By international comparison, New Zealand’s per-capita contribution to carbon emissions is very high. But so, too, is its potential to mitigate climate change by planting forests to sequester large amounts of carbon quickly.
There is sometimes passionate debate about how best to do this. Should we continue establishing radiata pine plantations or focus instead on planting New Zealand native trees?
Arguments for and against each option exist – but there is also a third way that could achieve the best of both worlds: planting radiata pine forests that are not harvested, but instead transitioned over time into native forests through targeted management.
We need to cut emissions drastically. But we must also remove CO₂ from the atmosphere as much as possible, especially over the next 20 years. A transitional forest model is a powerful way to help achieve this.
Farming carbon using trees
As trees grow, they absorb CO₂ from the atmosphere and lock the carbon into wood, leaves, roots and soil.
The New Zealand Emissions Trading Scheme (ETS) provides income from growing trees to store carbon. It is a key tool for meeting domestic and international climate change targets, including the 2050 target set by the Climate Change Response Act 2002.
A newly planted native forest will absorb approximately 40 tonnes of atmospheric CO₂ per hectare over ten years. By contrast, an exotic radiata pine forest will achieve five to ten times this amount over the same period.
In other words, absorbing a given quantity of carbon during the early stages of reforestation will take five to ten times more farmland using natives. Because of this enormous advantage of exotics over natives, there is a place for exotic carbon farming.
Some object to pine planting on purely aesthetic grounds – they don’t like the look of radiata forests. And we agree there are some places where pine is inappropriate for the landscape. But the urgency to mitigate climate change means we need to turn as much unprofitable pasture into forest as possible.
Radiata forests are also criticised for being monocultures that lack biodiversity. But the pasture they replace is also a monoculture with even less biodiversity. Planting trees on pasture also reduces gross emissions by reducing animal stock and, therefore, methane emissions.
We can’t plant too many trees
A year of emissions in Aotearoa, New Zealand, equals 78.8 million tonnes of CO₂ equivalent, based on 2020 figures. To offset this for ten years would require planting roughly 20 million hectares of pasture in native trees and then waiting ten years for them to grow.
The total area of Aotearoa is 26.9 million hectares, with 3 million of those being mountains. Therefore, another treeless country of a similar size would be required to offset its emissions using native trees alone fully. Using radiata pine would require 2 to 4 million hectares.
At an individual level, just one return trip from Auckland to London for one person will produce approximately 11 tonnes of CO₂ emissions. To offset this would require planting over a quarter of a hectare (almost an acre) of native trees and waiting ten years for them to grow.
On current projections, Aotearoa must purchase 100 million tonnes of offshore carbon credits to meet its international commitments. According to Treasury calculations, this will cost between NZ$3.3 billion to $23 billion between now and 2030.
The country cannot offset all its emissions by planting trees, native or exotic. Reducing emissions is the priority. But from a climate perspective, we cannot grow too many trees.
Restoring biodiversity over time
One of the criticisms levelled at exotic carbon forests is that the carbon storage is not permanent because of the shorter lifespan of pine. But pine plantations in New Zealand can keep accumulating carbon for at least a century if they’re not harvested.
Also, the carbon storage is permanent if exotic forests are transitioned into self-sustaining native forests. This process occurs naturally but can and should be accelerated by targeted management.
Because radiata pine needs much light to grow, its seedlings will not establish beneath its canopy. Therefore, pine will naturally decline over time and gradually be replaced by native forests, which occur naturally but take many decades.
To provide crucial structural and species diversity and to expedite the transition process, native trees requiring plenty of light need to be planted, and pine trees need to be thinned. This is unlike commercial harvesting, so the problems associated with forestry “slash” do not arise.
Fruiting natives will attract birds and enhance seed dispersal. At the same time, the income from carbon credits through the ETS can be used for further plantings and to fund intensive animal pest control – a critical step towards rebuilding native forests.
Eventually, this strategy will provide permanent carbon storage and capture that continue way beyond a century. But within decades, we would also see the return of large areas of highly biodiverse native forests.
Sebastian Leuzinger, Professor of Environmental Science, Auckland University of Technology and Len Gillman, Professor of Biogeography, Auckland University of Technology
- This article is republished from The Conversation under a Creative Commons license. Read the original article.
