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TECHNICAL REPORT
GREENHOUSE ALLIES PROJECT
Measurement of carbon sequestration
in small non-industrial forest plantations.
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Summary
The New England, Northern Rivers, Mid North Coast and Lower Hunter Regional
Plantation Committees, Northpower*, and Southern Cross University jointly
received project funding in 1999 under the Greenhouse Allies Program of the
Australian Greenhouse Office to:
- build capacity among the private commercial tree growers in northern New South
Wales to measure the sequestration of greenhouse gases by their plantations;
- raise awareness of the importance of expanding private plantation forestry activities
in greenhouse mitigation strategies;
- form linkages to ensure appropriate support is provided to the growers to develop
their carbon mensuration capacity; and
- field trial the newly-developed Greenhouse Challenge Vegetation Sinks Workbook
(VSW) (Forestry Technical Services, AACM International & Clean Commodities Inc.
for the Australian Greenhouse Office 1998).
This study was the first use of the VSW in Australia since its release late in 1998, and
has provided an opportunity for refinement. The resulting data on carbon
sequestration will be useful for the National Carbon Accounting System, and,
perhaps, carbon credit trading by forest owners, subject to the further development
of the Kyoto protocol and domestic policies. Carbon trading, however, is outside the
scope of this study.
The results of this study provided carbon acquisition figures for native species and
selected exotic species in non-industrial plantings on farms in northern New South
Wales. The Australian Greenhouse Office aimed to achieve the following from this
project:
- some simple but reasonably accurate estimates of carbon presence in ecosystem ‘pools’
(trees and roots, soil, litter, other vegetation and wood products);
- a better understanding of the difficulties experienced by small landholders in
measuring for carbon; and
- a refinement of the equations and estimates in the VSW.
This was achieved through:
- measuring a broad range of plantations;
- harvesting six species;
- a comparison of modelled and measured carbon figures;
- monitoring the level of accuracy, precision and time taken for the various measurement
techniques;
- liaison with small landowners and farmers; and
- consultation with expert advisors and a literature review.
As owners of farm forests do not generally measure and monitor their trees, this
project also aimed to encourage tree growth monitoring by the owner.
This overview contains a summary of the technical component of this project, in
particular a report on the field trial of the VSW, new records of the carbon stored in
tree and soil pools, a comparison of the results obtained using various techniques for
estimation of the tree pool, and comments on the requirements for building the
capacity of private commercial tree growers to participate in the sequestration of
greenhouse gases.
The plantations assessed in this project were supplied by a number of ‘Greenhouse
Allies’, landowners with existing plantations and an interest in the sequestration of
carbon. A total of twenty five Allies from Newcastle to the Queensland border
participated in this project. The plantings were limited to those established after 1990,
as required for inclusion under Article 3.3 of the Kyoto Protocol.
The major ‘pools’ of carbon measured were those in the soil and the trees. At the
majority of sites, a pair of soil pits were dug to 1m or to bedrock, one in the
plantation and one in an adjacent paddock. These pits were usually not replicated at
each site but were used to give an idea of the pattern of soil carbon content in the
region. At six of the sites, trees were harvested and their biomass measured both
above and below ground. At the remainder of the sites the trees were measured in
situ. These non-destructive measurements were converted to biomass and thence
carbon content by (a) using the calculations proposed in the VSW, (b) using the
relationships established from the harvested species, or (c) using existing data in the
literature.
The standing carbon on a property was estimated by treating the various ages,
species, or site types present in a planting separately and then combining the
separate estimates to obtain an overall estimate and the confidence interval for the
estimate.
Testing the VSW Wood density
The wood density of the (young) plantation trees varied from those in the published
literature, sometimes significantly (Figure a). As the wood density is used to convert
volume to biomass this can make a big difference to the estimated biomass of a
planting and hence the carbon stored. The wood density figures varied slightly
according to the position on the tree from whence they came, but a height of 1.3 m
(the position of diameter at breast height) appeared to be an appropriate median
point.
