TESTING CARBON FARMING OPPORTUNITIES FOR SALINITY MANAGEMENT
A Scientific Report on the ‘Pilot to Test Carbon Driven Solutions to Salinity Project’
ISBN: 978-0-9923323-1-0
AUTHORS
Peter Ritson 1, Mike Clarke 2, Adele Killen 2,3 , Sarah Jeffery 3
1 [email protected] ; 2 Department of Agriculture and Food, Western Australia; 3 Northern Agricultural Catchments Council.
Copyright: This Testing Carbon farming Opportunities for Salinity Management: A Scientific Report on the ‘Pilot to test Carbon Driven Solutions to Salinity Project’ is © NACC 2015.
ACKNOWLEDGEMENTS
Funding for the study was provided by the Western Australian Government’s State Natural Resource Management Program.
The study was a collaborative project between the Northern Agricultural Catchments Council (NACC) and the Department of Agriculture and Food, Western Australia (DAFWA).
The following landowners generously allowed access to revegetation projects on their farms for measurements and sampling, including destructive sampling of some trees and shrubs: Paul Andrews (Perenjori), Gary Butcher (East Pithara), Chad Eva (Three Springs), Fiona Falconer (Coorow), David McFarlane (East Pithara), Terry McGlew and Barry McGlew (Perenjori).
Competent assistance with the extensive fieldwork (sampling and measurements) was provided by the following: Kim Brooksbank (DAFWA), Mike Clarke (DAFWA), Sally Fenner (NACC), Adele Killen (NACC), Katrina Kingston (NACC), Lizzie King (YYCMG), Heather Legge (NACC), Callum Love (NACC), George Morris (DAFWA), David Nicholson (DAFWA), Peter Ritson (DAFWA/FPC), Dene Solomon (YYCMG), Shelley Spriggs (NACC), Caroline van Tilborg (CarbonQuest Australia), Julia White (MCC), Alex Winter (DAFWA/FPC), and Byron Yeo (DAFWA/FPC).
Adam Peck (DPAW) provided a weigh trailer that was used to weigh some of the stems and crowns of trees felled for biomass determination.
Photographs were taken by Mike Clarke (DAFWA).
Peter Ritson (FarmWoods) compiled this Scientific Report with assistance mainly from Mike Clarke (DAFWA), Sarah Jeffery (NACC) and Adele Killen (NACC).
DISCLAIMER
The information contained in this publication is provided for general information purposes only. Neither the Northern Agricultural Catchments Council, nor any of the contributors, accept responsibility for any error or omissions it may contain, whether caused by negligence, or for any loss however caused, or sustained by any person who relies on it.
TABLE OF CONTENTS
Glossary and Codes and abbreviations ...... 1 Summary ...... 3
INTRODUCTION ...... 5
METHODS ...... 6 Site selection ...... 6 Carbon inventory ...... 9 Allometrics ...... 15 Carbon calculations ...... 19 Salinity measurements ...... 22
RESULTS AND DISCUSSION ...... 23 Species ...... 23 Allometrics ...... 24 Carbon stocks ...... 32 Salinity ...... 38 Species comparisons ...... 42
CONCLUSIONS ...... 50 Study design...... 50 Allometrics ...... 50 Effect of salinity on carbon stocks...... 51 FullCAM ...... 51 Species ...... 52 Recommendations for further study ...... 52
REFERENCES ...... 54
APPENDICES ...... 56 Appendix 1. Protocol for stem diameter measurements on trees ...... 56 Appendix 2. Estimates of species tree carbon stocks by salinity categories...... 58 Appendix 3. Method for projection of tree carbon stocks...... 61
GLOSSARY AND CODES AND ABBREVIATIONS
Glossary A mathematical equation that relates the growth of one part of an organism Allometric equation to another part or the whole organism, e.g. height to weight; stem diameter of a tree to total tree biomass. Biomass Sample Trees selected for destructive sampling to develop allometric equations. Trees (BSTs) Carbon Farming An Australian Government scheme that allows farmers and land managers Initiative to earn carbon credits by storing carbon or reducing greenhouse gas emissions on the land. These credits can then be sold to people and businesses wishing to offset their emissions. Carbon The removal of carbon dioxide (CO 2) from the atmosphere and storage of sequestration the carbon (C), e.g. in plants. Carbon stocks The amount of carbon in something, e.g. the amount of carbon in a tree may be measured in kg, the amount of carbon in a planting may be measured in t/ha. Diameter For multi-stem trees, the diameter equivalent at the measurement height is equivalent calculated as the diameter of a circle with area equal to the sum of cross- section areas of all stems at that height. Emissions A proposed Australian Government scheme that will allow applicants to bid Reduction Fund to provide offsets (emission reductions) in an auction process.
