Ecosystems
DOI: 10.1007/s10021-011-9436-8
Ó 2011 Her Majesty the Queen in right of Canada
Carbon, Water, and Energy
Exchanges of a Hybrid Poplar
Plantation During the First Five
Years Following Planting
Tiebo Cai,1 David T. Price,1* Alberto L. Orchansky,2 and Barb R. Thomas3,4
1
Natural Resources Canada, Northern Forestry Centre, 5320-122 Street NW, Edmonton, Alberta T6H 3S5, Canada; 2Micrometeorology
Consultant, Edmonton, Alberta, Canada; 3Alberta-Pacific Forest Industries Inc, Boyle, Alberta T0A 0M0, Canada; 4Department of
Renewable Resources, University of Alberta, 751 General Services Building, Edmonton, Alberta T6G 2H1, Canada
ABSTRACT
Eddy covariance was used to measure above-canopy exchanges of CO2 and water vapor at an
operational plantation of hybrid poplar (variety
‘‘Walker’’) established on marginal agricultural
land in east central Alberta, Canada. Winter ecosystem respiration (Re) rates were inferred from
seasonal changes in the normalized respiration rate
at 10°C (R10) for the growing season and observations of soil CO2 concentration measured with
solid-state probes. Over five consecutive growing
seasons following planting, gross ecosystem production (GEP) increased each year, ranging from
21 g C m-2 y-1 in year 1 to 469 g C m-2 y-1 in
Received 24 June 2010; accepted 22 March 2011
Author Contributions: TC performed the data analysis presented in this
article and drafted the methodology and results. DTP conceived the study,
located the site and contributed significantly to writing the article. ALO
turned the concept into reality, set up and maintained the eddy covariance system and other instrumentation, and organized data sets. BRT
assisted greatly with site selection, provided expertise on hybrid poplar
plantation establishment, coordinated management operations at the
study site and contributed to the writing.
*Corresponding author; e-mail: [email protected]
year 5. During this period, the annual carbon balance shifted from a net source of greater than
330 g C m-2 in year 1 to approximately C-neutral
in year 5. Total carbon (C) release over 5 years
likely exceeded 630 g C m-2. Intra- and interannual variations in temperature and soil water
availability greatly affected annual C balance each
year. GEP and Re were particularly sensitive to
temperature during spring and to soil water availability in summer: year 5 was notable because a
cold spring and accumulating drought caused
growth and carbon uptake to fall well below
their potential. Annual evapotranspiration (ET)
increased slightly with leaf area, from 281 mm
in year 1 to 323 mm in year 4, but in year 5 it
declined, while exceeding total precipitation (P).
This trend of increasing annual ET/P suggests that
annual GEP could become increasingly water-limited in years with below normal precipitation, as
the plantation achieves maximum leaf area. Measured canopy albedos did not change appreciably
over three winters, suggesting that estimates of
increased radiative forcing resulting from afforestation in high latitudes could be exaggerated in
regions where fast-growing deciduous plantations
are managed on short (20-year) rotations.
Key words: plantation; hybrid poplar; net ecosystem exchange; chronosequence; carbon sequestration; drought; land-use change.
T. Cai and others
INTRODUCTION
Afforestation of marginal or abandoned agricultural
land has been a common practice in many parts of
the world, usually to create or increase local supplies of wood fiber. In Canada, however, afforestation has occurred on a relatively small scale,
limited primarily to private land and the protection
of farm soils, notably through the efforts of individual farmers creating shelterbelts of trees made
available for the past 100 years through the Prairie
Farm Rehabilitation Administration (now Agriculture and Agri-Food Canada’s Agroforestry Development Centre of the Agri-Environment Services
Branch). Planting of extensive areas of previously
unforested agricultural land has been carried out in
eastern Canada (notably in Ontario, Québec and
New Brunswick) since the 1960s, but is a relatively
recent initiative in western Canada. An important
objective of this new activity is to compensate for
shortfalls in timber supply resulting from productive forest being converted to other land uses. In
Alberta, perhaps 5,000 km2 of boreal forestland
have been lost to oil and gas exploration, and for
coal and oil-sands extraction (for example,
Schneider 2002; Schneider and Dyer 2006). Forestry companies affected by such losses are keen to
supplement their operational area by planting
nearby marginal agricultural land if economically
feasible. As remaining forests are assigned to protected areas, or lost to other land uses, the importance of afforestation can only increase. In Canada,
much effort is currently focused on plantations of
hybrid poplars in Ontario and Québec, as well as
the three Prairie Provinces, established either on
agricultural land, or on recently harvested forest
sites, and managed on rotations of about 20 years
(for example, Saurette and others 2006; Arevalo
and others 2009).
