Author

Scott Wood, University of Notre Dame Australia

Development of a System to Optimise Water Recharge and Timber Production from Pinus pinaster Aiton Plantations on the Gnangara Water Mound

Date of Award

2011

Degree Name

Doctor of Natural Resource Management

Schools and Centres

Arts & Sciences

First Supervisor

Dr Syd Shea

Second Supervisor

Dr Louis Caccetta

Abstract

Degree Name:

Doctor of Natural Resource Management

In 1996, the Western Australian Government decided to progressively liquidate 21,000 hectares of Pinus pinaster Aiton plantations located 20 kilometres north of Perth on the Gnangara Water Mound. The decision was made for a number of reasons but one of the principal concerns was the conflict between water use and timber production. The decision to liquidate the plantation over a prescribed period while meeting a legal requirement to produce timber for a laminated veneer lumber plant provided the opportunity to integrate the large database on plantation growth and water recharge and use operational research models to determine the optimum liquidation scenario.

Pinus pinaster Aiton growth on the Gnangara Water Mound on sites with shallow water tables was found to be less than on sites with deep water tables. This indicates that the trees on these shallow sites, with depths to water table of five to ten metres, are not accessing the water table. Consequently, it is concluded that it is the volume of the water in the unsaturated zone, beneath the trees that is the factor limiting growth not the depth of the water table. Leaf area index (LAI) can be derived from satellite photography except in stands with very open canopies. The relationship between LAI and the extent to which groundwater is recharged is critical. The relationship between Leaf Area Index and Basal Area was not linear as has been assumed in previous recharge models but there was a strong relationship between tree growth and Leaf Area Index. LAI is subject to a number of influences which include amongst other species, age and stand density.

Site maximum LAI for Pinus pinaster Aiton was 3.3 on the Gnangara Mound. Leaf Area Index regrowth for Pinus pinaster Aiton on the Gnangara Mound after thinning for a whole stand could be explained by:

LAI regrowth = z0 + a(SPH) + b(AGE) + c(SPH*SPH) + d(AGE*AGE) (where SPH is stems per hectare and AGE is time since planting). The LAI growth curve of unthinned young (old) stands was sigmoidal and explained by LAI = 3/(1-81^((3.5+12/2-year)/12)). This pattern is probably caused by increasing reductions of water availability. Because of this LAI pattern for young stands (old) it was hypothesized that the water recharge /age relationship for these stands is a negative sigmoid. That is it inversely mirrors the growth rates of LAI. Consequently the proportion of total rainfall used by plantations would increase with increasing LAI until it reaches the site maximum LAI and then plateaus. Combining the recharge relationship with LAI growth for the same ages of young stands (below 20 years of age) results in a negative linear relationship where percentage recharge = -15*LAI +45. This differs for the LAI to recharge relationship used in PRAMS( Perth Regional Aquifer Model) which proposes that above LAI 1.5 there is a complete extinguishment of recharge. The hypothesised negative linear relationship of LAI to water recharge is more readily able to explain the LAI to age pattern for young stands(old) whereas a full extinguishment at LAI 1.5 is not able to explain the ongoing LAI growth to 3.3. It is not conceivable that a plant would double its transpiration and rainfall interception area if it has already reached the limitation of full site water usage as proposed in PRAMS. There will always be some uncertainty in the LAI to water recharge relationship because it is impossible to measure directly. Sensitivity analysis was used to accommodate any departure from the actual relationship. LAI does have a strong inverse relation to water recharge. Consequently, LAI minimization within a model would have the same effect as maximizing the water recharge.

A Gnangara Mound Model was developed to provide a high level decision support tool that could effectively evaluate the relationship between water recharge and timber production. The model required a number of scenarios to test sensitivity to growth rates, recovery percentage of Laminated Veneer Lumber (LVL) from gross volumes, increasing volume requirements and shorter liquidation periods. Changing the objective to one of minimizing LAI (equivalent to maximizing ground water recharge) was tested to determine if it would give a significantly different result to that obtained with a maximize volume objective.

The effect of a number of harvesting regimes and other management options on the change in average per annum water recharge derived from the Gnangara Mound Model are listed below:-

• Increasing the frequency of prescribed burning native vegetation, increased recharge by 33.5 GL/yr.

• A reduction of private and public abstraction from 370 to 304GL/yr, increased recharge by 66 GL/yr. • Harvesting regime for Sustainable timber volume supply to 2026, increased recharge by 45.5 to 55 GL/yr.

• Harvesting regime for Minimize LAI with 100000 m3 minimum annual timber volume until 2026, increased recharge by 47.6 to 58.2 GL/yr.

• Harvesting regime for Sustainable timber volume supply to 2016, increased recharge by 50.5 to 62 GL/yr. An equal area of Banksia, at LAI 1, to the pine plantations would recharge annually 39.4 to 36.2 GL. A continuing lower rainfall of 100mm less than long term average would mean a reduction of recharge annually of 107 GL to that achieved prior to 1970.

Within the constraints of the State Agreement Act a small increase in water recharge could be achieved by using a harvesting schedule with an objective that minimizes LAI provided the volumes from 2008 to 2026 are no lower than 117000 m³ per annum.

It is also possible to combine this scenario with maximizing the liquidation of Gnangara plantation rather than the Pinjar and Yanchep plantations within the next 5 years. The combination of these two scenarios would result in the best water outcome (3.2 GL per annum) within the timber production constraints of achieving a minimum of 117000m3 per annum.

Early liquidation of the plantation will increase the water recharge but the water outcomes are small (5-7 GL per annum) in comparison to the timber volume loss and potentially could result in significant costs. Far greater savings could be achieved at a lower cost by using other management strategies such as reduction in Banksia water use through prescribed burning or less water extraction for private use. Small gains in water production would be lost by only a small reduction in annual rainfall due to continuing climate variability.

Publication Details

Wood, S. (2011). Development of a System to Optimise Water Recharge and Timber Production from Pinus pinaster Aiton Plantations on the Gnangara Water Mound [Doctor of Natural Resource Management]. The University of Notre Dame Australia. https://researchonline.nd.edu.au/theses/56