Mallee Typically short (2 to 5 m tall at maturity) woody plant, with many stems originating from a lignotuber. Sample plots A small area of land in which measurements are made on trees.
Shrub Typically very short (less than 2 m tall) woody plants. May have one, a few, or many stems.
Tree A tall (more than 2 m height at maturity) woody plant. In this study both tree form and mallee form were classed as trees. Tree form Typically tall (more than 5 m height at maturity) woody plant with one or few stems.
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Codes and Abbreviations BST Biomass Sample Tree C Centigrade. (°C indicates degrees centigrade.) CEA Carbon Estimation Area CFI Carbon Farming Initiative cm Centimetre
CO 2-e Carbon dioxide equivalent CVI Crown Volume Index DAFWA Department of Agriculture and Food, Western Australia DAH Diameter at Ankle Height DAHe Diameter at Ankle Height equivalent DBH Diameter at Breast Height DBHe Diameter at Breast Height equivalent DKH Diameter at Knee Height DKHe Diameter at Knee Height equivalent DPAW Department of Parks and Wildlife ECa Apparent soil electrical conductivity ECe Electrical conductivity of a saturated soil extract EM Electromagnetic Induction ERF Emissions Reduction Fund FPC Forest Products Commission FullCAM Full Carbon Accounting Model G Parameter in the FullCAM TYF that defines the age of peak growth rate. g gram ha Hectare Kg Kilogram Lat. Latitude Long. Longitude m Metre MCC Moore Catchment Council MSEP Mixed species environmental planting MSEPT Mixed species environmental planting temperate mm Millimetre mS/m Millie Siemens per metre NA Not available NACC Northern Agricultural Catchments Council NAR Northern Agricultural Region NLR NonLinear Regression PSP Permanent Sample Plot R2 Coefficient of determination R:S Root to shoot ratio TC Tree Carbon stock TYF Tree Yield Formula TSP Temporary Sample Plot t Tonne YYCMG Yarra Yarra Catchment Management Group
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SUMMARY
An emerging prospect for farm revenue from revegetation of saline and other lands that are marginal or non- productive for agriculture is the sale of carbon credits. Australian Government schemes for carbon credits include the Carbon Farming Initiative (CFI) and the proposed Emissions Reduction Fund (ERF).
Therefore, this study aimed to assess the potential for woody vegetation (trees and shrubs) established on and around salt-affected lands in the Northern Agricultural Region (NAR) to remove carbon dioxide from the atmosphere and store (sequester) the carbon in new growth.
Revegetation plantings on six farms were selected for study after the landowners expressed an interest in participating in the study. Criteria for selection included: a minimum of 5 ha planted on and around salt-affected land; a variety of native tree and shrub species planted; and that the planting be at least 10 years old. Across the six farms a total of nine sites were selected for study.
Objectives of the study were:
1. Obtain estimates of the average amount of carbon accumulated (carbon stocks) in each of the nine sites, and each of the 50 native species that were recorded across the sites. 2. Relate the carbon stock estimates for sites and species to salinity levels.
To those ends, a carbon inventory and salinity measurements were carried out on the study sites in 2013. Some key steps were as follows.
1. A network of 237 sample plots was established across the sites. 2. To indicate salinity level at each sample plot, apparent electrical conductivity (ECa) was measured with the GEONICS EM38 instrument. 3. All trees and shrubs in the sample plots were measured (total 5,170 trees and shrubs). For trees, stem diameter(s) were measured and, for shrubs, crown width and height was measured to calculate Crown Volume Index (CVI). 4. A sample of 313 trees and shrubs were also measured, then harvested (destructive sampling) to measure the biomass (and, therefore, carbon stock) of those Biomass Sample Trees (BSTs). 5. From the destructive sampling data, biomass prediction equations (allometric equations) were calculated for each species to estimate carbon stock from the non-destructive measurements (stem diameter or CVI). 6. These carbon inventory data were processed to calculate estimates of the carbon stock (kg) of each tree/shrub in the sample plots, then carbon stocks (t/ha) for each sample plot and the average and total carbon stocks for each site.