There is also the potential to use tree plantations
to reduce atmospheric greenhouse gas (GHG)
concentrations, but comprehensive research is
required to determine the true climate change
mitigation benefits that accrue. Under the terms of
the Kyoto Protocol Article 3.3, it is now possible to
claim credits for reductions in anthropogenic GHG
emissions resulting from afforestation—defined as
the establishment of forest, through human activities such as planting, on land that has not previously supported trees for at least 50 years (Natural
Resources Canada 2004). This is seen by Canadian
forestry companies as an additional economic
incentive for afforestation programs and has led to
interest in determining the complete carbon (C)
balances of hybrid poplar plantations.
It should be clear that large areas of land would
need to be afforested to create a significant C offset
to Canada’s GHG emissions. Then it must be
determined how new tree cover affects the accumulation of carbon in soils and litter, because a
commercial forestry operation will presumably
plan periodic harvesting in which much of the
aboveground woody biomass is removed from the
site. (Afforestation for conservation purposes may
also occur, but the area of such planting is likely to
be small and limited to relatively slow-growing
indigenous species.) Assuming the logistical problems can be solved (that is, if a major afforestation
program can be maintained for several decades),
then there are some important concerns about the
potential environmental impacts of such a program. Foremost among these, in regions where
water supplies are limited (such as the prairie
ecosystems of western Canada, for example, Hogg
1994; Hogg and others 2005), it is possible that
extensive plantations will make less water available
to neighboring crops or grazing lands and/or reduce
stream flows. Second, global scale modeling studies
have suggested that a shift from grassland to forest
cover in the northern mid-latitudes would result in
a significant reduction in surface albedo, particularly in winter, that would contribute to a net
surface warming of comparable magnitude to the
cooling resulting from the net GHG removal (Bonan and others 1992; Betts 2000; see also Bala and
others 2007; Montenegro and others 2009). We
would expect changes in albedo resulting from
establishment of deciduous poplar plantations to be
less dramatic than those suggested by studies focused on evergreen conifers, but their effects on
surface energy balance and climate should still be
quantified.
An ongoing study designed to address some of
these questions is in progress at an operational
300 ha plantation of hybrid poplars established on
agricultural land in east-central Alberta. Carbon,
water, and energy exchanges of the plantation
have been monitored since early 2005 using the
eddy covariance technique, together with standard
meteorological and soil measurements. The present
study provides a unique opportunity to assess
effects of interannual variations in climate on
trends in the carbon and water balances during the
five growing seasons from 2005 to 2009.
METHODS
The study site is located at 54.346°N, 111.522°W, in
the aspen parkland/southern boreal forest region of
Canada, near Ashmont, Alberta. Elevation is 622 m
Carbon, Water, and Energy Exchanges of a Hybrid Poplar Plantation
in gently rolling terrain. Soils are predominantly
moderately well drained Luvisols with a mesic
moisture regime and silty-clay to silty-clay-loam
texture in the upper 30 cm. Approximately 300 ha
of this land was used for crops (barley, alfalfa, hay)
until 2004, after which it was cultivated in September 2004 and treated with pre-emergent herbicide in May 2005. Just prior to planting in early
June 2005, a solar-powered eddy covariance (EC)
system and climate monitoring station were installed at the approximate center of a 90 ha block
(located within the 300 ha area). The entire area
was planted on an accurate 3 m grid with 1 m tall
rooted cuttings of Populus deltoides 9 P. petrowskyana
(var. ‘‘Walker’’). The selected tower site is close to
ideal for EC measurements, with uninterrupted
uniform fetch of 200–300 m in all directions on
very flat ground and genetically identical roughness elements (trees) growing on a regular grid.
