Development of growth and yield calculator
Tāne’s Tree Trust manages a national database on planted native trees and shrubs – the Tāne’s Tree Trust Indigenous Plantation Database – comprising measurements of 15,000 planted native trees and shrubs nationwide from 5 years to 110 years old. Measurements are from over 100 Permanent Sample Plots (PSPs) and growth plots established in planted native stands from Southland to Northland (Nationwide Survey of Planted Native Trees).
Analysis of the database indicates that some of our major native tree species planted on selected sites and with good management grow significantly faster than natural stands; this has substantial implications for productivity rates and carbon sequestration for planted stands that exceed current carbon estimates for native forestry in default tables based on regenerating native shrublands.
Tree measurements
Trees and shrubs were measured in plots of known horizontal area established within each planted stand. Bounded Permanent Sample Plots (PSPs) were sited within representative internal areas of each stand to reduce any forest edge effects. Temporary non-bounded inventory plots were used in shrub stands.
All stems were counted and stocking obtained by dividing the stem count by plot area and converted to a per hectare basis. The diameter over bark (DBH) at breast height (1.4 m above ground) was measured for all stems within each plot. Heights were measured for a sample of trees within each plot. For shrubs, because they often branch heavily from ground level or are multi-leadered from near ground level, stem diameters were taken at 10 cm above ground level.
Data analysis
Sigmoidal growth curves of the Chapman-Richards form were fitted using nonlinear regression. Fits of models using either separate slope or asymptote parameters for each species were tested. For both height and DBH, with models using separate asymptote parameters for each species performing best. For the height model, an intercept of 0.5 m representing height at planting was used. For the DBH models, a zero intercept at age 5 years was used, except for beech species where an intercept of 2 years was used, based on the average age at which breast height is achieved by native conifers and broadleaves.
Carbon equations (Beets et al. 2012) were then used to estimate the carbon contained in each measured tree. This system firstly estimates stem volume from the predicted DBH and height, and multiplies this by a tabulated value of wood density for each species to obtain stem biomass (or dry weight). Other equations are then used to estimate carbon in branches, foliage and roots. Total biomass carbon iss then converted to CO2 equivalents.
Model assumptions
Various assumptions are built into the calculator. Height and DBH growth rates are based on those measured in Tāne’s Tree Trust Indigenous Plantation Database. However, growth can vary greatly depending on site quality. The calculator allows users to account for site quality by specifying the site as Poor, Average or Good. Site quality adjustments in the calculator are based on the range of variability in growth represented in the database. Alternatively, growth curves can be calibrated or scaled for a particular stand using a stand measurement provided by the user. The database provides little information on the natural mortality which can be expected to occur in any forest stand. Currently, the calculator assumes an annual mortality of 0.6% for all tree species, and 3% for shrub species.
Future development
While the data is limited for many species especially for older stands increased planting over the last couple of decades are being added to the TTT Indigenous Plantation Database that are boosting the numbers of the major native conifer and hardwood tree species. These are established either as single species stands established at a regular stem density or in mixed species stands. Increasing numbers of stands are including nurse shrub species with trees interplanted.
The aim is to eventually provide species specific models on a regional basis as sufficient data is added to the database. This will allow the development of a family of growth curves for the key native tree species from slow to fast growing sites.
References
- Beets PN, Kimberley MO, Oliver GR, Pearce SH, Graham JD, Brandon AM. 2012. Allometric Equations for Estimating Carbon Stocks in Natural Forest in New Zealand. Forests 3: 818-839.
- Bergin DO, Kimberley MO. 2012a. Performance of planted native conifers. Tane’s Tree Trust Technical Handbook. Technical Article No. 10.2. 8p.
- Bergin DO, Kimberley MO. 2012b. Performance of planted native shrubs. Tane’s Tree Trust Technical Handbook. Technical Article No. 10.4. 8p.
- Kimberley MO, Bergin DO, Beets PN. 2014. Carbon sequestration in planted native trees and shrubs. Tane’s Tree Trust Technical Handbook. Technical Article No. 10.5. 12p.