The estimates of carbon stocks in species and sites were projected forwards or backwards from the measurement ages (11 – 22 years) to age 15 years to ‘age standardise’ the results and facilitate comparisons between species and sites. This was done using the national carbon accounting model, FullCAM.
Some key results and conclusions were as follows.
• Effect of salinity on carbon stocks
Across all sites, carbon stocks in the revegetation decreased with increasing salinity beyond ECa 200 mS/m. There was no statistically significant difference between salinity categories of ECa < 100 mS/m and ECa 100 – 199 mS/m. For these categories the average Tree Carbon stocks at age 15 years ( TC 15 ) was 42 t CO 2-e/ha. However, TC 15 declined to 29 t CO 2-e/ha (68%) for sample plots with ECa 200 – 299 mS/m, and to 12 t CO 2-e/ha (29%) for sample plots with ECa > 300 mS/m.
These levels of carbon stocks indicate modest carbon credit returns from plantings with a similar mix of species. For example, if carbon credits are worth $20 per t CO 2-e, then, after 15 years, the returns from planting on non-saline (ECa < 200 mS/m) land would be $8,400 per ha. These returns could be increased by planting the more productive species identified in this study
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Planting on saline lands (e.g. ECa > 200 mS/m) would provide lower returns from carbon credits. However, if degraded land with little or no commercial value for agriculture, then carbon credits may provide some financial returns from the land.
• Species
There were substantial differences between species in the growth (carbon sequestration) rates. Therefore, if tree planting is done for a carbon purpose, large gains can be achieved by selecting the more productive species.
Tree form Eucalyptus species such as E. sargentii and E. spathulata , had clearly superior carbon sequestration rates. These species accumulated at least double the carbon stocks compared to the best performing mallee or shrub species in any of the salinity categories.
However, other species may be of more value for other benefits besides carbon revenue. Thus, mallee species such as E. kochii subsp borealis and E. loxophleba subsp lissophloia were originally selected for leaf oil production. Tree form species can provide fence posts and other farm timbers, and Melaleuca species can provide material for brushwood fences in residential areas. All local native tree and shrub species will provide biodiversity value with some, such as Melaleuca shrub species that are often prolific producers of flowers and nectar, having particular biodiversity value. The saltbush ( Atriplex ) species provide fodder for sheep and other livestock, and other salt-tolerant species, such as Casuarina obesa may also have fodder value. All revegetation will provide multiple other benefits such as reducing soil erosion and the spread of secondary salinity and the aesthetic value of putting a green belt around ugly salt scalds.
• FullCAM
Of the two versions of FullCAM used to predict carbon stocks at the nine study sites at the ages they were measured, FullCAM 3.40 under-predicted carbon stocks, except for two sites. They were the Eva site that had the highest salinity level (average ECa 245 mS/m) and the Butcher saltbush site that had a moderate salinity level (average ECa 153 mS/m) but the planting had few trees and mostly saltbush ( Atriplex species) that were grazed annually.
The more recent version of the model (FullCAM 3.55) gave substantially higher predictions of carbon stocks (average 280% higher). Hence, FullCAM 3.55 over-predicted carbon stocks in all but one site. At the Eva and Butcher saltbush sites the over-prediction exceeded 500%.
• Methods
The strategy of studying existing revegetation projects established by landowners in the NAR was an effective way of evaluating carbon farming opportunities for salinity management. An alternative would have been to establish new plantings, e.g. demonstration sites and/or field trials. However, apart from the cost of establishing new plantings, a considerable disadvantage of such an approach would be that it is necessary to wait for around 10 years, or more, before meaningful results could be obtained.
Of course longer-term monitoring is desirable. To this end all sample plots were permanently marked and every second sample plot on the study site was left undisturbed as a Permanent Sample Plot (PSP), available for re- measurement. One of the recommendations for future study, if funding can be obtained, is to re-measure the PSPs around three to five years after the first measurements. This will indicate which species are still healthy and growing vigorously, and which species are declining, e.g. growth rates have slowed or high mortality rates.
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INTRODUCTION
Secondary salinity caused by rising saline water tables is a major problem in the Northern Agricultural Region (NAR) of Western Australia and elsewhere throughout the Wheatbelt of Western Australia.
