Monday, February 26, 2024

Ag Symposium shines light on exotic and native trees

Neal Wallace
Fir, poplar can aid transition to restoring indigenous forests.
One aim of the eventual biodiversity credit system will be to incentivise the planting of native forests over pines under the Emissions Trading Scheme.
Reading Time: 2 minutes

Carbon-sequestering exotic tree plantations can be a transition to restoring native forests, new research has shown.

Associate Professor Janice Lord from the University of Otago Botany Department said introducing a biodiversity carbon credit system for landowners who enhance the environment would encourage native afforestation.

Speaking at an Otago University Ag Symposium last week, she likened the exotic vs native carbon sequestration debate to a tortoise and hare race and noted there is little data on the long-term sequestration benefits of long-lived natives such as kauri, totara and broadleaf.

But because of the slow establishment of these species, Lord said, they “do not pay the bills for the first 20 years”.

Research shows native tree species can regenerate under exotic canopies, allowing for fast, short-term carbon storage as a forest transitions from exotic to native species.

The key to this transition is light penetration through the canopy and forest management.

Lord said very little grows under densely planted radiata pine, but under poplars and douglas fir, extensive and diverse varieties of native plants do grow.

Biodiversity credits could encourage this kind of native afforestation but she said credits could also be earned from carbon stored in the soil under native forests.

Professor Janice Lord likens the exotic vs native carbon sequestration debate to a tortoise and hare race.

Lord suggested landowners monitor carbon levels before and after native tree establishment for eventual inclusion in emission reduction plans, even if they are not yet earning credits.

Controlling feral herbivores is another tactic to improve carbon sequestration. It has been estimated that possums eat 21,000 tonnes of vegetation a day on conservation land, equivalent to 1.8t of carbon dioxide sequestration potential per hectare per year.

Researchers have looked at afforestation with natives using drone application, but also agricultural machinery such as cross-slit drills.

Lord said drones can deliver seed precisely over large areas and to places that are inaccessible or recently disturbed. 

But the method requires large volumes of seed, does not suit all species and is still vulnerable to weeds and herbivores.

Using agricultural machinery is expensive, requires site preparation and maintenance, is limited to areas accessible by machinery and requires weed and pest control.

The practice is most successful in areas that are rough and have humps and hollows and bracken.

Biodiversity credits must be linked to verifiable projects and activities, she said, and should not obligate landowners to supply large volumes of additional information.

South Otago farmer Simon Davies told the symposium that credits need to differentiate between outcome and action, but reward those who are making a difference.

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