Furthermore, the 90 ha block is surrounded by
200 ha of fields planted with var. Walker and two
other hybrid poplar clones of the same age.
Net radiation was measured continuously with
half-hourly averaging using a net radiometer (NR
Lite2, Kipp & Zonen) mounted at 1.78 m height,
and wind speed and direction were monitored
using a R.M. Young sensor mounted at 3.51 m. Air
temperature (Ta) was measured using an air temperature and relative humidity probe (Model
HMP45C, Vaisala Inc., Finland) mounted in a
ventilated radiation shield at a height of 1.5 m.
Rainfall was measured using a tipping bucket rain
gauge (RM Young 52203-L, Campbell Scientific,
Logan, UT) at 2.87 m height. The heights of these
meteorological instruments were not changed over
the 5 years of the experiment. Snowfall was not
measured, so daily total snowfall data were averaged from 14 nearby (within 90 km) climate stations operated by Environment Canada (notably
Lac La Biche) as a substitute for the snowfall at the
site. The climate normals (1971–2000) for growing
degree day sum (GDD) and precipitation (P) were
also obtained from the same climate stations. Soil
heat flux (G0) was measured using REBS soil heat
flux plates buried at 3 cm below the soil surface at
three locations. Averages of the measurements
from the three soil heat flux plates were taken as
the representative G0 of the site. Soil volumetric
water content was measured using time-domain
water content reflectometers (CS615, Campbell
Scientific) buried at 2, 10, 25, and 50 cm depth.
With the exception of the rain gauge, all meteorological sensors were sampled at 5-s intervals and
recorded as half-hourly means by a data logger
(CR23X, Campbell Scientific). Soil CO2 concentra-
tion was measured hourly using solid-state probes
(GMM221, Vaisala Inc.) buried adjacent to soil
moisture probes at 2, 10, 25, and 50 cm depth in
July 2005. In 2009, the soil CO2 sensors at the 10
and 25 cm depths failed, so a second set of Vaisala
CO2 sensors was installed about 5 m from the first
set, and the latter removed in June 2010.
The eddy covariance technique was used to
measure net ecosystem fluxes of CO2 (Fc), latent
heat (LE), and sensible heat (H). Instrumentation
consisted of a three-dimensional sonic anemometer-thermometer (CSAT3, Campbell Scientific) and
an open-path infrared gas (CO2/H2O) analyzer
(IRGA) (LI-7500, LI-COR, Lincoln, NE) mounted
on an instrument tripod. The EC sensors were
mounted initially at a height of 3 m in 2005. To
ensure that the sensors remained well above the
top of the growing tree canopy, the instrument
height was increased before the start of the growing
season in 2008, to about 4 m, and again to 5.36 m
in May 2009, when the instrument tripod was
replaced with a 6 m triangular tower. Incoming
and reflected photosynthetically active radiation
(PAR) (Qt) was measured using a pair of quantum
sensors (PAR Lite, Kipp & Zonen, The Netherlands)
initially mounted at 3 m on the instrument tower,
and raised in June 2009 to 4.50 m. Midday canopy
albedo was estimated from PAR reflectance, that is,
as the ratio of outgoing (reflected) PAR to incoming
PAR during the half-hours from 11:00 to 14:00 h,
recognizing that average PAR reflectance may differ slightly from the full spectrum shortwave albedo measured with paired thermopile pyranometers.
Also in 2009, a second net radiometer (CNR1,
Kipp & Zonen) was installed at 5.77 m. Half-hourly
net fluxes were calculated as F ¼ qa w 0 s0 , where qa
is the mean molar density of dry air and w0 s0 is the
covariance of instantaneous vertical wind speed
(w) and the scalar, s. The scalars for Fc, LE, and H
were CO2 mixing ratio (mol CO2 mol-1 dry air),
water vapor mixing ratio (mol H2O mol-1 dry air),
and ambient Ta, respectively, all measured at
10 Hz. The rate of change in canopy storage of
these scalars was approximated as Fs ¼ hm qa Ds=Dt,
where hm is the measurement height, Ds is the
difference between half-hourly means of s of the
current and previous half-hours, and Dt = 1800 s.
