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Biogeosciences, 8, 3687–3696, 2011
www.biogeosciences.net/8/3687/2011/
doi:10.5194/bg-8-3687-2011
© Author(s) 2011. CC Attribution 3.0 License.
Biogeosciences
Implications of albedo changes following afforestation on the
benefits of forests as carbon sinks
M. U. F. Kirschbaum1, D. Whitehead2, S. M. Dean3, P. N. Beets4, J. D. Shepherd1, and A.-G. E. Ausseil1
1Landcare Research, Private Bag 11052, Palmerston North 4442, New Zealand
2Landcare Research, P.O. Box 40, Lincoln 7640, New Zealand
3NIWA, Private Bag 14–901, Wellington, New Zealand
4Scion, Private Bag 3020, Rotorua, New Zealand
Received: 28 July 2011 – Published in Biogeosciences Discuss.: 24 August 2011
Revised: 5 December 2011 – Accepted: 5 December 2011 – Published: 15 December 2011
Abstract. Increased carbon storage with afforestation leads
to a decrease in atmospheric carbon dioxide concentration
and thus decreases radiative forcing and cools the Earth.
However, afforestation also changes the reflective properties
of the surface vegetation from more reflective pasture to rel-
atively less reflective forest cover. This increase in radiation
absorption by the forest constitutes an increase in radiative
forcing, with a warming effect. The net effect of decreased
albedo and carbon storage on radiative forcing depends on
the relative magnitude of these two opposing processes.
We used data from an intensively studied site in New
Zealand’s Central North Island that has long-term, ground-
based measurements of albedo over the full short-wave spec-
trum from a developing Pinus radiata forest. Data from
this site were supplemented with satellite-derived albedo es-
timates from New Zealand pastures. The albedo of a well-
established forest was measured as 13% and pasture albedo
as 20%. We used these data to calculate the direct radiative
forcing effect of changing albedo as the forest grew.
We calculated the radiative forcing resulting from the re-
moval of carbon from the atmosphere as a decrease in ra-
diative forcing of −104GJtC−1yr−1. We also showed that
the observed change in albedo constituted a direct radiative
forcing of 2759GJha−1yr−1. Thus, following afforesta-
tion, 26.5tCha−1needs to be stored in a growing forest
to balance the increase in radiative forcing resulting from
the observed albedo change. Measurements of tree biomass
and albedo were used to estimate the net change in radia-
tive forcing as the newly planted forest grew. Albedo and
carbon-storage effects were of similar magnitude for the first
Correspondence to: M. U. F. Kirschbaum
(kirschbaumm@landcareresearch.co.nz)
four to five years after tree planting, but as the stand grew
older, the carbon storage effect increasingly dominated. Av-
eraged over the whole length of the rotation, the changes in
albedo negated the benefits from increased carbon storage by
17–24%.
1 Introduction
Establishing forests has long been regarded as a useful green-
house mitigation option, as carbon is stored in trees rather
than residing in the atmosphere where it would lead to global
warming. However, the usefulness of that strategy was ques-
tioned through studies that explicitly considered the radiative
forcing associated with changes in energy absorption vs re-
flection (albedo) at the surface (Brovkin et al., 1999; Betts,
2000; South et al., 2011). Those studies showed that, de-
pending on the extent of albedo changes, incident radiation
and carbon-storage potential at different locations, the ben-
efit of tree plantings could be much diminished, or even be-
come negative at sites with extended snow cover and poor
tree-growth potential.
Betts (2000), in particular, concluded that in large parts of
the temperate and boreal regions, the overall radiative forc-
ing from a decrease in surface albedo by afforestation was
equal in magnitude to the radiative forcing of increasing car-
bon sequestration. The albedo effect is more important at
high latitudes because albedo differences between evergreen
forest and pasture or cropland are greatly accentuated when
snow covers the ground. In more temperate regions, like
New Zealand, snow cover is less important, and albedo ef-
fects resulting from vegetation shifts are therefore likely to
Published by Copernicus Publications on behalf of the European Geosciences Union.
3688 M. U. F. Kirschbaum et al.: Carbon, albedo and climate benefits
be less important than in boreal regions with extended snow
cover.
Bala et al. (2006) found that notional afforestation of the
whole southern hemisphere at mid latitudes would result in
a warming of 1.1◦C due to the direct albedo effect. More
specifically, Bala et al. (2007) found that forested regions
in the tropical zone had significantly increased cloud cover,
and that in this zone, the related change in cloud albedo was
therefore sufficient to offset any changes in surface albedo
for little effect on the overall radiation budget.