Options for reducing or reversing the spread of salinity include the establishment of trees and shrubs such as salt tolerant Casuarina , Melaleuca , and Eucalyptus species on or around saline lands. Such revegetation can provide additional multiple benefits.
Community benefits include the protection and enhancement of biodiversity values and contribution to global efforts to mitigate climate change. In the NAR there may be considerable scope for corridor revegetation projects, e.g. planting along salinized drainage lines.
Direct on-farm benefits, besides managing the spread of salinity, include reducing soil erosion. For grazing enterprises, benefits may include providing shade and shelter for livestock, and fodder production if salt tolerant fodder shrubs, such as saltbush ( Atriplex ) species, are planted. Depending on the species planted, farm timbers, such as posts and rails may be obtained. For commercial harvest a prospective species may be the Western Australian native sandalwood ( Santalum spicatum ). Suitable host species for sandalwood include a wide range of other native species. Sandalwood may be planted around saline areas and on other degraded or non-productive soils such as acid wodjil soils. There is also interest in commercial harvest of multiple products from mallee eucalypts (Eucalyptus species), including eucalypt oils and bioenergy products, but this is dependent on markets being developed.
However, an emerging prospect for farm revenue from revegetation of saline and other lands that are non-productive for agriculture is the sale of carbon credits. Australian Government schemes for carbon credits include the Carbon Farming Initiative (CFI) and the proposed Emissions Reduction Fund (ERF).
Hence, this study aimed to assess the potential for carbon farming to be an economic driver for revegetation to provide multiple benefits, including managing salinity on farms. In particular the aim was to investigate the potential for trees and shrubs established on and around saline lands in the NAR to remove carbon dioxide from the atmosphere and store (sequester) the carbon in new growth.
On saline lands the growth rates and, therefore carbon sequestration rates, may be slow. However, if the land has no economic value for cropping, then revegetation and sale of carbon credits (carbon farming) may be economically worthwhile.
Of particular interest in this study was how carbon sequestration rates vary according to the species planted and the severity of soil salinity. The knowledge gained and information products developed will aid landholders, government and industry to make informed decisions on carbon farming on and around saline lands.
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METHODS
SITE SELECTION
Six farms where cleared agricultural land had been planted with woody vegetation were selected for study. Criteria for selection were:
1. Located in the Northern Agricultural Region of Western Australia. 2. Valley floor or lower slope planting; 3. Affected by salinity; 4. A variety of native species planted, preferably including tree and shrub species; 5. Minimum 5 ha planted; and 6. Minimum 10 years since planting.
Potential sites were identified through a call for expressions-of-interest from farmers and through referral from NACC and DAFWA staff. All nominated sites were inspected, resulting in the final selection of nine sites on six farms (Table 1, Figures 1, 2). Long term average annual rainfall at all sites was around 300 mm.
Table 1. Sites selected for study.
Landowner Site Year Age when Description /farm Planted measured (years) Andrew Andrew 1996 17 Perimeter planted around a saline flat. Main species were mallee eucalypts ( Eucalyptus ) with Melaleuca (Melaleuca ) shrubs on more saline land. Butcher Butcher 2002 11 Mainly saltbush ( Atriplex) fodder shrubs with scattered saltbush Eucalyptus on a saline flat. Butcher Butcher 1996 17 Mixture of trees ( Eucalyptus ) and shrubs (mainly waterway Melaleuca and Atriplex species) planted along a waterway. Eva Eva 2000 13 Salt tolerant shrubs and trees (mainly Melaleuca , Atriplex , and Eucalyptus species and Casuarina obesa ) planted as a wildlife corridor along a saline valley floor with tree species (mainly Eucalyptus ) planted on the adjoining lower slopes. Falconer Falconer 2002 11 A variety of shrubs (mostly Melaleuca species) planted on three waterlogged and salt-affected flats with trees (mainly Eucalyptus ) and shrubs (mainly Melaleuca ) on the adjoining lower slopes. McFarlane McFarlane 1990 23 The east paddock had 3-5 row belts with salt tolerant E Eucalyptus trees planted on a waterlogging and salt- affected flat. Approximately 40 m cropping alleys between the belts McFarlane McFarlane 1995 18 The west paddock had 6-9 row belts (mostly 6 rows) with W salt tolerant Eucalyptus trees and some salt tolerant shrubs (mostly Melaleuca and Acacia species) planted on waterlogged and salt affected flats and gentle slopes. Approximately 150 m cropping alleys between the belts. McGlew McGlew 1998 15 2-4 row belts of mallees, either E. loxophleba subsp mallee lissophloia or E. kochii subsp borealis . Narrow (approximately 10 m wide) unplanted alleys between the belts. McGlew McGlew 1994 19 Small block planting alongside a saline flat planted with Melaleuca salt tolerant trees and shrubs ( Eucalyptus and Melaleuca species and Casuarina obesa ).
