Highlights: Greenhouse gases
Measuring the components of soil respiration. Image – John Hunt
Impact 1 Terrestrial greenhouse gas emissions and removals are understood and quantified so that changes in relation to management strategies, land–use policies and global change can be predicted.
Managing and projecting changes in carbon storage by native forests
Projections of expected changes in the amount of carbon stored in our forests are required for New Zealand’s ongoing climate change negotiations. Detailed empirical (observed) data and supporting modelling exist for exotic production forests but the information for indigenous forests is much less clear. To remedy this, the Ministry of Agriculture and Forestry (MAF) contracted us to improve understanding of (1) carbon currently stored in native vegetation, (2) the rate at which carbon accumulates at present (‘business as usual’), and (3) how ‘management’ options could optimise carbon sequestration in native forests.
Human–induced disturbance (e.g. logging, clearing, and burning) has produced a significant shift in the composition and age–structure towards young or regenerating forest types. Such forests are likely to have less carbon on average than forests affected by purely natural disturbance (e.g. storms, earthquakes). While the natural disturbance is unavoidable, proper forest management could remove the effect of human disturbance.
LUCAS (the Land Use and Carbon Analysis System) data from the National Vegetation Survey (NVS) Databank was used to quantify the actual carbon stocks by current vegetation type and by region. Calculations take into account carbon in live biomass of tree stems, branches and roots; standing dead stems, coarse wood debris, and shrubs. Each of the >1300 plots was measured between 2002 and 2007.
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.
Modelling the carbon sink potential of New Zealand’s exotic forests
In a significant project for MAF, we 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 model generates ‘productivity surfaces’ (similar to a contour map – see facing page) for the whole country, showing in unprecedented detail the regions with expected high and low productivity. These predictions corresponded extremely well with actual measurements in stands of different ages.
Stand productivity was found to be particularly sensitive to mean annual temperature, with 12–15°C being optimal, and to annual precipitation, 500–2000 mm being optimal. Currently, temperatures are generally sub–optimal for growth, and precipitation sub–optimal. Soil fertility was generally adequate for most sites. Highest productivity was modelled for the moderately wet, warm northern and western regions of the North Island, and lowest for cold sites at higher elevation, for the dry eastern areas of the South Island, and the extremely wet sites on the West Coast of the South Island.
Simulations under likely future climatic conditions suggested increases in productivity of 15% by 2040 and 25% by 2090.
Revisiting carbon losses from soil
Soil respiration is the largest source of carbon dioxide (CO2) from terrestrial ecosystems, equivalent globally to 10 times that produced by burning fossil fuels. Therefore, any change in soil respiration with soil and climate warming could have a major impact on the rate of global warming. However, understanding soil respiration is complex because the CO2 comes from two distinct components (respiration of roots and associated soil microbes; and decomposition of soil organic matter and litter) that might respond differently to warming.
Previous studies have suggested that the temperature response of organic matter decomposition in undisturbed soils might vary from that measured by laboratory incubations. Using a novel stable isotope technique to measure the two components of respiration in an undisturbed soil under young radiata pine trees, we found respiration from roots increased as the soil warmed but decomposition of soil organic matter did not. However, in laboratory incubations of disturbed soil, soil organic matter decomposition was sensitive to temperature.
The results imply that current models of terrestrial carbon dynamics overestimate future losses of CO2 from undisturbed soils, at least in the short term, and hence are likely to lead to overestimation of the rate of global warming.
Forestry changes in New Zealand 1990–2008
Forestry changes were assessed for the Ministry for the Environment (MfE), using LUCAS, firstly, to document the methods and uncertainties of the LUCAS land use mapping project, and secondly, to collate land–use–change data 1990–2008 to help assess conditions and trends of ecosystem services.
Between 1990 and 2008, 75,000 ha (± 6%) of land were deforested and 579,000 ha (±2%) were forested; the change sequestered 140 million emission units of carbon. If the trends continue through the Kyoto commitment period, the sequestered carbon due to forest change will be worth $150 million per year to New Zealand.
Afforestation is taking place in hill country in the North Island, which has extra benefits for erosion control. In the South Island afforestation is evenly spread around the regions, except Tasman and Westland. Deforestation is primarily taking place in the central North Island due to conversion to dairy farming.
Accounting for the effects of erosion in the soil CMS
Landslide erosion has a significant effect on soil carbon stocks and needs to be accounted for in the soil Carbon Monitoring System (CMS) model. All of the eroded plots we sampled had significantly lower soil carbon stocks than comparable plots with no erosion. Furthermore, the loss of soil carbon persists for a long time – 70 years after the landslide occurred, soil carbon stocks were still well below the value measured for no erosion plots (by c. 40% for scars and 20–30% for debris tails).