Similar studies were conducted by Zhang and Walsh
(2007), Davin et al. (2007), Findell et al. (2007),
Shi et al. (2007), Montenegro et al. (2009) and Pongratz
et al. (2011), who confirmed the basic mechanisms relat-
ing albedo and global temperature changes, but differed in
their numerical findings. These reported differences partly
reflected differences in the scenarios used or regions cov-
ered by different workers, and partly reflect differences in as-
sumptions or the completeness of factors that were included.
The fourth IPCC Assessment report (Forster et al., 2007)
also summarised studies that had estimated the change in
surface albedo with the expansion of agriculture since pre-
industrial times and concluded that land-use change is likely
to have led to a radiative forcing of −0.2±0.2Wm−2,
leading to a global cooling of about −0.1◦C. Detailed
studies also showed that within this broad global pattern,
there is likely to be significant regional variation from 0 to
−5Wm−2depending on the nature of the specific land use
changes in different regions (Forster et al., 2007).
In quite a different study, Juang et al. (2007) combined
modelling and observations and concluded that converting
a grassland to pine or hardwood forest resulted in an over-
all cooling by about 1.5◦C, with a cooling effect of about
2.5◦C due to greater latent heat (evaporative) and sensible
heat exchange of forests offset by albedo-caused warming by
about 0.8◦C. This study highlights the apparent contradic-
tion between global and local effects. Forests can be cooler
than neighbouring pasture, primarily because of the effects
of evaporation, which cools the local environment. However,
that local cooling is not relevant in the global context as evap-
oration simply transfers heat from the evaporating surface to
the atmosphere. When the water vapour condenses as liquid
in clouds or rain, the heat is returned to the atmosphere. On
the global scale, the effects of evaporation and condensation
cancel each other out with no net effect. Only effects on the
radiative balance remain.
Conventionally, the net radiation, Qn, retained by a body,
like the Earth, is given by the conservation equation:
Qn=Qs↓ − Qs↑ + Ql↓ − Ql↑(1)
=(1−α)Qs↓ + Ql↓ − Ql↑
where the radiation components are Qs↓downward short-
wave radiation, Qs↑upward reflected short-wave radiation,
Ql↓downward long-wave radiation and Ql↑long-wave ra-
diation emitted from the body and αis the albedo of the body,
defined as the fraction of downward short-wave radiation that
is reflected from the body (Landsberg and Gower, 1997).
Considering the Earth as a whole, incoming long-wave
radiation and emission of short-wave radiation are negligi-
ble. Equation (1) can therefore be simplified and extended to
give:
Qs↓ − Qs↑ = (1−α)Qs↓ = − Ql↑ = f (T 4)(2)
where Tis temperature in Kelvin. This shows that, follow-
ing the Stefan-Boltzmann law, the amount of long-wave ra-
diation emitted from the Earth is a function of its temper-
ature raised to the fourth power. Any decrease in albedo
leads to increased energy absorption by the Earth. To re-
gain energy equilibrium, outgoing long-wave radiation from
the Earth must be increased to match the increased absorp-
tion of short-wave radiation, which can only be accomplished
through an increase in temperature.
Values of albedo for darker coniferous forests lie in the
range 8–15% (Jarvis et al., 1976; Breuer et al., 2003;
Hollinger et al., 2010), and values for more reflective
pastures are usually 5–10% higher (Breuer et al., 2003;
Hollinger et al., 2010). A change in land use from pasture to
conifer forest therefore results in a decrease in albedo, which
has a direct effect on the surface radiation balance resulting
from decreased reflected short-wave radiation (Betts, 2000).
The increase in Qnleads to an increase in radiative forcing,
resulting in warming of the atmosphere. In addition, land-
use change can have indirect effects on surface radiation bal-
ance through changes in evaporation that subsequently affect
cloud formation, with effects on both short- and long-wave
components (Bala et al., 2006, 2007).
Direct and indirect effects of changes in albedo associated
with the conversion of pasture to forest are described in Fig. 1
(based on a figure from Findell et al., 2007). Storing carbon
in vegetation reduces atmospheric CO2and thereby allows
more long-wave radiation to escape from the Earth, with con-
sequent negative radiative forcing (term A). This beneficial
effect of tree plantings is partly offset by the direct effects
of decreasing albedo (i.e., less reflection), which means that
more short-wave radiation is absorbed at the surface (term
B). In addition, there are indirect effects of afforestation
(terms C, D) because forests evaporate and transpire (termed
evapotranspiration) more water than grasslands (e.g., Zhang
et al. 2001; Beets and Oliver, 2007), principally because of
higher interception of rainfall by forest canopies than shorter
grassland canopies (Jarvis and McNaughton, 1986; Beets
and Oliver, 2007).