Half-hourly net ecosystem exchanges of CO2 (NEE,
positive value means upward flux), and of LE and
H, were calculated as the summation of their corresponding F and Fs. All measurements were
coordinate rotated, and Fc and LE were corrected
following Webb and others (1980). Because of
recognized problems operating the LI-7500 in cold
winter conditions (for example, Burba and others
T. Cai and others
2006), the EC system was generally operated only
from early May to early November each year,
bracketing the period when leaves emerged in
spring and most leaves had fallen in late autumn.
The IRGA was calibrated using reference gases and
a dewpoint generator (LI-610, LI-COR), three times
each season: before installation in spring, approximately in mid-July, and after take-down in
autumn. The EC system was operational usually
from mid-May to mid-November, except for 2005/
2006 when it was allowed to continue operating
throughout the winter. In general it was necessary
to estimate monthly ET and CO2 fluxes during the
winter months, recognizing that the very cold
conditions in this region severely limit both fluxes.
Monthly ET during the first and last winter
months was estimated from the previous month’s
(November) or next month’s (May) data, and
assumed to be constant throughout the intervening
period.
During the growing season, half-hourly ecosystem respiration was estimated using an iterative
5-day moving window technique similar to that
of Reichstein and others (2005). Because
NEE = Re - GEP, where Re and GEP denote ecosystem respiration and gross ecosystem production
(photosynthesis), respectively, and because at
night, GEP can be assumed to be zero, nighttime
NEE represents Re. In this study, nighttime and
daytime periods were defined as PAR less than 50
and PAR greater than or equal to 50 lmol m-2 s-1,
respectively. The iterative technique for estimating
Re has four steps. First, nighttime half-hourly NEE
measurements made in turbulent conditions (friction velocity, u* > 0.1 m s-1) were related to their
corresponding air temperature using an (R10 Q10) relationship, that is, Re = R10Q(Ta-10)/10
, where
10
R10 is the normalized respiration rate at 10°C, and
Q10 is the relative temperature sensitivity describing the change in Re for a 10°C change in Ta. Values
of R10 and Q10 were obtained by curve-fitting all
valid measurements obtained in consecutive 5-day
periods (‘‘windows’’) when the EC system was
operating. Second, the Q10 values from step 1 for a
given year were weighted using the inverse of their
standard errors (SE). The SE-weighted Q10 values
for 2005–2009 were 1.86, 2.12, 1.18, 1.66, and
1.36, respectively. The average of these SE-weighted Q10 values, that is, Q10 = 1.63, was chosen as
the representative Q10 for all 5 years. Using
Q10 = 1.63, the R10 values for the 5-day windows
defined in step 1, were recalculated and assigned to
the middle (third) day of each 5-day window.
Third, every set of five consecutive R10 values from
step 2 was averaged and assigned to the middle day.
Lastly, both nighttime and daytime half-hourly
values of Re were calculated using the (R10 - Q10)
relationship with the smoothed R10 values and Q10
of 1.63. GEP was then calculated as the difference
between daytime Re and daytime NEE. For winter
periods when the EC system was not operating, the
smoothed values of R10 were used to estimate Re
from weather data, assuming a minimum R10 of
0.5. Weather data were obtained at the tower site,
except for the first 5 months of 2005, when they
were estimated from observations at the nearby Lac
La Biche climate station, as explained previously.
RESULTS
AND
DISCUSSION
During the five growing seasons, the planted trees
grew from single-stemmed rooted cuttings measuring approximately 1 m into bushy saplings
occupying a diameter of 1–1.5 m. By the end of
2009, average tree height in the vicinity of the
tower was approximately 3.0 m, with some individuals approaching 5 m.
Climatic Conditions during 2005–2009
The 5-day mean Ta frequently dropped below
-20°C in winter but never exceeded 25°C in
summer, and generally exceeded 5°C only from
May to September (Figure 1A). The 30-year normal annual GDD (5°C base) at this site is approximately 1,400. We calculated the annual GDD sums
for this site for each year from 2005 to 2009 using
the Environment Canada algorithm: GDD =
(Tmax + Tmin)/2 - Tbase, where Tmax and Tmin are
daily maximum and minimum air temperatures,
respectively, and Tbase is the base temperature
(5°C). When (Tmax + Tmin)/2 is less than Tbase, the
daily GDD value was set to 0 (McMaster and Wilhelm 1997). Annual GDD for 2006 was greater
than normal, whereas those for 2007 and 2008
were very close to normal and 2005 and 2009 were
markedly lower than normal (Figure 1B).
