The forestry sector has scrambled to respond to claims by Ravensdown scientist Dr Ants Roberts that converting land to pine trees can damage soils, taking a generation to get them back to a point where they are once again suitable for pasture.
Late last year Roberts was reported to have said that it can take 20 to 30 years to get soils back to pre-forest pastoral health once it is in trees, due to the acidifying effect of trees on soils and the loss of organic matter that contributes to earthworm populations.
Roberts based his claims on work done with Ngai Tahu on land converted from forest to pastoral use.
Scion has responded to Roberts’ claims, with principal scientist Dr Peter Clinton saying most soils in New Zealand tend to be acidic in nature, having all developed under forests.
“When forest is cleared to make pasture, soils need to be made less acidic through application of lime to reach a pH level that is best for pasture. It is no surprise to see those changes reversing when forest is re-established,” Clinton said.
He said when soil health has been measured under pasture, pine and indigenous forest, researchers have found soils under pine to be more similar to those under indigenous forest than they are to soils under pasture.
Pine-planted soils also mimic indigenous forest soils in terms of water and nutrient losses.
Clinton pointed out soil pH is only one measure of soil health, while compaction is another factor that needs to be considered. It occurs under pastoral conditions from animals and machinery.
NZ Institute of Forestry president James Treadwell was scathing about the initial report’s claims, asserting the report was misleading and that all NZ soils tend towards being acidic, with the most acidic existing under kauri forests.
He cited Scion research that highlighted pines’ positive effect on coastal sand dunes’ topsoil development and pointed to the need for pastoral soils to receive regular lime applications to maintain their lower acidic levels.
The benefits of forests when it comes to lowering erosion and carbon sequestration mean NZ needs more forests not less, Treadwell said.
He pointed to the need for more wood timber in the future, given the move away from more carbon-intensive products, like steel and concrete.