The resultant surface radiation balance is complicated by
the resultant changes in the partitioning of available en-
ergy into latent heat (evaporation of water) and sensible heat
(terms C and D). Increasing latent heat flux reduces both the
amount of energy available as sensible heat and the local sur-
face temperature. That latent heat is released somewhere else
in the atmosphere when water condenses again, but the in-
creased amount of water vapour in the atmosphere absorbs
Biogeosciences, 8, 3687–3696, 2011 www.biogeosciences.net/8/3687/2011/
M. U. F. Kirschbaum et al.: Carbon, albedo and climate benefits 3689
Decreased albedo
Afforestation of pasture
Increased evaporation
Increased latent heat
Decreased surface temperature
Decreased
upward long-
wave radiation
Increased
low cloud
cover
Increased
water vapour
concentration
Indirect
Direct
Increased C stocks
Increased
upward long-
wave radiation
Decreased
atmospheric
CO2
Carbon
More short-wave
radiation absorbed;
positive forcing
B C D
wave radiation
cover
Increased net long-
wave radiation flux;
positive forcing
Less short-wave
radiation;
negative forcing
concentration
wave radiation
Decreased long-
wave radiation flux;
negative forcing
A
Fig. 1. Schematic representation of the direct (A, B) and indirect
(C, D) radiative effects associated with conversion from pasture to
forest. All the radiative fluxes described are at the surface and the
convention is that fluxes are positive for energy moving towards
the surface. Indirect effects are shown in the hatched box in this
diagram because the present work considers only the carbon and
direct albedo effects. Diagram modified from Findell et al. (2007).
some outgoing long-wave radiation, prevents it from escap-
ing from the Earth system (term C) and adds to global warm-
ing. The increased amount of water transferred to the atmo-
sphere must also be returned to the surface as precipitation,
which involves cloud formation. As low clouds tend to have
high albedo, they predominantly reduce the short-wave flux
to the surface (term D) and thus cool the Earth.
In summary, the direct effect of decreasing albedo with af-
forestation (term B) will increase radiative forcing at the sur-
face. However, increased evaporation (terms C and D) can
result in an increase or decrease in radiative forcing. The fi-
nal combined effect is determined by the relative sizes of the
two components (terms C and D). If the combined net surface
radiative balance is positive, then both latent and sensible
heat fluxes may increase and surface temperatures may in-
crease even more that that due to term B alone. These indirect
effects are difficult to compute and were beyond the scope of
the present study. However, other studies have shown that
under some conditions they may be quantitatively as impor-
tant as the direct effects (e.g., Bala et al., 2007).
Here, we provide an assessment of the impacts of chang-
ing albedo and carbon storage in trees on radiative forcing.
Our focus is on land-use change from pasture to Pinus radi-
ata forest at Puruki Forest, situated in New Zealand’s Cen-
tral North Island, where long-term measurements of forest
growth and energy exchange are available. Our conclusion
is based on comparison of these two opposing impacts on
radiative forcing as the forest develops. A quantitative as-
sessment of the indirect effects of changing land cover on
radiative forcing from changes in the hydrological cycle for
New Zealand requires regional-scale climate modelling and
is beyond the scope of the work reported here. Unpub-
lished regional climate modelling does suggest, however, that
for New Zealand, afforestation is likely to lead to some in-
crease in low cloud cover and cause a cooling to partly off-
set any warming from greater radiation absorption by forests
(S. M. Dean, unpublished).
2 Methods
2.1 Description of the Puruki site
Puruki is a 34.4ha catchment located in the Purukohukohu
Experimental Catchment at the southern end of the Paeroa
Range in the Central North Island of New Zealand (lati-
tude 38◦2504400 S, longitude 176◦1205100 E). Originally used
as pasture, Puruki was planted with P. radiata in 1973 at
a nominal tree density of 2200 trees ha−1. Daily measure-
ments of weather variables above the forest canopy began in
1976 at a mast installed in the Rua sub-catchment at Puruki,
where early mortality reduced stocking to 1483 trees ha−1
by 1976 (Beets and Brownlie, 1987). In 1980, the for-
est stand was thinned to 550 trees ha−1and the trees were
pruned to 2m height, with no further silvicultural opera-
tions undertaken thereafter. Following thinning, measure-
ments ceased in 1982, and a new meteorological tower was
installed about 30m from the original mast, and measure-
ments re-commenced from 1983.
Mean top height of the stand increased by approximately
1.5myr−1(Beets and Brownlie, 1987). Measurements of
changes in tree biomass with forest development were avail-
able from Beets and Pollock (1987). These direct biomass
measurements were supplemented with biomass predictions
obtained using the C Change model/300 Index growth model
(Beets et al., 1999; Kimberley et al., 2005). These models
utilise silvicultural information and ground data from inven-
tory plots, which were installed in the Rua sub-catchment
and measured over a full rotation. Stem volume per hectare
was estimated from tree height and diameter measurements
using generalised model corrections (Kimberley et al., 2005).
