Accurate carbon counting
6 September 2018
Which plantation tree stores the most carbon? Scion has the answer thanks to MPI-funded research.
Scion has been contributing to the Sustainable Land Management and Climate Change Research programme (SLMACC) since it was set up by the government in 2007. Usually referred to as ‘slee-mac’, the programme aims to find out how climate change might affect New Zealand’s natural environment, primary industries, and local communities.
MPI celebrated the 10 year anniversary of SLMACC yesterday with the launch of a commemorative booklet showcasing some of the research projects, including eight carried out by Scion.
Three of the case studies are about counting carbon stocks.
We need to know how much carbon our forests have stashed away, and how much extra they capture and store as they grow. This allows us to calculate how much of New Zealand’s greenhouse emissions can be offset, and how much carbon forest owners participating in the New Zealand Emissions Trading Scheme (ETS) have.
The amount of carbon in a tree is related to its total biomass – the above ground parts of the trees, the roots and the deadwood and litter on the forest floor. One way to measure this is to cut down and dig up the tree, sieve the soil, dry all the biomass and weigh it. This is very expensive (and fatal for the tree).
Over a century’s worth of growing experience and research on radiata pine enable accurate modelling of trees’ carbon content that also takes into account tree age and where it is planted.
This works well for radiata forests, but does it apply to other plantation species grown in New Zealand?
Work in 2010 set out to more accurately quantify the carbon stocks in Douglas-fir, which makes up 6 per cent of New Zealand’s planted forests.
Douglas-fir from 10 stands around the country were cut down, dug up and cut up in a massive biomass measurement exercise.
The research team found that the biomass in stems, bark and tree crowns were higher than the radiata pine model predicted, while root biomass predictions were similar. Adjustment functions have now been added to the basic radiata model to improve the estimation of the carbon in Douglas-fir.
Eucalypts have a reputation for fast growth and may be more suitable for carbon plantations than Pinus radiata.
A 2012 project investigated the growth and carbon sequestration rate of Eucalyptus regnans and of E. globoidea, which has naturally durable wood, to provide more accurate tools and models for predicting carbon sequestration.
Taking measurements across a wide geographical range, the research team found that trees grew fastest in warm areas. E. regnans grew more quickly and sequestered more carbon than E. globoidea, but the wood of E. globoidea was denser. Both species captured and stored sequestered more carbon than radiata pine.
Bringing results to the tables
Foresters with less than 100 ha of forest can use MPI reference tables to ‘look up’ the amount of carbon stored in their trees. Actual field measurements are needed forests greater than 100 ha in area.
The look up tables are a series of pre-calculated values of forest carbon stocks, by age, for a given forest type with the carbon stock values expressed in tonnes of CO2.
When they were first published, tables for alternative species were based on best guesses based on radiata pine data and limited in age and geographical range.
Scion’s research on Douglas-fir and Eucalyptus has improved the tables. Further work with Manaaki Whenua has led to updates that more accurately reflect tree growth rates and carbon sequestration coast redwood, cypress, E. fastigata, E. nitens and key regenerating indigenous species.
Field based measurements have addressed some gaps although other large gaps remain as some species are not widely planted. However, moderately strong relationships were found between the growth of radiata pine and the growth of other planted species, so it is possible to predict the growth of other species.
More diverse forests
Under the ETS, forest owners earn carbon units for each tonne of carbon their trees sequester. Knowing how much carbon our forests can capture and contain and how much future forests could capture, combined with the rising price of New Zealand carbon credits will hopefully encourage people to plant more trees.
The updated information on the carbon captured by alternative species, together with more accurate look up tables, will help land owners and investors make decisions about whether tree planting is a good use of their land and money, and about which species to plant. The benefits of diverse forests and land use will go beyond simple carbon capture and provide specialty timbers, opportunities for new products and economic development and benefit the environment.