Gully erosion has a minor effect on soil carbon stocks because gullies generally only occupy a small portion (<5%) of the landscape.
Further work on landslides is needed in parts of the country that have a long history of landsliding, and which have had significant post–1990 storms, to refine models of soil recovery on landslide scars and debris tails. This would complement work to establish a national overview of the land area affected by landsliding each year.
Ammonia from animal excreta
Gaseous ammonia (NH3) is emitted (volatilised) from animal excreta as the urea content breaks down. Some of this NH3 is redeposited elsewhere and transformed into other nitrogen compounds, including nitrous oxide (N2O); hence NH3 emissions are accounted for as ‘indirect N2O emissions’ in greenhouse gas inventories.
The value of the volatilised fraction of excreted nitrogen (the emission factor) used in New Zealand’s greenhouse gas inventory was recently reduced, as recommended by a review for MAF in 2008. To obtain further data to support this decision, MAF funded a 2.5–year paddock–based project to measure NH3 emissions from cattle excreta. We led the project in partnership with Lincoln University and AgResearch. In the first experiment, cattle urine was applied in a regular pattern of patches; in a second, urine and dung excreted by cattle over three days were left in situ. We estimated emissions from measurements of concentration of NH3 at five heights in the paddock and at one height upwind.
The volatilised nitrogen fractions were similar in both experiments. Volatilisation from dung peaked later and amounted to a smaller fraction than that from urine. Both experiments were conducted at the warmest time of the year and the emissions were high, but our findings support the continued use of the present emission factor.
The implications are that our findings support the reduction in reported N2O emissions by 5%, or c. 600,000 tonnes CO2–equivalent. At an assumed trade price of $25 per tonne CO2, this would represent a saving of $15 million in greenhouse gas emission liabilities.
Impact 2 Strategies for land use and asset management increase carbon storage, mitigate greenhouse gas emissions and balance environmental, economic and social benefits.
Methane biofilters for dairy effluent ponds
We have been developing and testing methane biofilter technology to reduce emissions from farm dairy effluent ponds. For about 20 months and with minimal maintenance, a prototype methane biofilter has been successfully consuming >95% of the methane in biogas from a covered effluent pond at a Massey University dairy farm. This work confirmed that selected methanotroph (methane–eating bacteria) communities in an appropriate medium with adequate air supply can function efficiently through all seasons. However, preliminary measurements from a Southland farm show limited application for cattle wintering barns because methane concentrations were too low for our current filter design.
We are now beginning to test a second biofilter prototype, at the Massey University dairy farm, that is intended to provide optimal methane consumption at minimal cost. Over the next year, we will be optimising filter performance, then we will focus on encouraging activity of the most active methanotrophs.
Interest in the methane biofilter has come from DEFRA in the UK, and Pork Australia in Sydney. Through collaboration with the University of Western Sydney, we have been invited to join a consortium to tackle Australian cattle emissions, using biofilters.
Regional GHG inventory framework used in planning
Wellington City Council (WCC) has adopted and advanced our regional greenhouse gas inventory framework and emissions profiles for potential 2020 scenarios. We worked with the WCC to develop potential scenarios around trends in population growth, GDP, fuel use, transport (including airport expansion), energy prices, and energy generation mix. The work demonstrated potentially large variations in the city’s emissions profile – a 90% national renewable energy target decreased Wellington City emissions by 67%, a 3% rise in GDP increased emissions by 47%. The ‘ground–breaking’ results have been used to educate councillors on the impact of current trends and development strategies and in the development of the Wellington City Climate Change Strategy and Wellington 2040 Strategy. The use of scenarios in developing local climate change strategy has attracted attention from other councils.
Climate change and Māori land business opportunities
Of the 1.5 million hectares of Māori land (half of which is marginal for farming), 37% could be eligible for Kyoto forest and a further 15% has potential for reversion to scrub and forest. We collaborated with Gisborne–based 37 Degrees South Aotearoa to identify climate change business opportunities and critical success factors for Māori–owned land. Of opportunities identified, Māori ranked carbon–forestry sinks as highest priority, followed by land–use change and land–use flexibility. Other options were energy and renewable energy from sustainable wood products; energy efficiency, biodiversity and environmental services; and lowest priority, nutrient use and budgets, measurement technologies, anaerobic digestion, methane, and nitrous oxide abatement.
Climate–related business opportunities, such as new carbon afforestation and biofuels projects, could generate hundreds of millions of dollars into the rural and Māori economies, including much needed new jobs. Afforestation and reforestation on marginal lands in the Gisborne–East Coast Region of the North Island would also have a large number of environmental benefits such as providing greater resilience for landscapes and communities in the face of any increased incidence of extreme weather events.