Planting trees on pasture can be good for soil health – Scion

Newshub story, highlighting changes in soils from converting pasture to plantation forests, has given a misleading impression of soil health, says Scion principal scientist Dr Peter Clinton.

Rather than damaging soil in New Zealand, planting trees on pasture restores soil to be similar to its original condition.

“Soils that have naturally developed under forest, like most soils in New Zealand, are acidic in their natural state,” Dr Clinton says.

“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’s no surprise to see those changes reversing when forest is reestablished.”

These changes have been well documented in long-term studies, Dr Clinton says.

“When we have measured soil health under pasture, planted pine forest and indigenous forest, we have found that soils under pine are much more similar to those under indigenous forest than they are to soils under pasture. In fact, lime needs to be regularly added to pasture soils to maintain the pH suitable for pasture growth.

“We see these similarities in a range of measures. Nutrient and water runoff under pine forest are much more like the nutrient and water runoff under natural forest than pasture too.”

Soil pH is only one measure of soil health that is different under forests.

“We see that soils under intensively managed pasture can be more compacted by animals and farm machinery than what we typically see under forested land.”

This can make it harder for roots to grow, and for soils to absorb water during heavy rain. In the worst cases, pugging of soil becomes a problem.

Most forests established under the Emissions Trading Scheme (ETS) have been on classes of land that have serious limitations for intensive agriculture but are well suited for forestry.

Additionally, well-managed plantations of trees on farms, exotic as well as indigenous, provide a range of benefits for sustainable and economic farm management.

Extensive research also shows that forested landscapes improve water quality and soil structure, reduce erosion, enhance biodiversity, and support regional economic development. Export earnings from forestry make a significant contribution to the national economy and locally grown timber is a key input to the New Zealand housing and construction industry.

Forests will also have an increasingly vital role to play in helping New Zealand meet its net-zero emissions targets by 2050 through carbon sequestration and providing feedstocks to meet growing demand for bioenergy, and for high-value products that offer an alternative to those made from fossil fuels.

“Forests will be at the heart of a low-carbon biological-based future and reduce greenhouse gas emissions on a scale unmatched by any current technologies.

“New Zealand needs to rapidly decarbonise the economy at the same time as establishing significant areas of new exotic and indigenous forest.

“Scion is committed to research that explores how current forest management practices need to be modified so that the value of forests can be optimised in terms of how they protect and enhance the environment to further benefit economic, social and environmental goals.”

  • NOTE:  The Newshub report which prompted this response was amended today to incorporate comment from Scion and the New Zealand Institute of Forestry.

The Newshub report said new research is showing just how much damage is being done to our land by pine forests.

The research suggests it can take 30 years for the soil to be suitable for pastoral use once again, after the trees are chopped down.

Ravensdown chief scientific officer Dr Ants Roberts has gathered statistics which he says may make farmers think twice about converting pastoral land to forest.

“It takes time and considerable time, 20 to 30 years, to get it back to full, what we would regard as soil health for pastoral farming,” he explained to Newshub. 

That’s because the soil in pine forests tends to be acidic, unlike the soil on a lifestyle block, and in the forest you don’t get nearly as much organic matter.

“You won’t find these in pine forests, these are pastoral earthworms,” Dr Roberts said, holding some up in his hand.

Additionally, forestry soil is looser and may be subject to erosion.

Experts always suspected that growing pines would have a huge impact on the soil but they didn’t know for how long.

Ravensdown and AgResearch scientists crunched the numbers from Ngāi Tahu land converted from forest to pastoral.

Dr Roberts said he was upset at the loss of top-quality pastoral soil when good pastoral land is converted to pine forests.

“Pastoral agriculture, the production of meat, wool, meat, fibre and milk is what drives the New Zealand economy. Yeah, we wouldn’t have half the stuff we have in this country if it wasn’t for agriculture earning overseas funds,” he said.

Sources:  Scion and Newshub




Author: Bob Edlin

Editor of AgScience Magazine and Editor of the AgScience Blog