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Quantifying interception associated with large-scale plantation forestry in the Northern Territory

Hutley, Lindsay B., Lancaster, Ian, Reilly, Don, O'Grady, Anthony P., Almedia, Auro, Kraatz, Maria, Smith, Sarah A., Bristow, Mila, Sawyer, Bruce and Yin Foo, Des (2012). Quantifying interception associated with large-scale plantation forestry in the Northern Territory<br />. : .

Document type: Research Report
Citation counts: Google Scholar Search Google Scholar

Author Hutley, Lindsay B.
Lancaster, Ian
Reilly, Don
O'Grady, Anthony P.
Almedia, Auro
Kraatz, Maria
Smith, Sarah A.
Bristow, Mila
Sawyer, Bruce
Yin Foo, Des
Title of Report Quantifying interception associated with large-scale plantation forestry in the Northern Territory
Publication Date 2012
Total Pages 99
Field of Research 300800 Environmental Sciences
Abstract In 2004, the Northern Territory Government signed an Intergovernmental Agreement as part of the Commonwealth’s National Water Initiative (NWI). The NWI identified large-scale plantation forestry as an activity which may be undertaken without a water access entitlement (WAE). However, as this has the potential to intercept significant volumes of surface and/or groundwater, under the agreement, the NT Government is required to assess the significance of these activities and apply appropriate planning, management and/or regulatory measures. Research undertaken as part of this report suggest interception of water from non-irrigated mahogany plantations is unlikely to significantly differ from native vegetation or improved pasture water use. However, this finding needs to be treated with caution as results from the modelling are subject to uncertainty, and more  work is required to confirm model findings.

In the mid-2000s, an introduction of Managed Investment Schemes (MIS) in plantation forestry resulted in the development of a plantation estate of Khaya senegalensis (desr.) A. Juss (African mahogany) in the Douglas-Daly catchment. These non-irrigated plantations have been developed predominantly on freehold lands previously cleared for improved pastures. It is anticipated that there will be an expansion of the African mahogany estate within the Daly River region of the Northern Territory. This report proposes the development of a policy framework for the management of non-irrigated plantations, such as those of the in the Douglas-Daly region, to address elements of the NT’s NWI obligations.

Mahogany plantations currently represent only a small proportion of the total catchment area (<1%), however the estate is currently ~13, 000 ha and is anticipated that this area could increase at a rate of approximately 2000 ha-1, potentially up to a total of 50, 000 ha. There are no existing mature and extensive plantation sites to directly measure water balance components. As such, a modelling approach was taken to examine future scenarios and potential impacts where a plantation estate is developed over time.

Water use characteristics of plantations

In order to assess the impacts of African mahogany plantation expansion on the water resources, a preliminary parameterisation of the growth model 3-PG2 was undertaken using literature, existing data and expert knowledge. Water balance of the dominant land types of the Douglas-Daly catchment, native savanna vegetation, improved pasture and mahogany plantations were simulated. The model agreed reasonably well with growth of African mahogany within the region, and was also in reasonable agreement with previous work examining the water balance of tropical savanna vegetation and improved pasture communities within the catchment(TRaCK program). A table of water balance components for the three land uses is given below.

Annual evapotranspiration (ET, total land surface water loss) from mahogany plantations was found to be similar to that observed in the surrounding tropical savanna vegetation and improved pasture, although there were marked differences in the partitioning of total ET and the seasonal dynamics of ET.

Mahogany plantations maintain canopy transpiration rates more than double that of the native Eucalypt trees species of the savanna (Summary Table). Isotopic analyses suggests, that young mahogany trees (~4 yo) are not utilising groundwater to maintain this high rate, and are able to meet transpirational requirements for soil store alone. In land units most appropriate for plantations, water table depth is typically well beyond the anticipated rooting depth (> 15 m). Mahogany tree plantings within 500 m of watercourses where water tables depth is approximately 5m-7 m, isotopic analysis suggested trees were not using groundwater. For trees established in low lying positions in the landscape this result may change as trees mature and root systems become deeper and more extensive.

Tree transpiration dominates ET in plantations, whereas understory transpiration and evaporation dominate the annual water balance of tropical savannas. The shift to higher tree water use on plantations is largely offset by the loss of understory vegetation, in particular, high water using grasses. On managed plantations, at canopy closure, shading results in very low vegetation cover beneath the tree canopy. As a result, up to 400 mm of moisture that would have otherwise been transpired by grasses remains in the soil profile which is available to support mahogany tree transpiration and growth.

Annual ET across the three land covers is similar at ~ 900-1000 mm y-1 (Summary table). Higher tree water use may result inb significant depletion of soil water stores, particularly in the late dry season. This may reduce deep drainage as ultimately a larger volume of water is required during the following wet season to replenish soil water resources. However, the impact of this dynamic could not be fully explored within the current modelling framework, but this may be an important dynamic in this system as a large component of dry season baseflow in the rivers of the catchment is derived from groundwater discharge. This is an ongoing research question that requires improved modelling.

There is considerable uncertainty in model outputs as there are few existing parameter sets for this species and little data on the growth and water use dynamics for model validation. Estimates of pasture ET are higher than previous measurements. It is also difficult to partition the drainage and runoff terms within the model. Water use monitoring is currently underway and preliminary data support 3PG-2 modelled African mahogany rates as compared with native species. Existing measurement regimes in complimentary projects will reduce modelling this uncertainty in the coming years.

In summary, while tree water use was found to be higher in African mahogany trees when compared with trees of native savanna vegetation, annual water use (evapotranspiration) was similar, at both plot and sub-catchment scale and impacts on water resources was deemed to be low.

This project was led by Charles Darwin University (CDU) and a Steering Committee, with representatives from the NWC, CDU, the NT Government’s Dept of Resources and NRETAS. CSIRO was engaged to make a preliminary assessment of the potential impacts plantation establishment and proposed expansion on the water resources of the region using a modelling approach. EcoLogical Australia was engaged to review previous forestry activities in the NT and develop a policy framework in collaboration with NRETAS staff.

Policy recommendations

Given a the low risk of impact on the water resources of the Douglas Daly Basin due to interception by non-irrigated African mahogany plantations, it is recommended that this land use does not require a water allocation licence under the NT Water Act.

Further work is required, however, to reduce the significant uncertainties associated with the outcomes of this research and to continue to inform the policy review process.

This policy applies to non-irrigated African mahogany plantations grown in the Daly basin. Application of the policy to other species requires research and modelling specific to that species. This approach, however, provides a useful and appropriate template for future research and policy development.

Further work is required to improve the modelling and this can be achieved by improvement in data availability such as;

1. spatial variability of rainfall and soil texture with depth within the catchment
2. establishment and regular measurement of a network of monitoring plots of differing age classes
    within the mahogany estate
3. improved parameterisation of the 3PG and other models, particularly with respect to biomass
    partitioning and controls on canopy conductance
4. An assessment of the contribution of groundwater to water use by mahogany plantations as trees
    mature
5. Long-term monitoring of stream flow in sub-catchments where plantation expansion is likely to
    occur, in particular Stray Creek.
 
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