Carbon in our native forests looks steady at first glance, but the way that carbon is stored is shifting. That could mean more emissions ahead. 

A new Manaaki Whenua–Landcare Research report commissioned by DOC analysed data from about 700 long-term forest plots remeasured over an average of 12 years. The researchers asked whether New Zealand’s mature native forests are holding, losing or gaining carbon, and whether browsing animals such as possums, deer and goats might be part of the story. 

How carbon pools are shifting and what that means for long-term storage 

Overall totals of carbon did not change, but the balance between carbon pools has shifted. Less carbon is being stored in living trees (known as live stem carbon) while more is building up in dead trees (called coarse woody debris). This change is mainly caused by the death of large trees, which hold a lot of the forest’s carbon. For now, the extra carbon stored in dead wood makes up for the losses from living trees, where mortality is now higher than growth and new recruitment combined, so total carbon appears stable. However, dead wood will eventually rot and release much of its carbon back into the air as carbon dioxide. Unless that loss is balanced by new growth and recruitment (young trees growing into big trees) our forests risk becoming net sources of carbon emissions rather than long-term stores. 

The change is most noticeable in forests where kāmahi is one of the main tree species, such as kāmahi–podocarp and kāmahi–southern rātā forests, which have lost significant amounts of their live stem carbon. These forests cover about 750,000 hectares across Aotearoa. Among all tree species, kāmahi showed the biggest drop in live carbon. This supports earlier research by Paul et al. (2021), which found significant losses of total carbon in kāmahi–podocarp forests. Kāmahi is a preferred food of both possums and deer. 

Are possums and deer to blame? 

Short answer: we still do not know for certain. Overall, none of the factors tested; herbivore activity, abundance of tree species preferred by deer or possums, forest type, climate or soil fertility, could explain changes in the total amount of carbon stored in living trees. 

That said, the report found some hints that browsing animals are part of the picture: 

  • Species favoured by possums generally gained less carbon than species they avoid. 

  • In a few plots, high possum activity was linked with little or no recruitment, suggesting possums may limit future growth in some forest types. 

  • Where ungulates such as deer had high activity, carbon in small stems of preferred species declined  while small stems of avoided species increased. There was no clear evidence that ungulates changed total live stem carbon across plots. 

  • Loss of carbon through tree mortality did increase where ungulate preferred species were abundant, but overall recruitment and growth were not affected. In many places, species that ungulates avoid appear to compensate for declines in those they prefer. 

These results point to a complex set of interactions. Browsers can change plant communities by reducing recruitment of preferred species and shifting forests towards avoided species. Over long time frames this could result in arrested succession or a replacement canopy made up of smaller or less dense wood species, which would store less carbon in the long run. Such legacy effects could take decades to become obvious in overall carbon numbers. 

Limits of the evidence 

The report emphasises that the tools used to estimate herbivore presence, such as chew cards and pellet counts, are fairly coarse. They are useful indicators but cannot precisely measure how many animals there are or what they are eating. Many tree species are eaten by more than one browser making their impacts difficult to untangle, and some of the most damaging impacts likely occurred before these forest measurements began. That makes it hard to assign cause with confidence from these data alone. More precise, long-term studies are needed. 

What researchers recommend and what ZIP is doing 

The report highlights priorities that ZIP shares and is already working on through the Native Carbon Initiative, including: 

  • Understanding the long-term impacts of possums on forest carbon by taking advantage of areas where they have been eliminated 

  • Remeasuring plot networks with and without sustained animal control to assess mortality and recruitment over longer time scales 

  • Improving measures of herbivore abundance 

The Native Carbon Initiative is initially focusing the majority of our efforts on kāmahi-podocarp forests. We’ve begun research to see if we can measure small carbon changes and show that they’re caused by controlling introduced animals. We are partnering with research organisations and government agencies to answer these questions. 

If you would like to read more about ZIP’s programme or support work to protect native forest carbon, visit the Native Carbon Initiative.