24 June 1996 Lincoln research brings clearer understanding to agroforestry

Domestic and international interest in Lincoln University's agroforestry trial remains high as the pinus radiata trees enter their seventh year in the ground on the six-hectare experimental site three kilometres from the campus.

Research projects associated with the trial range over many areas from the trees' root systems to their crown and canopy "architecture" and the fundamental matter of their competitiveness in a pasture/grazing regime.

"Understanding the interactions between trees and pasture is fundamental to the development of agroforestry and farm forestry systems," says Dr Keith Pollock of Lincoln University's Department of Plant Science.

The Lincoln experiment is taking place on a Templeton soil type and includes radiata pine undersown with five pasture types and a bare ground understorey; an area without trees sown with the same pasture types; and four tissue cultured genotypes and a seedling line of radiata pine.

Trees were planted in mid-1990 at 1000 stems per hectare (1.4 x 7m) spacing and have been thinned annually to give 200 stems per hectare this year. The trees have had two prunings to date. The pasture was forage harvested in the first three years and has been grazed by sheep since then.

Lucerne has been the most productive pasture. Carrying capacity and sheep liveweight gains have been about 20 percent greater in the open in the open and under trees. Fifteen percent of the pasture in the trees is still bare ground due to the original herbicide spraying for tree establishment. To date sheep have not been on the site in winter.

Pasture growth near individual trees has been reduced. At this site over 80 percent of the rain comes from the southwesterly quarter which has created a significant dry zone on the north side of each tree. The shady area to the south of each tree combined with tree root competition for moisture has affected ryegrass to a greater degree than other pastures. As the tree crowns become higher off the ground and tree roots spread out these effects are expected to become less localised.

Tree growth has been slowest with lucerne. Monthly soil moisture measurements showed that lucerne depleted the soil moisture more rapidly which created a longer summer drought period for the associated trees. Tree growth slowed considerably when the volumetric soil moisture content in the top metre dropped below 15 percent.

Among other investigations conducted at the site has been the development of a water­ use model for dryland agroforestry systems.

The work of doctoral candidate Blair Miller, supervised by Dr Graeme Buchan of the Department of Soil Science, this experiment sought to identify water use differences between the radiata pine trees and the pasture understorey in a six year old agroforestry system with a ryegrass and cocksfoot pasture.

Total system water use was found to be the same for both pasture types. However the trees in the cocksfoot showed reduced transpiration rates indicating that the understorey evapotranspiration from the cocksfoot was higher than the ryegrass. Tree growth rates in the cocksfoot for the current season have been similar to those of the trees in the ryegrass.

These results indicate that the trees in the cocksfoot have a higher use water efficiency as a result of increased competition for that water.

Other current work at the site includes –

• The effect of pruning on above ground carbon allocation and tree physiology in radiata pine.
• Internal cycling of nitrogen in second order branches of radiata pine
• Comparison of root growth of two radiata pine genotypes with and without a lucerne understorey
• Shade effects on pasture grasses under radiata pine
• Phosphate uptake by radiata pine roots/mycorrhiza under different pasture types.

The Lincoln University agroforestry trial has a number of unique aspects – it's the only one in New Zealand in a dry environment; it’s the only one using a range of different pasture covers, and it's the only one using different genotypes of pine tree.

Ian Collins, Journalist, Lincoln University, Canterbury, New Zealand.