Stand volume estimates were then converted to stand dry
matter by multiplying wood volume by wood density. Ex-
pansion factors provided annual estimates of live-biomass
carbon stocks over the length of the rotation. Dead biomass
pools with their respective component decay rates were
also included, which became particularly important follow-
ing thinning. The same methods have been used for New
Zealand’s national planted forest carbon inventory (Beets et
al., 2010).
In these calculations, we did not include soil carbon
changes because detailed measurements at our experimen-
tal site had found no significant differences in soil carbon
between pine and pasture soils (Beets et al., 2002). More
generally, however, it is usually observed that soil carbon
stocks are reduced following reforestation of pastures (Guo
www.biogeosciences.net/8/3687/2011/ Biogeosciences, 8, 3687–3696, 2011
3690 M. U. F. Kirschbaum et al.: Carbon, albedo and climate benefits
and Gifford, 2002). Differences tend to be more pronounced
for conifer than broad-leaves species and increase with in-
creasing rainfall (Guo and Gifford, 2002), which could be
related to increased nitrogen leaching from sites with higher
rainfall (Kirschbaum et al., 2008). Hence, while it was ap-
propriate not to include soil carbon changes for our specific
site, soil carbon changes should be included for generic anal-
yses, or for other specific sites where changes in soil carbon
might have been measured.
2.2 Methods used to estimate albedo
Incident and reflected short-wave radiation were measured
using paired solarimeters (Solar Radiation Instruments SRI5)
installed above the canopy at 35m height. These instruments
have a spectral response range across the full short-wave
range from 250 to 3500 nm. Data were recorded as hourly av-
erages using a data logger (Campbell Scientific Inc., Model
CR7). Values of albedo were calculated from the ratio of
daily total reflected to incident short-wave radiation. Albedo
measurements started in 1977, four years after stand estab-
lishment when leaf mass was about 4.2t dry matter ha−1
(Beets and Pollock, 1987) and were continued through to the
end of 1992.
As the sequence of ground-based measurements at Puruki
did not extend back to the time before forest establishment,
pasture albedo was estimated from satellite measurements,
using data from the MODIS satellite. Data were collected
every eight days from 2000 to 2008. Pasture albedo was ob-
tained from locations in close vicinity of the forest site.
These data were processed with a BRDF (Bidirectional
Reflectance Distribution Function) to estimate albedo at the
known sun angle throughout the year and with reflectance
over the integral of all outgoing angles. Calculations were
done with radiation received either by direct beam radiation
from the sun at its known zenith angle or by diffuse radiation
(Dymond et al., 2001). Daily albedo was weighted towards
mid-day values based on estimated diurnally varying solar
radiation. Total short-wave radiation albedo was calculated
with the algorithm of Liang (2001).
To validate the procedure used for the present work, we
used Landsat 4 imagery that was available at finer spatial
resolution and at dates coincident with available ground ob-
servations. Four dates were selected with cloud-free con-
ditions that allowed a direct comparison with ground-based
measurements on the same days. Albedo was calculated for
the pixel that corresponded to the location of the tower in the
Puruki forest using atmospheric corrections and the BRDF
information derived from analysis of the seasonal patterns in
MODIS reflectance from similar pine forests.
2.3 Calculations of radiative forcing
2.3.1 The radiative forcing of carbon storage
The radiative forcing of an extra unit of carbon in the
atmosphere can be calculated from first principles. The
change in radiative forcing for a square metre of ground, Rm
(Jm−2d−1)can be calculated as:
1Rm=86400×5.35×ln1+1[C][C](3)
where [C] is the background atmospheric carbon dioxide
concentration, 1[C] is the change in atmospheric carbon
dioxide concentration attributable to land-use change, and
86400 is the number of seconds in a day. The conver-
sion “5.35” (Wm−2)converts from units of carbon diox-
ide to radiative forcing (Harvey et al., 1997; Ramaswamy
et al., 2001). This corresponds to radiative forcing of
about 3.75Wm−2for doubling carbon dioxide concentra-
tion, which is a mid-range climate sensitivity.
A change in carbon stocks, 1C (tC), needs to be con-
verted to the corresponding concentration change in the at-
mosphere, 1[C], considering that 1ppm in carbon dioxide
concentration corresponds to 2.123 GtC (Joos et al., 1996) so
that:
1[C] = 1C.2.123×109.(4)
The total radiative forcing over a year and for the Earth as a
whole, 1RE, can then be calculated as:
1RE=1Rm×510×1012 ×365 (5)
where 510·1012 m2(or 510 million km2)is the surface area of
the Earth, and 365 is the number of days in a year. With an
atmospheric carbon dioxide concentration of 390ppm, and
for the Earth as a whole, the radiative forcing, 1RE, of the
removal of 1 tonne carbon can be calculated as:
1RE=86400×5.35×ln
1+
1C2.123×109
390
(6)
×510×1012×365=−104 GJ tC−1yr−1.