The 30-year normal (1971–2000) annual P for
this stand was estimated as 461 mm. Summer
(June–August) rainfall normally accounts for about
50% of annual P, but the seasonal distribution
varied greatly among years (Figure 1C). Soil volumetric water content (hv) generally reached a
maximum (approximately 0.25 m3 m-3) immediately following snowmelt, which usually occurred
around mid-April. The timing of hv reaching minima varied among years depending on the rates of
soil water depletion and recharge (Figure 1D).
Values of hv measured from late fall to early spring
(un-hatched area in Figure 1D) were estimated
Carbon, Water, and Energy Exchanges of a Hybrid Poplar Plantation
Figure 1. Climate trends
at the hybrid poplar
plantation observed from
June 2005 to November
2009. A Five-day mean
air temperature; B
cumulative growing
degree days; C seasonal
precipitation totals; D soil
water content at 100 mm
depth. Thirty year climate
normals (1971–2000)
were estimated from
nearby climate station
records.
from the values obtained before soil temperature
fell below 0°C.
Seasonal Variation of Midday PAR
Reflectance
The 5-day averaged midday PAR reflectance was
generally high from early December to the end of
March due to snow cover (with significant decrease
due to early snow melting in 2006) (Figure 2). It
seems that stand growth during the first 5 years
following planting did not affect PAR reflectance
during summer, and there was also no obvious
trend in winter (though data for early 2009 were
lost due to a sensor failure). Recent global model
simulations have suggested that large-scale afforestation in mid/high latitudes would decrease surface albedo because forested lands are assumed to
absorb a greater amount of solar radiation than
grasslands or other regions carrying low profile
vegetation, particularly during periods of snowcover (Bonan and others 1992; Betts 2000). Hence,
the negative forcing on global warming through C
sequestration could be offset by the positive forcing
induced by the decrease in surface albedo (Betts
2000; Bala and others 2007; Bonan 2008; Montenegro and others 2009). These conclusions seem
inconsistent with Figure 2, possibly because the
model simulations assume that afforestation would
generally be with coniferous species. Incorrect
Figure 2. Seasonal trends in canopy albedo, measured
above the hybrid poplar canopy from June 2005 to
November 2009. Paired upward- and downward-oriented sensors were mounted initially at 3 m and raised to
4.5 m in June 2009.
parameterization of winter surface albedo may also
lead to significant errors in numerical weather
prediction models (for example, Sellers and others
1997; Betts and Ball 1997). Figure 2 also provides a
clear contrast with the much lower winter albedos
of a southern boreal aspen stand (referred to as
SOA) of 0.21, and of conifer stands (0.11–0.15)
measured by Betts and Ball (1997). SOA has two
distinct canopy layers: a 22-m trembling aspen
T. Cai and others
overstory and a 2-m hazelnut understory. Clearly,
shadow effects in older stands with more complex
structure are important, even in deciduous canopies and it would be premature to draw a firm
conclusion. However, afforestation with short
rotation deciduous species such as hybrid poplar
will likely cause a significantly smaller positive
forcing on climate warming, for two reasons. First,
the relatively small effects on winter albedo seen in
the present study, may continue for several more
years, though it is important to recognize that as
the trees mature and the crowns expand, winter
albedo may gradually decrease perhaps to values of
0.25–0.30 (that is, still higher than those reported
for SOA because we may reasonably assume a
woody understory will not develop). Second,
because the planned rotation length for hybrid
poplars in Alberta is approximately 20 years, the
fraction of land area with significantly reduced
winter albedo (that is, if older stands with larger
trees and branches, and possible shrub undergrowth, cause significant shading), will be much
smaller than in regions where evergreen conifers
are grown on rotations of 50 years or longer.
