Indigenous and plantation forests will help us get to net zero and reverse climate change – but we need to decide what type of trees will do the best job and where they will be planted.
Our nation of 5 million lives in a 27 million hectare subtropical paradise with fantastic potential for forestry. By expanding indigenous and plantation forests we can get to net zero emissions, absorbing and sequestering more greenhouse gases than we emit.
We have committed ourselves to net (allowing for forest absorption of carbon dioxide) greenhouse gas neutrality by 2050, and we now have to figure out how to get there.
Forests provide oxygen, carbon sequestration, enhanced and improved biodiversity, economic returns, employment, taxes, wellbeing & support of rural communities, wood, paper, erosion control, habitat, recreation, naturally durable woods to replace treated pine, heritage and cultural values.
Carbon sequestration must be part of a wider discussion and plan for Aotearoa, with government policy and financial support, to maximise these benefits.
In 2016 our Government and Opposition jointly commissioned the Globe study, by Vivid Economics, to explore how to get to greenhouse gas neutrality. The authors reported that we might reduce total emissions (approximately 80 million tonnes of CO2 equivalent annually) to zero by about 2085, but that also planting new forests would get us to net neutrality by 2050.
Dr David Evison, at the School of Forestry, produced a helpful graph with annual emissions on the y-axis, showing us what this means:
The red line shows what we can achieve in Aotearoa if we seek to reduce gross emissions to zero, while the green line shows the path to net greenhouse gas neutrality by 2050. Students of geometry will notice that the blue triangle has the same area as the triangle formed between the red and green lines, a triangle we can fill with carbon sequestered by forests.
We need new forests because although converting low vegetation to forest stores extra carbon in the landscape, forests do not increase their carbon forever; forestry can buy us time while we figure out how to live without emitting greenhouse gases, but it is not a long-term solution to climate change.
How many hectares of new forest we need, and costs of conversion and forest management, depend on what species we establish, where we establish them, and how we manage them. These are vitally important decisions.
Our Climate Change Commission recently released a report suggesting how we might begin reducing our gross emissions, but also that we should embark on a 17-year programme to plant areas of both exotic and native forests.
Our model estimates the annual carbon sequestration of those new forests assuming they would be planted on erosion-prone land, mostly in the North Island, and compared them to sequestration required over the next 60 years to keep us at net greenhouse gas neutrality after 2050 (assuming that we continue to reduce our gross greenhouse gas emissions to zero by the early 2080s):
The red line shows the gap to be filled, according to Vivid Economics. The purple line shows pine sequestration if it is unharvested and eventually converted to native forest (Dr Adam Forbes, an ecologist, demonstrated that we can plant rapidly-growing pine that is intolerant of shade and transition it to slower-growing but shade-tolerant native species).
Light blue shows sequestration or emission from pruned and harvested pine. Dark blue shows domestic carbon credits earned by pine forest owners under our Emissions Trading Scheme if long-term storage from growth and harvesting cycles is averaged. Brown shows sequestration by native forest using the Ministry of Primary Industry’s (MPI’s) “lookup” table of native forest carbon storage versus time after planting.
Several things are clear from the graph:
* None of the proposed forest plantings will get us to greenhouse gas neutrality by 2050;
* “Averaged” NZ carbon credit entitlements don’t properly represent medium-term impacts of periodically harvested forest on the atmosphere; and
* Establishing just under 300,000 ha of native forest will do very little to get us the greenhouse gas neutrality by 2050.
Several forestry options would fill the gap in our national accounts. We are likely to choose a mixture of options, but examining them in isolation helps us gauge their efficacy.
1 / Plant native forest
We could completely fill the gap with sequestration from planted native forest. Using MPI’s tables of annual carbon sequestration in native forest, we need 160,000 ha of new native forest each year for the next 25 years, for a total of 4 million ha of new native forest. Our native forest estate would expand from 6.5 million ha to 10.5 million ha, covering almost 39 percent of Aotearoa.
