A report on the impact of deer on native forest carbon levels suggests fewer deer may not necessarily deliver an increase in forest carbon sequestration.
Undertaken by Manaaki-Whenua Landcare Research and commissioned by the Game Animal Council, the report found there was not a particularly strong case for culling deer numbers on the grounds this would increased carbon storage ability in forests.
It has been suggested by both government ministers and conservation groups that reducing deer numbers is one way to address New Zealand’s net carbon emissions footprint more quickly, by allowing more carbon to be stored in preserved forest.
In June, Climate Change Minister James Shaw told Farmers Weekly he was committed to having farmers’ efforts on pest and predator control acknowledged as a positive contribution to carbon sequestration on farm.
But the report has found the issue is more complex and often more counterintuitive when it comes to how deer browse and select plants for grazing in native forests.
It found attributing declines in forest carbon to wild animals is a fraught process because the evidence base and supporting data have seldom been collected in a scientific, empirical manner.
Report co-author and principal scientist in ecosystem ecology Dr Duane Peltzer confirmed the evidence is weak, and in fact the carbon storage potential of forests has been relatively stable for some time.
“The research shows that in some forest types, the presence of deer may have a small negative effect on carbon stocks, while in other forest types, deer will either make no difference or actually promote more carbon storage.”
The report found the effects of possums on carbon storage is significantly greater, given their treetop, leaf-eating behaviour, which tends to have a more direct impact on the mortality of large trees.
However even here the report states conclusions on outcomes on total forest carbon can vary widely. In some cases, the presence of possums actually accelerates the rate of woody species succession, by removing competitive species like grasses through grazing.
Generally what limited evidence there is has found non-palatable native species tend to double down on their growth rates when more palatable possum- and deer-friendly species are reduced in forest areas.
Peltzer said the report also found significant declines in carbon storage are far more likely to come from other factors, including large scale landscape disturbances like earthquakes, weather or even pathogens.
“There is evidence that deer management can have an influence on carbon storage in successional forests recovering from such disturbances,” he said.
“While potential carbon gains from deer and ungulate management are limited and variable, there is far greater evidence to suggest gains can be achieved in biodiversity, particularly among highly palatable species in the browse tier.”
Grant Dodson, chair of the Game Animal Council, said the report is important for helping ensure the right decisions are made when it comes to pest management.
Increasing pressure on future government budgets will inevitably affect pest management control, meaning management programmes have to be targeted to achieve the best possible outcomes for the dollar spent.
“This report makes clear that those outcomes are mostly in improving biodiversity.”
He said management for improved biodiversity protection is not the same as management in the expectation of increasing carbon storage.
The report concludes that significant carbon gains may occur in certain situations such as when herbivore control promotes the establishment of high-biomass wood species.
It states while carbon gains from herbivore control are unlikely to provide the silver bullet solution for conservation funding at a national level, there is potential in certain areas of NZ’s native bush areas for conservation benefits and carbon gains to go hand in hand.
Dodson said the case for site-based management strategies remains, where a balance between indigenous biodiversity and game animals’ community value is struck.
“The Game Animal Council is fully committed to achieving better outcomes for biodiversity as well as supporting effective measures to address climate change. However, wise investment decisions need to be made,” he said.