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Figure a: Wood density at 1.3 m (diameter at breast height) taken for species occurring in the
plantations being measured for the Greenhouse Allies project. These are given as basic densities
(oven dry) and are compared with figures from Bootle (1983) for mature trees where available.
Harvest results
The results of the biomass harvests of the six species sampled are presented in a
number of ways: (i) as a Harvest Index, (ii) as a root:shoot ratio, and (iii) in the
development of correlations between various components of tree biomass and the
diameter at breast height over bark (dbhob, taken at 1.3 m). The Harvest Index is a
forestry measure of the biomass of the tree bole, or log, in relation to the total
above-ground biomass, while the root:shoot ratio is a measure of below-ground
biomass to above-ground biomass. These ratios are used in the VSW and similar
assessment procedures to estimate biomass from non-destructive measurements
such as diameter at breast height and the height of a tree across a range of species.
Correlations (also termed allometric relationships) between easily measured field
parameters such as dbhob and tree biomass obtained in harvests such as these are
useful, as total tree biomass is difficult and expensive to measure in the normal
course of events. Often these correlations are assumed to be species-specific,
implying a suite of harvests for each species studied: the general applicability of such
correlations across species was tested as part of this project.
The six species analysed showed a significant variation around the published (VSW)
estimates of Harvest Index of 0.68 for Pinus radiata and 0.7 for eucalypts, and the
root:shoot ratio of 0.2 (Table a). This variation could be due to a number of actors,
including the young age of the trees (more leaf to stem than in older individuals) and
the species or site.
Table a: Harvest indices and root:shoot ratios for the six species harvested. The Vegetation Sinks
Workbook (VSW) recommends a Harvest Index of 0.68 to be used for Pinus radiata and 0.7 for
eucalypts. There are no figures for rainforest species. The VSW recommends a root:shoot ratio to be
assumed to be 0.20 for all species. Means and standard deviations are shown.
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The allometric relationships for the harvested individuals were very reliable, and,
within the range of diameter at breast height encountered for those species,
provided a reliable estimate of biomass. It was interesting to note that the difference
between the allometrics for the species harvested was not definable, and one
equation could have been used for all six, regardless of whether they were eucalypts,
silky oak or radiata pine (Figure b). This deserves further testing, particularly across
age classes, as, if generally applicable, it could improve the estimation of standing
biomass for a wide range of species, and negate the need for a limitation of species
for which carbon accounting can apply.
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Figure b: The relationship between total biomass (dry weight) and the diameter at breast height
over bark (dbhob) when all the non-coniferous species are amalgamated, and with the coniferous
species.
Comparison with other allometrics
The estimate of standing carbon using the VSW is purposely conservative, and
proved, in this project, to be around half the amount of standing carbon estimated
using either harvest allometrics derived in this study or other equations from the
literature (Figure c). The data shown here is for the two best studied species in the
literature, Pinus radiata and Eucalyptus grandis. The similarity between the harvest
estimates and the estimates using equations published in the literature is
encouraging. With less well-known species, such as E. microcorys (tallowwood) and
Grevillea robusta (silky oak) no equations exist in the literature and the match
between our harvest estimate and that using equations for other species is less good.
These data are significant in demonstrating the advantage of using relationships
specifically derived for the region, species and age classes rather than conservative
default values.
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Figure c: Comparison between estimates of standing carbon by plot for Pinus radiata (top) and
Eucalyptus grandis (bottom) using: (i) the modified Vegetation Sinks Workbook estimate described
in this report; (ii) the relationship between biomass and dbhob obtained from harvest figures; and
(iii) the relationship between biomass and dbhob obtained from published allometric equations
(Pinus radiata - Baker et al. 1984, and E. grandis - Cameron et al. 1989). The numbers shown are for
stratum and plot within each site.
Plantation estimation
There are two components to this calculation: (i) the assessment of standing carbon,
and (ii) the assessment of the precision of the estimate. The calculation of the carbon
contained in the tree pool of the plantations showed some substantial carbon stored
in the trees, particularly in the Northern Rivers area (Table c). This is very promising,
especially given some of the small areas involved. The amount of carbon stored was
highly dependent on the age of the planting.
