increase in the amount of light intercepted per unit leaf area.

Recommendation – Maximise water use efficiency by matching nutrient supply to demand and reduce stocking to minimise drought risk while at the same time reducing harvest costs per unit volume.

Looking towards the Stirling Ranges (WA) from near one of our long-term study site

For more information contact Dr Don White.

Have our genetic improvement programs selected for faster or more efficient water use?

Nathalie Long is a Murdoch University PhD student in Programme One who started working on this difficult question late in 2007. There is a lot of evidence for a range of plantation species, including Eucalyptus globulus, of variation between clones and families in physiological traits associated with improved water use efficiency at the leaf scale.

At the same time, Greg Dutkowski and others have reported increased drought tolerance moving from west to east in the natural distribution of Eucalyptus globulus. In his invited paper at the EucProd meeting, Nuno Borralho highlighted the limitations of genotype trials based on single tree and row plots. It is possible that growth improvement through breeding is actually the result of faster water uptake rather than improved water use efficiency and some of the growth benefits may not be realised in broadscale commercial plantations. Among other things, Nathalie proposes to quantify the relationship between competition index and water use efficiency expressed at the leaf and stand scale for a number of selected genotypes.

For more information contact Nathalie Long.

Can we design water-use-efficient plantation estates?

Most existing experiments and published results on water use efficiency in plantations are at plant or plot scales and rarely at the catchment scale. This is due to limitations of available data and the lack of appropriate modelling frameworks that link forest growth and carbon balance with hydrology. Although modelling the relationships amongst wood, carbon and water at multiple scales is a complex task, there is a clear need for this information.

The National Water Initiative compels water regulators to account for the effect of a range of activities including large-scale plantation establishment. It does not however provide the technical knowledge or tools for objectively quantifying the trade-offs inherent in plantation establishment. State water regulators are developing recommendations and / or policy, either by scaling up from estimates of plantation water use made at the plot scale or by scaling down from the catchment scale estimates of plantation water use made using generic curves or similar catchment-scale models. Both approaches are potentially flawed, particularly when applied at intermediate scales.

In this new project we intend to:

  • test for convergence between catchment scale and aggregated stand scale models of plantation water use using streamflow data from two gauged catchments in which there has been large scale plantation establishment
  • develop and field-test a model of plantation growth and water use linked to a hill slope scale model of distributed flow
  • use these modelling approaches to quantify the water use efficiency of carbon sequestration and wood production and the effect of site characteristics including landscape position.

We intend to work in two contrasting environments with one experimental catchment in Tasmania and one in Western Australia. In partnership with water authorities and plantation growers we will select two catchments where substantial plantation establishment has occurred and test the capacity of existing modeling frameworks to predict observed carbon and water balances at a range of scales. We will quantify the relationship between plantation growth, carbon sequestration and water use from the compartment to the catchment scale.

Through our research we aim to broker a shared understanding of the water impacts of plantations to help underpin state and national policy on plantations and water.

For more information contact Auro Almeida or Jenny Carter.