Carbon stocks and sequestration from small tree patches in grassland landscapes in Aotearoa-New Zealand

Small patches of trees add structural heterogeneity to grassland landscapes, support biodiversity, and provide essential ecosystem services including carbon stocks and sequestration, erosion control, and shade provision. In Aotearoa-New Zealand (A-NZ) small patches of trees have further cultural significance within Te Ao Māori – the worldview of the indigenous Māori people. However, economic pressures have driven the management of grasslands either towards removing tree cover to enhance agricultural productivity, or large-scale conversion of grassland to production forests. Protecting existing small patches of trees, and encouraging their establishment across grassland landscapes, is challenging since the current extent and benefits of these patches is not known. This study demonstrates the critical contributions of small tree patches (those less than one hectare in area) in A-NZ grassland landscapes. Using a high-resolution tree cover dataset, we mapped 1,639,015 small tree patches in grasslands across A-NZ, with grassland small tree patches covering a total land area of between 185,589 ha and 187,928 ha. We used a probabilistic simulation approach to estimate carbon stocks and annual sequestration, revealing a potential aboveground carbon stock between 11.6 and 29.3 million tonnes (Mt) of carbon and an annual biomass sequestration of 0.3 Mt to 0.8 Mt. Despite their current exclusion from the national carbon emissions market, economic valuation suggests a liability of between $NZ 86.9 million and $NZ 8.6 billion (€47.9 million to €4.7. billion) if the small tree patches were felled, and a market value of between $NZ 2.0 million and $NZ 237.6 million (€1.1 and 130.9 million) annually for sequestration. Existing policies, including afforestation incentives, do not encourage protection or establishment of small patches of trees. Significant policy adjustments are required to recognize, protect, and incentivize the conservation and establishment of small patches of trees within grassland landscapes.Small patches of trees in grassland landscapes are overlooked by current climate policy and their benefits are rarely quantified.Small tree patches stored and sequestered substantial carbon, equivalent to between 2.9% and 7.8% of agricultural emissions.In addition, the patches support adaptation through co-benefits including erosion control, shade and shelter, biodiversity, and have cultural benefits and importance.Inclusion of small patches of trees in national emission reductions policies and schemes could further incentivize their protection and establishment.