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Figure 1. Location of the six farms in the Northern Agricultural Region of Western Australia. Road centre line dataset Copyright © Western Australian Land Information Authority, trading as Landgate (2014).
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Figure 2a. ‘Andrew’ site Figure 2b. ‘Butcher saltbush’ site Figure 2c. ‘Butcher waterway’ site
Figure 2d. ‘Eva’ site Figure 2e. ‘Falconer’ site Figure 2f. ‘McFarlane E’ site
Figure 2g. ‘McFarlane W’ site Figure 2h. ‘McGlew mallee’ site Figure 2i. ‘McGlew Melaleuca’ site
Figure 2. The study sites
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CARBON INVENTORY
A carbon inventory of all sites was carried out in April to June, 2013. Generally the methods followed the Carbon Farming Initiative (CFI) ‘Farm Forestry’ methodology (Department of the Environment, 2014a, c). Key steps in the process were:
1. Establish a network of ‘sample plots’ throughout all sites. 2. Measure stem diameters or crown dimensions of each tree or shrub in the sample plots. 3. Harvest a sample of the trees and shrubs to determine the amount of biomass and, therefore, carbon in each of those Biomass Sample Trees (BSTs). 4. Calculate regression equations for each species that relate the ‘carbon stock’ in a tree or shrub to the easily measured stem diameter or crown dimensions. 5. Process the inventory measurements to calculate estimates of the carbon stock (kg) in each tree or shrub in the sample plots, then carbon stocks (t/ha) for each sample plot and the average and total carbon stocks for each site.
These steps are described in more detail in the following sections.
Sample plot location
The sample plots on a farm planting were located at the grid intersections of a randomly placed and oriented square grid. This is known as ‘systematic sampling with a random start’ (Schreuder et al., 1993, pages 53-56).
Figure 3. Sample plot location at the Eva site. Aerial photography Copyright © Western Australian Land Information Authority, trading as Landgate (2014).
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For example, Figure 3 indicates the sample plot network at the Eva site. In that case the grid size (selected to achieve 30 – 40 sample plots) was a square 1.35 ha (116 m x 116 m) grid. The location of one plot (the anchor point) was determined from random Easting and Northing coordinates from the range for the site, i.e. the ‘random start’. The randomly selected orientation for the grid was 11 degrees from true north.
The Excel spreadsheet used for the sample plot location is suitable for application to other sites and is available from NACC (contact Sarah Jeffery, [email protected] ) or FarmWoods (contact Peter Ritson, [email protected] ).
For the belt plantings at the McFarlane East and West sites ‘systematic sampling with a random start’ was also used but this involved sampling at regular intervals along the length of the belts (Figure 4). From the southern end of the far eastern belt, a path running up and down the belts was determined. Then, from a random start along the length of the first belt, plots were located every 1500 m of planting line. For example, where the plots were four rows wide the plots were located every 1500/4 = 375 m of belt, or where the plots were six rows wide the plots were located every 1500/6 = 250 m of belt. The first plot was located at a randomly selected distance along the first 1500 m of planting line.
Figure 4. Sample plot location at the McFarlane site. Aerial photography Copyright © Western Australian Land Information Authority, trading as Landgate (2014).
The target was to establish at least 30 sample plots on each farm planting and the actual number varied from 30 (Andrew) to 46 (Falconer).
Sample plots
Circular sample plots (Figure 5) were established in block plantings and in the McGlew mallee belt plantings. As the McGlew mallee belt plantings had narrow unplanted alleys between the belts the circular plots covered both belts and pasture alleys at that site.