Energy Balance Closure during
2005–2009
The half-hourly eddy covariance measurements of
LE, H, and G0 from June to September were
converted to ratios of measured Rn. Summation of
these three ratios for daytime periods for 2005 to
2009 produced totals of 1.12, 1.02, 1.05, 1.02, and
1.04, respectively (Figure 3), indicating extremely
good energy balance closure (EBC), generally
within 5% of measured Rn. The relatively poor
EBC in 2005 possibly resulted from spatial heterogeneity in G0 which formed a larger proportion
of the total energy flux, because there was minimal vegetation cover that year. Nighttime EBC
was relatively poor in all years (but the average
ratio was still 0.89), with the main uncertainty
apparently in the LE term (data not shown). Our
EBC closure was significantly better than at most
eddy covariance sites, where values of 0.8–0.9 are
typically reported in the literature (for example,
Wilson and others 2002). This probably demonstrates that with near-ideal site conditions, that is,
flat and relatively homogeneous terrain, adequate
fetch and genetically identical trees arranged on
an accurately spaced grid, the EC technique can
produce very reliable results. Further this greatly
increases our confidence in the estimates of annual carbon and water balances presented in the
following sections.
Figure 3. Annual averages of measured energy balance
components during growing seasons from June 2005
until November 2009. Component fluxes are half-hourly
soil heat (G0), sensible heat (H), and latent heat (LE)
fluxes summed and expressed as a ratio of the measured
net radiation. A value of 1.0 for the sum would imply
perfect energy balance closure. The relatively large
imbalance in 2005 was attributed to greater spatial variability in the contribution of G0 to total energy balance
that year, when vegetation cover was minimal.
Diurnal Variation of Net Ecosystem
Exchange (NEE)
Half-hourly NEE measurements made in turbulent
conditions (u* > 0.1 m s-1) from June to September inclusive, in each year were bin-averaged,
and their diurnal variation plotted in Figure 4A.
Diurnal NEE in 2005 was invariably positive (C
source). From 2006 onwards, however, the growing season average daytime NEE became progressively more negative, suggesting an increasing C
sink capacity during the day (Figure 4A). Average
NEE values at noon were +2.25, +0.18, -1.27,
-3.77, and -4.78 lmol C m-2 s-1 for each year
from 2005 to 2009, respectively.
Falge and others (2002) have reported maximum
half-hourly NEE for boreal/cold temperate (for
example, aspen), temperate (for example, beech),
and warm temperate (for example, oak-hickory)
broadleaf deciduous forests of -12, -19, and
-26 lmol m-2 s-1, respectively. Coursolle and
others (2006) reported maximum NEE for recently
disturbed (3–9 years), young (14–26 years), and
intermediate-aged (36–60 years) coniferous stands
were -1.55, -4.79, and -12.35 lmol m-2 s-1,
respectively. The maximum daytime and nighttime
NEE for SOA in August 2003 were approximately
-10 and +5 lmol m-2 s-1, respectively (Coursolle
Carbon, Water, and Energy Exchanges of a Hybrid Poplar Plantation
Figure 4. Ensemble half-hourly values of net ecosystem carbon exchange (NEE) from June to September over five years
at the hybrid poplar plantation. All nighttime data when u* < 0.1 m s-1 were rejected. A Twenty-four-hour trends
showing progressive increase in daytime C uptake (negative NEE) over the five growing seasons; B same data as A binned
by measured quantum flux densities (PAR).
and others 2006), although this site experienced
severe droughts from 2001–2003 (Krishnan and
others 2006). Our maximum half-hourly NEE in
2009 was only about 50% of that reported for SOA,
suggesting large future potential growth for this
stand.
Response of daytime NEE to PAR is shown in
Figure 4B, which was obtained by bin-averaging
half-hourly PAR and its corresponding NEE values
using a bin width of 100 lmol m-2 s-1. When Re
and GEP are approximately equal, NEE becomes
insensitive to the environmental controls on Re and
GEP. The response of NEE to PAR in 2005 was almost exponential, largely reflecting the dominance
of Re and its exponential response to increasing soil
and air temperature (which are generally positively
correlated with PAR). NEE was least sensitive to the
increase in PAR in 2006 because C uptake through
photosynthesis was in approximate balance with
respiratory C loss. From 2007 onwards, the average
responses of NEE to PAR increasingly resemble a
standard hyperbolic photosynthetic light response
curve, suggesting that photosynthesis became the
dominant component of NEE. However, when PAR
exceeded 1200 lmol m-2 s-1, NEE became generally less negative (that is, decreasing C uptake) with
the further increase in PAR, presumably because
photosynthesis had already became light-saturated
but respiration continued to increase exponentially with temperature (Figure 4B). The maximum
net C uptake (most negative NEE values) usually
occurred at PAR levels around 900 lmol m-2 s-1
(Figure 4B) or around 9:00 am when photosynthesis was near saturation but temperature was still
low and Re was not close to its daily maximum
(Figure 4A).
