Afforestation versus Reforestation in New Zealand: Effects on Regional Climate

Land use and land cover change (LULCC) are significantly affecting the regional greenhouse gas concentrations in New Zealand (NZ). In NZ with the arrival of the Europeans in the 19^th century the native forests were reduced to 24% of the land area. Reforestation occurred, especially in the North Island with planting of 1.8 million hectares (mha), 89% are Monterey pine trees. Another 1.2 mha are being planted through to 2027 and to date 88% of reforestation is done with Monterey pines. These trees emit relatively large amounts of monoterpenes whereas the NZ natural forests do not. There is extremely limited afforestation (regrowth of natural forest that has been cleared) as these much of these areas have been reforested with exotics. Any changes in the regional atmospheric chemistry associated with the plant-emitted gases can directly impact the lifetime of methane. We used a regional climate-air chemistry coupled model to investigate these changes. Results indicate that for NZ the extra monoterpenes released from the area of Monterey pine plantations increase the methane lifetime in the forest canopies from 12.4 to about 15.0 years. The longer lifetime of methane increases the Global Warming Potential over a 20-year period (GWP₂₀) of methane from 84 to 111 (relative to carbon dioxide). Such changes in GWP₂₀ for methane mean an underestimation of its contribution to regional warming, increasing from 42 to 50% of total greenhouse gas emissions in carbon dioxide equivalent. The increase in warming caused by methane has dramatic implications for NZ emissions reduction targets. These results show the significant impacts of changes in LULCC on region GHG emissions and climate.