However, we’ve only rarely approached 100,000 ha/year of new plantation forest in the past. We would need to expand our nursery facilities and conduct intensive research to improve the efficiency and scale of our seed collection, storage, nursery practice and forest establishment for a variety of species.
A conservative estimate of the cost after that research is $20 billion spread over 25 years. We have approximately 1.3 million ha of erosion prone land where it makes sense to farm carbon, and so we might also displace 2.7 million ha of hill country farmland for the new native forest.
This forest would not necessarily look like the image above, but could in many cases be restored, diverse native forest in threatened habitats where original forest types have been eradicated.
2 / Unharvested exotic forest ultimately converted to native forest
If we established rapidly growing exotic species (such as the large trees in the image above) to fill the gap in our accounts then we could use Dr Forbes’ research findings to eventually convert them to slower growing native forest.
This conversion might take many decades, but he explored options for accelerating the process. In the absence of local, native seed sources we’d need one hectare of native planting for every 10 hectares of exotic forest.
Simulations suggest that we might need as little as 700,000 ha of such forest to fill the gap in our accounts, with 32,000 ha planted/year over 22 years. This could be on erosion-prone land and is feasible with little additional research.
It would cost about $1.7 billion spread over 22 years. With appropriate policy it could earn valuable carbon credits on erosion-prone portions of farms while farming continued to maintain rural communities.
3 / Mostly harvested exotic forest
We collaborated with Dr Evison to devise a planting programme that would fill the gap in our carbon accounts while allowing harvests of pine. The way to avoid periodic deficits in carbon accounts after harvesting is to spread the planting over the average period of a commercial crop rotation (about 27-29 years).
Gradually increasing planting from 20,000 to 90,000 ha/year over 29 years on a total of 1.6 million ha of land, with 75 percent of the area pruned, periodically harvested and replanted, and 25 percent transitioned to native forest and never harvested, would cost about $3.2 billion over 29 years.
However large financial returns from NZ carbon credits and harvesting would make this the most financially valuable option so far considered. Our area of exotic plantation would almost double, becoming just over 12 percent of our land area, but 1.6 percent of the national land area occupied by exotic species would ultimately be transitioned to native forest.
4 / Improved pest control in our existing native forest and extended rotations in existing plantations
Committing to net greenhouse gas neutrality by 2050 is very different from previous climate change commitments involving a reference year, such as 1990 or 2005, because carbon sequestration in forests existing prior to reference years did not earn NZ carbon credits.
Greenhouse gas neutrality involves no past reference year and so changing the way we manage our existing 6.5 million ha of native forest and 1.7 million ha of exotic plantation forest might improve our carbon accounts.
Increasing rotation ages by one year in our plantations, for instance, might add over 30 million tonnes of CO2-e to their long-term average carbon storage. Current carbon credit prices provide plenty of incentive to add years to rotations.
Pest management in some types of native forest will protect them and increase carbon storage, and because the areas of these forests are vast, this may help fill our carbon account gap and could pay for more pest control. We need experiments and permanent sample plots in native forests accompanied by remote sensing, and more biomass assays of native species, to quantify this.
Some urban myths about radiata pine forests need to be countered, such as:
a) they are responsible for most wilding outbreaks (they are not);
b) they so acidify the soil that nothing will grow under them (untrue); and
c) they are always devoid of biodiversity (they aren’t).
Dangerous global heating, is already deadly and causing significant damage. In 2021 the world has now warmed by 1.2 degrees above pre-industrial temperatures of 1850. It remains to be seen if the world can limit global temperature increase to 1.5 or even 2 degrees, but in Aotearoa we can reach greenhouse gas neutrality by 2050 by working strategically together.
Forestry has an essential role to play, and we must consider the alternatives outlined here and make the right choices. The way in which we use forests to reach our goal is up to you as a citizen, influencing government policy, along with owners of the land where forests might grow and store carbon.
