Research into trees as ‘pumps’ holds promise for the future

Press, 18 February 1983, Page 24

Research into trees as ‘pumps’ holds promise for the future

Research at the Water and Soil Science Centre at Aokautere near Palmerston North into the capacity of trees to act as “living pumps" may ultimately save pastoral farmers, and the country, millions of dollars. Of New Zealand's total hill country of nearly 9.5 million hectares almost 5.5 million hectares is devoted to pastoral farming. Every year some part of this hill country grazing is eaten into by wet weather slips and other forms of erosion.

The immediate loss to farmers has been put at more than $2O million a year, without taking into account the reduction in future pasture productivity. For example, during the 1977 wet winter in the Wairarapa extensive soil slippage occurred and pastoral hill country losses in annual income in the first year over a 1400 square kilometre area were about $1 million. Catchment board officers estimated that 100 farmers experienced drops in income of 15 per cent or more. A characteristic of this loss of pasture was not merely that from the slip area itself but also that which was buried by the slip material. Field studies have shown that earlier slip scars in the Wairarapa recovered their grass productivity quite rapidly over the first 20 years—to 70-80 per cent of neighbouring uneroded soil—but very little further recovery has been detected on scars up to 80 years old. Most slips are triggered by wet weather—either by very intense rainstorms or by above average ranfalls continuing over periods of weeks or even months. Soil scientists consider that if they can reduce the moisture level in the soil there will be less likelihood of slips occurring.

One technique for achieving this is through the use of trees and their known ability to soak up water from the soil and breathe it into the atmosphere through their leaves—the process of evapotranspiration. Ministry of Works scientists at Aokautere are engaged on research into the comparative rate at which different species of trees transpire. Root systems, too, are being studied since they have the capacity to bind the soil together. Two methods are employed to measure the rate at which trees exude water. This means that comparative sets of data can be correlated. The more valuable of these is by use of lysimeters, developed by Dr Ross Edwards relatively quickly in 18 months after a period of study overseas.

Although lysimeters have existed for more than 40 years it is the full automation of weighing operations that Dr Edwards developed in collaboration with the D.5.1.R., again based on other weighing installations. As solid state physics improves so do automation and accuracy.

The lysimeter consists of a metal tub one metre deep and with a diameter of 1.8 metres, filled with soil containing a growing tree. This sits in a close-fitting lined hole in its test site and on top of a carefully calibrated cantilevered weighing arm.

As the tree draws moisture from the soil and breathes it into the air the lightening load is accurately weighed electronically and the results transmitted to a recorder several hundred metres away in Dr Edwards’ office.

The electronic equipment also ensures that the soil in the tub is rewatered when a certain level of dryness is recorded.

Dr Edwards is now monitoring lysimeter data for four trees, a radiata pine, a poplar, a willow and a eucalypt. Daily and seasonal rates of transpiration vary considerably and over a year provide an accurate picture of the characteristics of each tree.

Transpiration rates are quite significant. During one 24 hour period in November the pine transpired as much as 54 litres, the poplar 45 litres, the willow 21 litres and the somewhat smaller eucalypt 20 litres.

The other method of moisture data collection from the trees is by means of electric probes inserted into the trunks to measure the rate at which the sap rises. This can be accurately related to the rate of transpiration.

If, as expected, it can be established over a period that the probe technique is

accurate and reliable then it can be applied to full field trials (particularly on hill slopes) of many different tree types and ages to give scientitsts the sort of answers they are seeking in different environments. According to Dr John Hawley, scientist in charge at Aokautere, considerable research benefits have come from solid state physics. “Electronic instrumentation for data collection has all got, cheaper and more

accurate and rugged, and we I are taking advantage of it," I he says. t Dr Hawley says farmers > have been planting trees in [ hill country to control ) landslips for many year, but there is still no certainty about the effects of the trees t on the soil. > It had still be be properly > established whether the real value came from the - strenght of the roots or from j the drying out of the ground. ? “Historicallly. the ap-

proach has been to tackle the problem ‘from the outside' by planting trees and waiting to see whether they prevent slips. “The difficulty is that you have to wait for a major storm and then make comparisons between the treated (planted) area and neighbouring untreated areas with equivalent rainfalls. Variations in rock type, soil type, steepness, aspect, land management and other factors have made these comparisons maddeningly inconclusive.

"So we are looking at the problem 'from the inside'—at the development of failure conditions within the soils.

“By placing instruments half a metre into the ground and observing the pattern of moisture extraction by the trees, we have a chance of showing what can be expected from particular tree species on particular types of soils,” says Dr Hawley. The research will not eliminate completely the slips due to wet weather, but should help to control the rate of slipping and thereby retain the productivity of the soil, one of our most valuable .resources, and keep New Zealand green a little longer.