With carbon storage in trees, it is also necessary to con-
sider possible carbon-cycle feedbacks. In essence, the car-
bon stored in trees lowers the atmospheric carbon dioxide
concentration, which in turn reduces the carbon dioxide up-
take by the natural global carbon pools, especially the oceans
(Kirschbaum, 2003, 2006). The atmospheric carbon dioxide
concentration is therefore reduced by less than the amount
of carbon stored in trees, and the difference increases over
the time of carbon storage. These are important considera-
tions for the impact mitigation potential of biospheric carbon
removal.
The calculations in the following analysis are done both
with and without consideration of these carbon-cycle feed-
backs, as either approach can be the most meaningful in its
Biogeosciences, 8, 3687–3696, 2011 www.biogeosciences.net/8/3687/2011/
M. U. F. Kirschbaum et al.: Carbon, albedo and climate benefits 3691
specific context. These processes are described in detail by
Kirschbaum (2003, 2006), and that formulation is also used
here.
2.3.2 Radiative forcing due to albedo change
The change in daily radiative forcing due to a change in
albedo, 1Rd, can be calculated as:
1Rd=Qs↓ ×1a ×(1−αatm)(7)
where Qs↓is total daily downward solar radiation, 1a is
the difference in albedo over the whole short-wave spectrum
between two different land-use types and αatm is the propor-
tion of short-wave radiation absorbed by the atmosphere. We
used an average global value for atmospheric absorption of
20% (Kiehl and Trenberth, 1997).
In principle, this calculation can be done at any scale, but
for comparative purposes, scaling the numbers to a hectare
is useful. Since radiation is usually given in units per square
metre of ground area, a value of radiation needs to be con-
verted to a hectare basis. Since radiation also follows a dis-
tinct seasonal cycle, it is necessary to sum these calculated
values over at least a year. Hence, the difference in radiative
forcing for a hectare over a year can be calculated as:
1Ryr =
365
X10000×1Rd(8)
where 10000 is the number of square metres in a hectare,
and 365 is the number of days in a year.
Calculated changes in albedo-based radiative forcing are
a direct function of the average radiation received at specific
locations and of the difference in albedo between two land-
use types. This, in turn, is strongly affected by the extent
of snow cover, when albedo changes between land-use types
are most pronounced. In New Zealand, snow cover generally
plays a role only in the higher alpine areas, but not in typical
conifer-growing regions.
Average annual incoming radiation changes with latitude
and with the extent of cloudiness. In New Zealand, an-
nually averaged mean daily radiation ranges from about
11–12MJm−2d−1in the wet and cloudy parts of the far
south of the South Island to about 15MJm−2d−1over most
of the North Island (Leathwick et al., 2002). There is a
distinct seasonal cycle, with minimum values in June (5–
6MJm−2d−1)and maximum values in December and Jan-
uary (c. 22MJ m−2d−1).
Using a mean annual incident solar radiation of
13.5MJm−2d−1and an albedo difference between two land-
use types of 7% (i.e., albedo values of 13% for forest and
20% for pasture as observed in our study – see below), this
translates into an annual radiative forcing, 1Ryr, of:
1Ryr =13.5×0.07×(1−0.2)×10 000×365 (9)
=2759GJha−1yr−1.
Fig. 2. Albedo measurements based on Landsat 4 satellite observa-
tions plotted against ground-based albedo measurements on match-
ing days. Two measurements were obtained in summer and two in
winter as indicated in the Figure. The dashed line is a 1:1 line.
For the specific calculations done here, we used the differ-
ence between daily measured albedo from our forest canopy
and satellite derived measurements of pasture albedo for the
same time of the year. Using Eqn. (7), the difference in
albedo between pasture and forests was then multiplied by
incoming radiation measured on respective days to calculate
the radiative forcing effect for specific days. This ensured
that seasonal variations in both albedo and incoming radia-
tion were appropriately combined.
3 Results
Calculation of the radiative forcing of albedo changes crit-
ically depend on the magnitude of changes in albedo with
land-use change. In our work, we had satellite derived es-
timates of pasture and forest albedo and ground-based mea-
surements of the albedo of the developing forest at our test
site. Within the respective observational periods, there were
four dates with Landsat 4 data that allowed a direct compar-
ison of satellite and ground-based albedo estimates. They
consisted of two summer and two winter observations and
thus allowed an assessment of both absolute values and the
extent of seasonal variation (Fig. 2).
