Greenhouse Gases
Equipment used to collect the soil surface CO2 efflux in order to measure its isotopic composition.
Outcome: Improved measurement and mitigation of greenhouse gas emissions from the terrestrial biosphere
Modelling the carbon sink potential of New Zealand’s exotic forests
In a significant project for Ministry of Agriculture and Fisheries, our researchers modelled wood productivity of Pinus radiata across New Zealand in response to the wide range of environmental variables that affect tree growth and carbon accumulation.
The CenW model generates ‘productivity surfaces’ (similar to a contour map) for the whole country, showing in great detail the regions with expected high and low productivity. Measurements came from stands of different ages of up to 32 years, grown at different stockings, and subjected to thinning and pruning at various stages throughout their growth. Predicted growth rates corresponded extremely well with actual measurements in these stands across the wide range of conditions.
Stand productivity was found to be particularly sensitive to annual air temperature and total annual precipitation, with optima reached at 12–15°C and 1500–2000 mm, respectively.
‘Current temperatures are therefore generally sub-optimal for growth in all regions other than Northland, and precipitation is close to optimal. Soil fertility was also generally found to be adequate for most sites. Highest productivity was modelled for the moderately wet and warm northern and western regions of the North Island. Lowest growth rates were modelled for cold sites at higher elevation, for the dry eastern areas of the South Island, and also for the extremely wet sites on the West Coast of the South Island,’ says researcher Dr Miko Kirschbaum.
The model also allowed researchers to assess the likely growth rates under future climatic conditions. They ran model simulations under three different greenhouse gas emissions scenarios and under climate-change predictions from 12 different global circulation models. When researchers ran the model holding CO2 concentrations constant, they found slight growth reductions in the warmer north of the country and increases in the cooler south. For the country as a whole, there was only a minor change in predicted wood productivity, Dr Kirschbaum says.
‘When increasing CO2 concentration was also included, productivity responses were generally positive, with average increases in wood production of 19% by 2040 and 37% by 2090. These responses varied regionally, ranging from relatively minor changes in the north of the country to very significant increases in the south, where the beneficial effect of increasing CO2 combined with the beneficial effect of increasing temperatures. These relatively large positive responses to CO2, however, can only be realised if the current high fertility levels in most commercial plantations can be maintained. There is also still some scientific uncertainty with respect to the extent of the CO2 response, especially under water-limited conditions.’
Managing and projecting changes in carbon storage by native forests
Meanwhile, MAF contracted our researchers to improve understanding of carbon currently stored in native vegetation, the rate at which carbon accumulates at present (‘business as usual’), and how ‘management’ options could optimise carbon sequestration in native forests.
This is because projected changes in the amount of carbon stored in our forests are required for New Zealand’s ongoing climate change negotiations, and while detailed empirical (observed) data and supporting modelling exist for exotic production forests, there is little known about indigenous forests.
Researcher Fiona Carswell says human-induced disturbance – such as logging, clearing, and burning – has produced a significant shift in the age-structure and composition towards young or regenerating forest types. Such forests are currently active sinks of carbon dioxide. MAF is interested in whether they can be managed to absorb more carbon dioxide. LUCAS (Land Use Carbon Analysis System) data from the National Vegetation Survey Databank (NVS) was used to quantify the actual carbon stocks by current vegetation type and by region. Calculations take into account carbon in the live biomass of tree stems, branches and roots; standing dead stems; coarse woody debris; and shrubs. Each of the 1300 plots was measured between 2002 and 2007.
Dr Carswell says the effect of human disturbance on total carbon content is being quantified with a complementary mix of plot measurements and satellite data. Plot-based variables include the presence/absence of grazing (managed stock only), clearing, mining, fire, logging, and the presence of tracks. From the data we can assess the level of ‘naturalness’ of the vegetation cover at each location.
Data are being integrated to model potential carbon stocks in the absence of all such disturbances. Our research will consider the scale at which optimal management could maximise gains or minimise losses in carbon sequestration (beyond current management) over the next couple of decades. This will help MAF to make an informed decision about ‘business as usual’ sequestration in indigenous forests, what actions are needed to increase the sequestration to optimal levels, and what the risks of reversals are.