Carbon models for planted natives
To determine the potential of a species for carbon sequestration, tree measurements and models for predicting carbon sequestration from these measurements are required. Tāne’s Tree Trust has obtained growth data from native New Zealand tree plantations throughout the country. Refer to articles in Section 10 of the Tāne’s Tree Trust Technical Handbook – e.g. Bergin and Kimberley (2012).
These planted stands cover a wide range of commonly planted native shrub and tree species established for 10-100 years. Tree measurements have been used to develop growth and carbon models to determine typical carbon sequestration rates of planted New Zealand native tree and shrub species. Refer to Kimberley, Bergin and Beets (2014).
The Intergovernmental Panel on Climate Change (IPCC) suggests forest carbon should be estimated for five carbon pools – Above Ground Biomass, Below Ground Biomass, Dead Wood, Litter, and Soil Organic Matter. The carbon calculator only models the first two of these pools (above and below ground biomass). However, change in carbon in the other pools is likely to be minimal for several decades after planting. Forest litter and dead wood will eventually build up but they will only form a fraction of the biomass of the living trees producing them. Soil organic carbon also only changes gradually. Litter and fine root material generally decays rapidly and releases carbon dioxide back into the atmosphere although there can also be some transfer of carbon into the soil organic pool resulting in a gradual increase of soil carbon under trees.
Over 120 planted stands of native trees and shrubs were inspected nationwide and assessed for growth. Predominantly in the North Island, they ranged from lowland coastal sites to inland sites over 500 m above sea level. While most native tree plantations were of a single species, virtually all shrub and small tree plantings consisted of a mixture of species.
Almost 10,000 planted trees and shrubs were measured during the survey, ranging in age from 3-110 years. Most stands were less than 50 years of age. Stand density averaged 1900 stems/ha for high forest trees and 3500 stems/ha for shrubs and small trees which are often used for establishing an initial cover on open sites.
Measurement of planted stands
In larger stands of planted trees, Permanent Sample Plots (PSPs) were established within representative areas of known age and management history. PSPs were established using circular or square plots of up to 400 m2 in area. From 1-6 plots were established at each site, depending on the range of species and ages and extent of the stand. In smaller stands of trees and in mixed-species shrub or small tree plantings, unbounded inventory growth plots were used.
Age of stands since planting was determined from stand records and landowners. Stand stocking was calculated from the number of trees within bounded plots or estimated in inventory plots using a sample of intra-tree distances. DBH (diameter at breast height - measured 1.4 m above ground level) was measured for tree species and heights measured for a sample of trees using a Vertex Hypsometer. Due to multiple leaders and heavy branching from near ground level, root collar diameter (RCD - diameter measured 10 cm above ground level) measurements were usually used for shrub and small tree species. However, DBH was measured in some older stands of small tree species.
For the PSPs, diameters of all planted trees and shrubs within plots were measured. For inventory plots, diameters were measured for a representative sample of up to 30 plants of each of the major species.
Native species planted
Over 60 different native tree and shrub species were encountered during the Tāne’s Tree Trust survey of planted stands. Species were classified into one of four species groups:
- conifer trees
- other hardwood trees
- shrubs and small trees
Of the more than 10,000 trees and shrubs assessed during the survey, 6600 were conifers, 800 beeches, 900 other hardwood trees, and 2200 shrubs and small trees.
Species planted included the major timber tree species and a wide range of smaller tree species and shrubs, the latter comprising hardy early successional species used to assist establishment of native trees. The most commonly planted native species represented in the Tāne’s Tree Trust Indigenous Plantation Database include:
|Species group||Common or Māori name||Botanical name|
|Beeches||Red beech||Fuscospora fusca|
|Silver beech||Lophozonia menziesii|
|Black beech||Fuscospora solandri|
|Other tree hardwoods||Taraire||Beilschmiedia tarairi|
|Small trees and shrubs||Wineberry, makomako||Aristotelia serrata|
|Cabbage tree, ti kouka||Cordyline australis|
|Lacebark, houhere||Hoheria species|
|Mahoe, whiteywood||Melicytus ramiflorus|
|Mapou, red matipo||Myrsine australis|
|Lemonwood, tarata||Pittosporum eugenioides|
|Lowland ribbonwood, manatu||Plagianthus regius|
|Fivefinger, whauwhaupaku||Pseudopanax arboreus|
Estimating carbon sequestration in native forests
Equations for predicting carbon sequestration for native trees have been derived by SCION for the Ministry of the Environment using biomass and tree measurements from a database of felled or fallen trees (Beets et al., 2012). These equations predict the carbon stored in a tree from measurements of tree diameter and height. These can be converted into a carbon value per hectare by multiplying by the stand stocking.
The equations were developed mostly from old-growth natural forest stands. The only biomass study of a native plantation in New Zealand appears to be a study conducted by SCION of trees thinned from a 69-year old Taranaki kauri stand. The estimate of carbon dioxide sequestration obtained from this stand was 1306 t/ha. Applying the carbon equations to the tree height and diameter measurements from the stand gave a very similar estimate of 1326 t/ha, giving confidence that the carbon equations developed from natural stands can also be applied to planted native stands.