Both the assessment of plantation carbon and the confidence of the estimate depends
greatly on the accuracy of the plantation area and stratum area, and this calculation
could not be conducted for several plantings because of difficulties in this regard. The
age of the planting also affected the accuracy of the estimate, as plantings less than
two years had consistently high confidence intervals (low precision), making the
assessment unreliable. This error was also the result of inappropriate sample size,
and the selection of optimal sample size and replication needs further work,
particularly for mixed species plantings. The poor confidence of some of the
estimates of property carbon could be greatly improved by (i) an accurate estimation
of planting and stratum areas, and (ii) more and/or bigger plots. This can be
improved in the future using the knowledge obtained in this project.
Table c: The standing carbon estimated using the VSW protocol for six properties in northern NSW
together with a 90% confidence interval for the estimate. Note data for Coward, Johnston, and
NSW SF Walcha are not listed here due to lack of data on stratum areas, and Greening Australia,
Armidale is not listed due to lack of full data provided. The plantings of Jones, Dickson, Thomas and
Williams were too young to be measured at this stage.
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Soils
The soil was assessed at only one set of paired pits per site, so little can be said about
soil conditions within any of the sites. The data show, however, that most of the
carbon in the soil is held between 0 m and 0.5 m. Several of the sites are deeply
ripped, and significant carbon lies between 0.3 and 0.5 m. For rapid and economic
assessment, therefore, and providing replication, samples could be taken to that
depth and a good estimate of the total carbon at a site be gained. An example of the
pattern of carbon is shown in Figure d for the Envirocom planting at Mebbin in the
Tweed Valley.
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Figure d: Soil profiles of percentage carbon content (left) and estimated carbon content for
plantation and paddock soils of one property in the Northern Rivers region of New South Wales.
A comparison of the soil carbon to 0.5 m at the Allies’ properties shows a significant
amount of carbon held in the soils in some areas ranging from 50 tonnes of carbon
to nearly 400 tonnes of carbon per hectare (Figure e). The median value lies around
150 tonnes per hectare. Any variation between plantation and paddock cannot be
discussed as the degree of variability was found, in one replication of pits in a
paddock, to be up to 43%.
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Figure e: Total soil carbon to 0.5 m depth for all sites and situations measured in northern New
South Wales as part of the Greenhouse Allies project. No variation is shown as each of these is a
singular measurement.
Total carbon acquisition rate per property
The amount of carbon fixed per year for the measured properties gives us a good
idea of the potential of the plantings in greenhouse gas reduction. An estimate of the
amount of carbon stored in the year immediately prior to the measurement of the
trees in 1999 or early 2000 gives a good idea of the possible rate of acquisition for
these young plantings both now and in the future. This last year is taken, as, when
the plants are young (for example 0 to 4 years old), there is little actual growth while
the plants become established, so an estimate incorporating the growth in those
years as an indication of future potential would be misleading. An estimate of the last
year’s growth is obtained using the exponential growth pattern of all plant (and
indeed animal) life (Waring and Phillips 1970, Charles-Edwards et al. 1986):
This function describes the growth of the plant from its early establishment in the
paddock (the lag phase) to the exponential phase where it grows most vigorously.
This relationship is not likely to be suitable for older trees undergoing linear growth,
or for those in the maturation phase, when the plant begins to reach its final size,
which might be expected in many of these plantings start at a minimum of 30 years
of age. Using this equation, the amount of carbon acquired during 1999 shows a
marked effect of age, but also an effect of, presumably, site quality (Table d). Sites
like Envirocom, showing a carbon acquisition rate of 13.95 tonnes ha-1 of carbon
between 4 and 5 years of age, may be compared with 7.85 t ha-1 acquired at the
Moody property by trees of the same age, commensurate with the much lower
rainfall at the Moody property. Internal stratum effects are not shown here, but
considerable variation between strata on the various properties was evident,
particularly in the case of Fayle, where one stratum, in a gully, had a very high
growth rate in comparison to other strata. These factors may well prove most
valuable in future planning for plantation establishment and management, once
error affects like accurate stratum areas are corrected.
