Will wide-spaced silvo-pastoral plantings maintain soil carbon stocks?

Abstract

Silvo-pastoral systems are common globally but not so in New Zealand, where there are limited examples within pastoral landscapes. Pastoral soils in New Zealand have relatively high soil organic carbon (SOC) stocks by global standards, and tree planting may reduce these. This study aimed to investigate whether wide-spaced tree planting into pasture would maintain SOC stocks after 25-27 years, extending the data previously reported at 14-16 years after planting on two North Island pasture sites. Two deciduous tree species were planted in the late 1990s in part-Nelder experimental designs – poplars (Populus deltoides × P. nigra) at a summer-moist site, and alders (Alnus cordata) at a summer-dry site. Soil OC stocks to 600 mm depth were re-measured in 2023, under open pasture and two tree stem densities at each site, along with pasture herbage accumulation and micro-climate variables. Soil OC stocks had continued to decline under the poplars but were relatively stable under the alders. Pasture production was c. 60% of open pasture under the poplars but not reduced under wide-spaced alders (120 stems/ha). The nitrogen-fixing capability of the alders may be an important function in this silvopastoral context, offering nitrogen input for additional biomass C sequestration without compromising pasture productivity and SOC stocks.