The comparisons between the two different methods gave
comparable results for both summer and winter measure-
ments, with just a slight difference by about 0.4% between
satellite and ground-based measurements. This level of un-
certainty is small relative to the size of the difference between
land uses. It allowed us to use the satellite data with confi-
dence in estimating the albedo of pasture sites in the vicinity
of our forest site.
www.biogeosciences.net/8/3687/2011/ Biogeosciences, 8, 3687–3696, 2011
3692 M. U. F. Kirschbaum et al.: Carbon, albedo and climate benefits
Fig. 3. Changes in stand biomass carbon (a) and albedo (b) mea-
sured at Puruki, and satellite-derived albedo estimates for pastures.
Pasture albedo is plotted for 1973 (when the stand was established),
but actual measurements correspond to the mean of observations
from 2000 to 2008. Also shown are 365-day running means of for-
est albedo, and a linear interpolation from the pasture data at the
end of 1973 to the beginning of the trend line for the forest data.
Biomass data include estimated carbon stocks in litter and roots.
Comparing the radiative forcing of the removal of 1 tC of
−104GJtC−1yr−1(Eqn. 6) with that of a 7 % albedo change
(2759GJha−1yr−1, Eqn. 9), it became possible to calculate
the required amount of carbon storage needed, B, to balance
the increase in albedo from increased radiation absorption as:
B=1Ryr1RE=2759−104= −26.5 tC ha−1.(10)
Hence, forests in New Zealand need to remove about
25tCha−1before the radiative cooling effect of carbon stor-
age outweighs the warming effect of increased absorption of
short-wave radiation. At Puruki, it took about 4.5 years be-
fore the stand had accumulated 25tC ha−1(Fig. 3a). Growth
and carbon accumulation was slow for the first few years as
trees were still expanding their crowns and increased their
ability to intercept and utilise most incoming radiation.
Thereafter, growth in stand carbon stocks proceeded more
rapidly before reaching a temporary halt as the stand was
thinned. Thinnings at Puruki were not removed from the site
so that carbon stocks did not decrease, but for a few years
after thinning, there was no increase in carbon stocks as the
decay of dead stems on the forest floor and decaying coarse
roots balanced the on-going carbon accretion in the remain-
ing, and vigorously growing, trees.
Measured albedo at the Puruki site (Fig. 3b) followed a
distinct seasonal cycle due to the changing angle of the sun,
which changed the proportion of incident radiation being
absorbed or reflected. In winter, with a more oblique sun
angle, a slightly higher proportion of incident radiation was
reflected (Fig. 2). Albedo also changed greatly with stand
development from a typical albedo for pasture of about 20%
(based on satellite observations) to about 12–13% when the
forest was 8 years old (Fig. 3b).
These differences in albedo (Fig. 3b), together with obser-
vation of stand-carbon storage over time (Fig. 3a), were used
to calculate changes in radiative forcing of the two compo-
nents (Fig. 4). Albedo forcing displayed a strong seasonal
cycle, primarily due to seasonal changes in incident solar ra-
diation which was slightly increased further by the season-
ally changing albedo (Fig. 3b), with highest values over the
winter months (Fig. 2). Numbers were positive, indicating
that the change in albedo from pasture to forest constituted
positive radiative forcing, with a warming effect.
Carbon-based radiative forcing followed the changes in
carbon stored in the forest. Initially after tree planting, there
was a lag for the first few years as trees slowly established
themselves at the site, followed by a decrease that was tem-
porarily halted in 1980 when the stand was thinned, and then
a sustained growth period thereafter. Overall, carbon-based
radiative forcing was negative, with a cooling effect.
Consideration of the feedback effects via adjustments in
the carbon pools in the global carbon cycle reduced the over-
all beneficial effect of carbon storage. In essence, carbon
removal from the atmosphere and its storage in trees reduces
the inherent carbon uptake by natural carbon pools especially
the oceans (Kirschbaum, 2003, 2006). The atmospheric car-
bon dioxide concentration is therefore reduced by less than
the amount of carbon stored in a stand of trees. As that car-
bon uptake takes time, especially uptake by the less accessi-
ble pools like the deep oceans, the feedback effect becomes
stronger over time, and the difference between calculated
benefits with and without carbon cycle feedbacks widens. By
the end of the rotation, the carbon cycle feedback reduced the
beneficial effect of carbon storage in trees by about one third
(Fig. 4).
The magnitude of radiative forcing from albedo changes
and carbon storage were comparable for the first few years
after stand establishment, but stand biomass continued to in-
crease long after albedo changes had reached their maximum
so that the carbon storage effect dominated for older stands
(Fig. 5). The difference was less pronounced if carbon-cycle
feedbacks were included. In that case, the on-going increase
in stand-level carbon storage led to progressively less global
benefit because stand-level storage was increasingly offset by
adjustments in other carbon pools.