Table d: The carbon acquisition rate for the last year of growth for each of the measured properties
using a fitted exponential growth model with time since establishment. Note that the rate per year
is highly dependent on an accurate stratum area estimate.
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this is equivalent to a CO2 fixation rate of 47.37 tonnes ha-1 yr-1. In addition to this, the
carbon on the properties studied is mainly held in the soil, with, in some instances,
one hundred times the amount of carbon held in the top 50 cm in comparison to the
plantation. This, in older plantations with increasing biomass, is reduced to ten times
the amount of carbon held in the soil than in the plantation. This illustrates the
desirability of management practices which minimise early soil carbon losses, for
example minimising tillage.
As an exercise, these data can be projected to estimates of carbon acquisition per
year for the national carbon accounting commitment period between 2008 and 2012.
These dates are the years between which the national emissions of carbon are to be
assessed and compared with 1990 levels: we, as a nation, have been allowed 108% of
1990 levels by this period.
In order to estimate what sort of growth these plantations may be doing in 2008 and
2012, a little ‘crystal ball gazing’ needs to take place. We can assume we have two
measurements, one at planting (an average seedling) and one at the measure taken
in 1999 (or early 2000). Using the exponential growth equation above, we have
estimated the rate of growth in 1999 and converted that to carbon acquired. Taking a
conservative assumption that after 1999 growth continues at the same rate as in
1999, we can estimate a standing carbon in 2008 and 2012 (Figure f). This is a
conservative estimate, as some increase in growth rate might be expected to occur
after 1999, particularly for the younger plantings (such as those less than 5 or 6 years
of age in 1999) but without further measurements any variation is risky. Indeed,
some trees may die, or the planting may be thinned in this intervening period. The
large interval between 1999 and 2008, without any information between, needs to be
filled, and then a more realistic estimate can be obtained. A measurement in, say
2004, of the plantings would be a good time to test the projections.
Figure f: A graph of the carbon acquisition per property observed from planting date to
measurement in 1999 of plantings at four properties in northern New South Wales using the
exponential growth equation. This is extended to the national carbon accounting commitment period
between 2008 and 2012 by taking the 1999 growth rate and applying it to every subsequent year.
Conclusion
Using the approaches outlined in the Greenhouse Challenge Vegetation Sinks
Workbook and standard field sampling methods, the amount of carbon held in
small-scale farm forestry plantations on the north coast of New South Wales can be
estimated with good reliability. The reliability depends on good planting area figures
and is poorest for plantings less than 2 years of age. The potential for error in mixed
species plantings appears higher than in monocultures, but this can be improved by
further refining the measurement protocols.
New material developed for this project which will improve the use of the VSW and
may alter the approach taken includes:
- wood density figures for young plantation trees–the age of the tree affects the
basic density;
- allometric relationships between diameter and biomass for six species on farms;
- root:shoot ratios and Harvest Indices for six species;
- the discovery that the allometric relationships for all harvested species are the
same across species and species types within the age classes studied;
- identification of suitable published relationships for the harvested species;
- the component of the soil profile in which most carbon is held to determine the
most effective measurement depth (0.5 m); and
- establishment of a methodology for assessment of property biomass and carbon,
and the confidence of the estimate.
Useful additional work which could be undertaken to refine procedures and
estimates identified to date include:
- good replication of soil conditions at (at least) one site;
- robust diameter and height relationships for all species encountered;
- accurate planting areas and stratum areas;
- optimum sampling strategy, particularly for mixed-species planting (plot size and
number);
- carbon content in wood (variation around the 0.5 VSW figure);
- more allometric equations for diameter at breast height to biomass; and
- Site categorisation according to quality.
Once these gaps have been filled, a robust knowledge-base and methodology should
be available to landholders.
* 2003: Country Energy
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