15 June 1992 Urgent Need for Ecological Management Plan among South Westland Rimu says Lincoln Scientist

An ecologically viable management plan for logging among south Westland's remaining rimu stands is needed urgently, says Lincoln University soil scientist Peter Almond.

Clearfelling of the production estate is due to finish this year and pressure is being directed at the rimu trees in the North Okarito and Saltwater Forests which, since 1984, have been gazetted for sustained yield production.

A lecturer and doctoral candidate at Lincoln University, Mr Almond is working with forest ecologist Dr Richard Duncan, formerly of the Forest Research Institute, to produce a study of the relationship between soil characteristics and forest vegetation.

Mr Almond hopes that the project in Saltwater Forest will lead to a means of managing the forest that ensures a balance between employment, economic and conservation interests, one that is based on the most efficient and least damaging use of the environment.

"To sustain the forest as a diverse, viable ecosystem, future management should match as closely as possible the natural on-going processes in the forest," says Mr Almond.

"Virgin indigenous forest does not resemble a plantation and should not be managed as one or to create one.

"The very wet climate of Westland means that on all the older land surfaces soils are strongly leached and very infertile. Land cleared of forest is very difficult to turn into productive farmland and often turns to swamp. The most productive use of these areas is the sustained production of high quality, high value native timbers."

Attempts at sustainable harvesting of lowland rimu forest began in south Westland in the early 1970s with selective logging. The forest was treated as a plantation and a portion of the volume from a stand was removed before moving on. It was hoped that by the time the whole forest had been treated in this way a portion of the initially cleared stands would have regenerated.

However, damage to site and stand by machines designed for clearfelling meant that mortality in the stands after logging was greater than regeneration.

It is now recognised that in these lowland rimu forests with their very wet, infertile soils, trees tend to replace themselves in small areas as cohorts formed after stand disturbance, probably as a result of windthrow.

"Research into the nature of forest growth and replacement has not, however, taken into account the effects of soil," says Mr Almond.

"There appears to be a strong relationship between soil type and stand type, suggesting that soil controls the rate or nature of stand replacement."

Studies also highlight the fragility of the forest ecosystem. Because the soils are very wet, any surface modification through logging or roading can lead to exacerbated surface ponding and subsequent tree death. The removal of the forest canopy during clearfelling also results in extra rainfall striking the soil surface. Once exposed to sunlight and extra rainfall, vital nutrient sources in the soil litter and topsoil horizons are lost forever.

The basis of Mr Almond's study of Saltwater Forest soils is an understanding of the history of landscape evolution in south Westland. Over the past two million years, Westland has experienced many advances and retreats of glaciers originating in the Southern Alps. Each advance has left behind debris and river deposits on which the soils are now formed.

Understanding the chronology of events allows an appreciation of the important influence of time on soil formation and soil patterns.

"Within this framework the nature of the soils can be studied in terms of their nutrient status and physical qualities as a rooting medium," says Mr Almond. "Some soils are drier and better bearers of trees than others."

Mr Almond began work on this area when employed by the New Zealand Forest Service. He is about one-third of the way into a five-year project which should add to the understanding of the glacial history of Westland and its influence on vegetation.

The findings will not be directly applicable to other rimu forests because of the unique soil of the Westland stands – wet and impoverished.

Mr Peter Almond, Soil Science Department, Lincoln University, Canterbury, New Zealand