These calculations gave an offset of the carbon storage
benefit through albedo changes by the end of the rotation
by an average of 10% if the carbon stored in the forest was
considered alone (Fig. 5c) and 15% if carbon storage by the
carbon cycle feed-backs was included (Fig. 5d). Integrated
over the whole length of the rotation, the offset was 17% if
only carbon storage in the forest was considered and 24% if
carbon-cycle feedbacks were included.
Biogeosciences, 8, 3687–3696, 2011 www.biogeosciences.net/8/3687/2011/
M. U. F. Kirschbaum et al.: Carbon, albedo and climate benefits 3693
Fig. 4. Calculated radiative forcing due to changes in albedo and
carbon storage. Albedo-based forcing is calculated daily from ob-
served albedo and incident solar radiation. Carbon dioxide is dis-
tributed globally so that there are no daily variations. Carbon-based
calculations were done with considering stand-level carbon storage
only (stand), and after calculating carbon cycle feedback effects
(with C cycle). The stand was thinned in 1980, which accounts for
the discontinuity in the slope of the lines depicting carbon-based
radiative forcing.
4 Discussion
The detailed measurements available from Puruki allowed
quantification of the offset of the carbon storage benefit by
albedo changes that accompanied land-use change from pas-
ture to exotic forest. Carbon storage in forests had the net
effect of lowering the atmospheric carbon dioxide concentra-
tion and through that, total radiative forcing, but the decrease
in albedo resulted in the absorption of more short-wave ra-
diation, with positive radiative forcing. These two effects
were of comparable magnitude in young stands, but in older
stands, the carbon-storage effect became much greater than
the albedo effect. Under New Zealand conditions, it could
therefore be concluded that the albedo changes negated the
benefit from carbon storage over a rotation of a conifer forest
by 17–24%, depending on the inclusion or omission of the
consideration of carbon-cycle feedbacks
These values are smaller than those calculated by Betts
(2000) and Bala et al. (2006) for calculations at higher lati-
tude, with the key differences being the extent of snow cover
and the projected forest growth rates in respective regions.
Betts (2000), for example, used a 5 % difference in albedo
between forested and non-forested land under snow-free con-
ditions, which was less than the 7% difference found at Pu-
ruki. However, the values used by Betts (2000) increased to
an albedo difference of up to 55% with snow present (25 %
for snow covered forest and 80% for snow-covered open
land).
Betts (2000) also assumed carbon storage potentials be-
tween about 100tCha−1for cold or dry regions in Siberia,
Canada or China and 200tCha−1in more temperature
Fig. 5. Net radiative forcing effect combining albedo and carbon-
storage effects. Calculations were done for either stand-level carbon
storage (panels a, c) or with inclusion of carbon cycle feed-backs
(panels b, d) and are expressed as either net radiative forcing per
hectare (panels a, b) or as a percentage offset on the carbon storage
benefit by the associated albedo changes (panels c, d).
regions of Europe and North America. The Puruki stand
reached carbon storage of 100tCha−1after about 10 years
and 200tCha−1after 20 years (Fig. 3). Hence, the differ-
ences in snow cover and biological growth potential between
global regions accounts for the difference in the net balance
between carbon storage and direct albedo effects. Under
New Zealand conditions, the radiative forcing changes re-
sulting from albedo changes therefore only diminish but do
not negate the climate-change benefit from tree plantings.
The exact magnitude of the offset depended on a num-
ber of factors, most importantly, on the difference in albedo
between the different land-cover types. We confirmed that
satellite and ground-based measurements agreed well at our
forest site. Using the same algorithms for deriving pasture
albedo from single reflectance measures during a satellite
overpass, we could derive the matching albedo of pastures
in the vicinity of our measured forest.
Breuer et al. (2003) summarised albedo values from dif-
ferent authors and found albedos for coniferous forests to be
generally between 11–14% and grassland albedo between
19–27%. Hollinger et al. (2010) similarly reported grass-
land albedos of 18–21% and 11% for evergreen needle-leaf
forests in temperate locations. Our measured values for New
Zealand pastures were generally at the low range of global
observations, probably owing to the general good growing
condition of most New Zealand pastures, with few times of
grass drying off or developing bare patches due to moisture
limitations or excess grazing. There are also no periods of
extended snow cover, especially not in the vicinity of our
forest site with the typical mild weather of New Zealand’s
North Island.
Albedo of forests also changes with age (Fig. 3b). Amiro
et al. (2006) observed albedo to decrease with increasing for-
est age, whereas our observations indicate lowest albedo at
www.biogeosciences.net/8/3687/2011/ Biogeosciences, 8, 3687–3696, 2011
3694 M. U. F. Kirschbaum et al.: Carbon, albedo and climate benefits
about 10 years of age, but a slight increase for older stands
which coincided with thinning, which may be the directly
responsible factor. These albedo changes are only slight but
significant nonetheless as even a 1% difference in albedo can
translate into a significant difference in radiative forcing.
