The trouble with pines
Trees ‘‘suck’’ carbon dioxide from the air in a process called sequestration. The faster the tree grows, the more efficient it is at sequestering carbon. The planting of trees is frequently praised as a solution to the build-up of atmospheric carbon dioxide, the main greenhouse gas contributing to our changing climate.
To reach carbon neutrality by 2050 new forests must be planted. ‘‘Failing to plant billions of trees in coming decades would be an expensive mistake,’’ Stuff national correspondent Charlie Mitchell wrote. But the composition of these forests has been heavily debated.
Pinus radiata, a humble pine tree, grows like a weed. But they have been adopted in the fight against climate disruption due to their ability to grow quickly and hoover up CO2 while still young. Plantations can then be cut down and sold, providing an immediate economic benefit.
By age 27, a typical pine plantation in New Zealand will be harvested, having sequestered up to 779 tonnes of carbon, more than twice what a hectare of indigenous forest sequesters at double the age. This data is from a Ministry of Primary Industries report which aims to help small forestry owners calculate carbon credits.
However, the in-depth research into radiata sequestration is not matched for indigenous forests, and therefore the idea pine trees could be a more viable solution to carbon sequestration cannot be taken for granted. Pine tree data in the MPI report is sourced from pines all across New Zealand, while indigenous forest data is lumped together to represent all our native species. Is it fair to say pine trees are more capable of sequestration when the appropriate research isn’t being accounted for on both sides of the story?
However, radiata might not be the answer to our carbon prayers. Net sequestration of pine forests does not account for emission sources such as forest management (roading, pruning, planting etc), the transport of harvested logs, and the processing and manufacture of forestry products, which occur with every rotation of pine tree forestry. Over 83 per cent of the New Zealand plantation harvest is exported, emitting thousands of tonnes of carbon along the way. Therefore, modelling by Landcare researcher Larry Burrows and Di Lucas, Director of Lucas Associates, has shown average carbon stocks from pine plantations will decline until the process emits more carbon than pine trees sequester.
Pine plantations also damage our native biodiversity. Clear-felling damages the lower levels of forest which host our native creatures. Disruption to the land removes carbon from the soil, and makes it more vulnerable to rain and erosion.
Pine trees aren’t our saving grace in the form of an exotic tree. But trees could still have an answer for us pining humans. While at the beginning of their life they have a slower sequestration rate than pines, the total carbon stocks achievable from permanent native forests exceed those of native plantations. However, we do not need to start destroying our pine plantations. Dr Adam Forbes from Forbes Ecology has conducted research on gradually replacing pine plantations with native forest by using the exotic trees as a nurse canopy for natives.
It allows native species to establish by creating a micro-climate similar to native bush, and avoids mass clear-felling. Eventually, the land use moves from pine forest to native forest. In this way, we could get the best of both worlds; reinstate our native forests and reap the benefits of pine sequestration.