The Crotty Creek Project. The effects of strip thinning Eucalyptus regnans on forest growth and water yield. Second progress report – Post-treatment phase.  Ed. P. J. O’Shaughnessy, R. Benyon and A. Lucas.  1993.  74pp. (unpubl.)

SUMMARY

The Crotty Creek Project is a cooperative experiment managed jointly by Melbourne Water and the Department of Conservation and Natural Resources (CNR). The project also forms part of the Melbourne Water Catchment Hydrology Research Program. The first Progress Report published in 1981 described the establishment of the experiment and the commencement of the treatment phase. This Second Progress Report describes the method of treatment application and the initial results for forest growth and streamflow.

The 122 ha Crotty Creek catchment is forested with 1939-origin Mountain Ash (Eucalyptus regnans). Monitoring of streamflow and rainfall began in 1976. The experimental aim was (and is) to examine the effects on streamflow and forest growth of reducing the basal area of the forest by 50% using 35 m wide alternate cut and retained strips.

These strip thinning treatments were applied between 1979 and 1985. The logging contractor was generally happy with the production rates, which were improved by the introduction of rubber-tyred skidders. Over the six years of treatment the average yield of sawlog was 164 m³/ha and of pulpwood 289 m³/ ha.

Measurement of eight permanent forest growth plots showed that in the five years following treatment, gross stand growth was reduced by 33% and net stand growth was reduced by 18%. A growth response in a 5 to 10 m band along either side of the cut strips partly compensated for the 50% reduction in growing stock. Monitoring of eucalypt regeneration on the cut strip showed that initial stocking was marginal at the outset, that stockings progressively deteriorated to insignificant levels with time and that the regrowth of regeneration was half that of unsuppressed regrowth. Damage to retained stems was minimal.

Rainfall and streamflow monitoring has continued throughout the life of the experiment. The Soil Dryness Index (SDI), the Climatic Index (CI) and REGMOD models were used to identify streamflow responses to treatment. Allowing for higher rainfall in the post-treatment phase and two years of unexplained high flows in the control catchment, the post-treatment increase in mean annual streamflow was 31% when estimated using the SDI approach; 21% and 26%, respectively, estimated by the other two approaches.

Analysis of streamflows using double mass curves supported this result. This analysis also showed that the ratio of base flow to stormflow did not alter. In seasonal terms the average monthly flow between April and September dropped by 0.7% of the total annual flow and increased by the same percentage for the October to March period.

An economic analysis using discount rates of 0% and 1% showed that strip thinning at age 43 and clearfelling at age 80 years rather than clearfelling only at 80 years would result in losses in net present timber values. However for discount rates of 2% or higher, strip thinning resulted in an increase in net present timber values.

The loss of wood value at discount rates of 0% and 1% would be compensated by pricing streamflow at $36 ML and $11 ML respectively. When a constant area of forest was considered, the loss in timber volumes if strip thinning at age 43 and clearfelling at 80 years was a regular practice would require a compensating water value of $36 ML.

It can be concluded that for catchments subject to multiple use, strip thinning has the potential to increase the combined economic output from the catchment. For catchments currently unavailable for timber harvesting strip thinning has the potential to increase streamflow without detriment to water quality.

It is reasonable to assume that the water yield increases found for 43 year old E. regnans stands would in general be applicable to other ash type stands. The study did not investigate the effects on flora and fauna values of strip thinning operations and this would be required prior to any management decisions being made.

It is recommended that in order to fully determine the impacts on streamflow and forest growth, monitoring of streamflow and forest growth continue for another 10 years under the guidance of the Crotty Creek Working Committee. A further report will be produced in 1997.