The magnitude of the albedo offset also depends on
whether the effect was integrated over the whole life of a ro-
tation, or was calculated only at the end of the rotation. The
offset was obviously greater when it included the early stages
of stand establishment when both effects were of compara-
ble magnitude. For longer rotation lengths, the contribution
of later growth stages became progressively more important
as the carbon storage effect became greater than the albedo
effect.
The offset percentage was also different for calculations
done with or without consideration of carbon cycle feed-
backs. Whether those carbon cycle feedbacks should be in-
cluded depends on the context within which specific ques-
tions are being asked. Calculations of stand-level carbon
changes give the relevant net radiative forcing change at-
tributable to a particular forest. However, perturbation of
the global carbon budget by forests leads to a change in the
global carbon cycling that partly negates the initial stand-
level change in carbon storage. So, to calculate the net effect
of a land-use change, it is relevant to include the net effect
of planting forest with its consequent adjustments in natural
carbon pools.
For a comparison of forest planting and fossil-fuel man-
agement, the same carbon-cycle feedbacks apply to both.
The emission of 1tonne of carbon from fossil fuels leads
to an increase in atmospheric carbon content by less than
1tonne because some of that carbon will be absorbed by the
oceans. To calculate the benefit of saving the emission of
1 tonne of fossil fuels against the benefit of storing 1 tonne of
carbon in the biosphere, the comparison therefore needs to be
consistent in either including or excluding carbon cycle feed-
backs in both instances. As these feedbacks are not generally
included in calculating the effect of fossil-fuel management,
it would be appropriate to base comparison on stand-level
calculations.
In the present work, we were not able to quantify the in-
direct effects of afforestation resulting from changes in the
temperature profile between the surface and the atmosphere,
and those resulting from increased evapotranspiration from
forest canopies and consequently increased cloud formation.
Previous work (e.g., Bala et al., 2007) has indicated that the
indirect effect can be of a similar magnitude (and opposite
sign) as the direct, ground-based albedo changes, especially
in more tropical locations. For New Zealand, we cannot cur-
rently quantify whether, or to what extent, indirect effects
may negate the direct albedo effect on radiative forcing.
5 Conclusions
The case study presented here dealt with the direct albedo
changes and the associated change in the absorption of short-
wave solar radiation. This was compared against the global
effect of reductions in atmospheric carbon dioxide concen-
tration due to increased carbon storage. Our calculations
showed that under New Zealand conditions, plantation estab-
lishment had a significant overall net cooling effect. While
albedo changes reduced the overall tree-planting benefit, car-
bon storage was of greater quantitative importance. This
finding contrasts with the findings for boreal regions where
albedo changes can be quantitatively more important than
carbon storage, giving a net warming effect of tree planta-
tions (Betts, 2000).
The main difference between the findings for different re-
gions relates to the role of snow, which can greatly accentuate
differences in albedo between vegetation types (Betts, 2000).
Snow plays an important role in the boreal zone, but only a
minor one in New Zealand’s conifer-growing regions. The
growth rate of trees and their carbon-storage potential is also
much greater in New Zealand than in more slowly growing
boreal forests. That shifts the relative benefit in favour of tree
plantings. On the other hand, annual incident solar radiation
is also greater in New Zealand than in the boreal zone so that
the same albedo difference caused greater radiative forcing
in New Zealand than in regions receiving less short-wave ra-
diation.
However, irrespective of the quantitative importance of
albedo in specific regions and circumstance, our study, like
previous ones (e.g., Brovkin et al., 1999; Betts, 2000; Bala
et al., 2007, 2007; Pongratz et al., 2011), has shown that
albedo changes can be important enough to be considered in
an overall assessment of the radiative forcing consequences
of a change in land use (Schwaiger and Bird, 2010; South
et al., 2011) and for urban design (Akbari et al., 2009). Cli-
mate change cannot be mitigated efficiently if management
response strategies consider only some radiatively important
factors (i.e., carbon storage) while ignoring others (i.e., di-
rect effects on the Earth’s energy balance). Optimal manage-
ment of the Earth’s biosphere for climate-change mitigation
can only be achieved if all relevant factors are considered,
quantified and included in an overall assessment.
Acknowledgements. This work was supported by the New Zealand
Ministry of Agriculture and Forestry. We would like to thank Anne
Austin, Kailash Thakur and several reviewers from the Ministry
of Agriculture and Forestry for many useful comments on the
manuscript.
Edited by: G. Wohlfahrt
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M. U. F. Kirschbaum et al.: Carbon, albedo and climate benefits 3695
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