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Tree centre
Boundary tree
Survey mark
Plot boundary
Figure 5. Example of a circular plot in a block planting. Boundary trees (at ends of rows in the plot) were marked with flagging tape to indicate which trees were in the plot. The survey marks were retained as permanent markers for the plot centres.
Belt plots (Figure 6) were established on the McFarlane belt plantings. Plot length was calculated so that the area covered by each belt plot was 0.04 ha (400 m 2).
x x x x x x x x x x x x
x x x x x x x x x x x x x A C B
x x x x x x x x x x x x x x
x x x x x x x x x x x x x x
Centre line x Tree crown (x Survey mark = centre of stem base) Plot Standard boundary margin
Figure 6. Example of a belt plot in a 4-row belt planting. Distance AC = Distance BC = half plot length. The standard margin distance was 2 m. Plot length (AB) was calculated to give the required plot area.
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Table 2 has some details of all sample plots. Plot areas were determined for each planting so that there were at least 20 planting positions in each plot, i.e. at least 20 ‘trees’ if 100% survival.
Table 2. Sample plot designs
Site Plot Plot area Comments type (ha) Andrew Circular 0.03 Butcher Saltbush Circular 0.02, 0.04 Nested plots, inner (0.02 ha) and outer (0.04 ha). Butcher Waterway Circular 0.04 Eva Circular 0.03 Falconer Circular 0.02 McFarlane E Paddock Belt 0.04 McFarlane W Paddock Belt 0.04 McGlew Mallee Circular 0.04, 0.06 Plot areas were 0.04 ha in E. kochii belts and 0.06 ha in E. loxophleba belts. All plots included belts and pasture alleys. McGlew Melaleuca Circular 0.03
Nested circular plots were established at the Butcher Saltbush site as the site had a high density of saltbush ( Atriplex species) shrubs (planted and regeneration from seed) and a low density of tree form Eucalypts. The saltbushes were measured in the inner 0.02 ha plots while the Eucalypts were measured in the 0.04 ha plots.
Some plots were edge plots, i.e. the centre point for the plot was in the planted area (including the 2 m standard margin) but some of the plot was outside. Usually for edge plots there was only one straight edge. In these cases the mirror (mirage) method was applied. The mirage method was first proposed by a German (Schmid-Haas) in 1969. For English language descriptions and discussions of the method see Loetsch et al . (1973), Beers (1977), Gregoire (1982), AGO (2002), or Mason (2005).
However, two of the edge plots (both in belts) were corner plots, i.e. at least two straight edges in the plot. In those cases the area weighted method was applied as the mirage method was not practical to apply. If there is more than one edge in a sample plot, the area weighting method (also known as the direct weighting method), although not bias free, is the least biased of alternative methods (Beers, 1977; Loetsch et al., 1973).
Tree measurements
As discussed below, stem diameter was measured on trees (tree form and mallee form) and Crown Volume Index (CVI) measured on shrubs. The objective was to establish a simple measurement for each species that could be used to indicate the biomass (carbon stock) of a tree or shrub. See Carbon Stocks, pages 32-37 for more descriptions of the allometrics methods.
Stem diameter measurements
See Appendix 1 for the detailed protocol for stem diameter measurements.
Stem diameters were measured at one of three heights (Table 3, Figure 7). Generally, stems were measured at the greatest height possible where there was at least one stem with a diameter greater or equal to 10 mm.
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Table 3. Stem diameter measurement heights.
Measurement Measurement Height Diameter at Breast Height (DBH) 1.3 m Diameter at Knee Height (DKH) 0.5 m Diameter at Ankle Height (DAH) 0.1 m
Figure 7. Stem diameter measurement with a diameter tape.
On tree form species live stems only at the measurement height were measured.
On mallees the diameters of all live stems and branches at the measurement height were measured. This was because it is often difficult to distinguish between stem and branches in mallees. The following species were defined to be mallees: E. comitae-valis , E. horistes , E. kochii , E. loxophleba and E. subangusta . All other tree species (see list in Table 4) were measured as tree form species. Note that the E. loxophleba planted on the study sites were E. loxophleba subsp lissophloia . These were classed as mallees but, on the study sites, usually had only two or three stems, i.e. not the many stems typical of the mallee form.
Where there were multiple stems (or multiple stems and branches on mallees) to be measured all stem diameters were measured and a diameter equivalent calculated as the diameter of a circle of area equal to the sum of cross-section areas of all stems and branches measured at that height