EFFECT OF TEMPERATURE AND
PRECIPITATION ON GEP AND ECOSYSTEM
RESPIRATION (RE)
The effect of temperature and precipitation on GEP
and Re were assessed by comparing data from June
and August, respectively. In June, GEP totals were
approximately 0, 26, 51, 106, and 79 g C m-2 for
2005–2009, respectively (Figure 5A). Hence, in
Figure 5. Trends in monthly carbon balance over five
years during June and August compared to A growing
season heat sum, and B August precipitation.
T. Cai and others
2007 and 2008, June GEP approximately doubled
from the previous year, but in 2009 it actually
decreased by 25% relative to 2008. The abnormally
low temperature in the early growing season of
2009 clearly delayed leaf emergence and development, and the GDD sum to 1 July was reduced by
about 30% compared to normal, all of which
reduced GEP (Figure 5A). The corresponding June
Re values for these 5 years were 61, 55, 56, 76, and
70 g C m-2, respectively. In contrast to GEP, the
rate of increase in June Re was much slower. In
2009, the abnormally cool spring caused June Re to
decrease, compared to 2008, by approximately 8%.
The impact of cool temperatures on GEP was three
times larger than that on Re, which resulted in NEE
in June 2009 being only 30% of that in 2008. If the
trend of year-over-year increases in annual GEP
and Re had continued from 2008 to 2009, we estimate these terms would have been approximately
220 and 103 g C m-2, respectively, so the stand
may have sequestered 117 g C m-2 less than its
potential in June 2009 alone. Because GEP and Re
usually respond to changes in temperature in the
same direction (Van Dijk and Dolman 2004; Cai
and others 2010), the net C uptake is determined
by which of them is more enhanced or decreased
by temperature. These results confirm other
observations that in the early growing season, GEP
generally is more sensitive to temperature than Re,
whereas the reverse occurs in summer and autumn
(for example, Piao and others 2008). Consistent
with our results, Black and others (2000) reported a
significant increase in C sequestration by a boreal
deciduous forest in years with early springs, and
Cai and others (2010) reported that warmer and
drier conditions stimulate Re more than GEP in late
growing seasons in a boreal peatland ecosystem.
Figure 5A demonstrates the important effects that
interannual climate variability can exert on vegetation phenology and hence on terrestrial C cycling
(for example, Menzel and others 2006; Myneni and
others 1997).
Figure 5B shows that rainfall in August 2008
was about 50% above normal, whereas August
rainfall in the other 4 years varied around 70%
below normal. In 2005, August Re was slightly
higher than in either of 2006 or 2007, although
the latter 2 years were warmer (Figure 1A–B).
This result was likely due to microbial decomposition of residual labile material in 2005 from
previous years’ agricultural crops. Although this
heterotrophic component of Re probably decreased in 2006, and 2007, the autotrophic
component (that is, plant respiration) increased
each year as the trees became established—hence
there was relatively little change in Re over the
first 3 years (Figure 5B).
August Re for 2005 to 2009 were 60, 50, 54, 90,
and 100 g C m-2, whereas GEP totals were 14, 29,
43, 113, and 150 g C m-2, respectively (Figure 5B). Somewhat similar to the results for June,
GEP in August increased in each of 2008 and 2009,
relative to the previous year, by approximately
163% and 33%, respectively. In comparison,
increases in August Re during 2008 and 2009,
compared to the previous year, were 67% and
11%, respectively, or approximately one third of
GEP. Therefore, it appears that soil water availability had a greater influence on GEP than on Re,
possibly because the plant hydraulic pathway (for
example, xylem embolism resistance) (Sperry
2003) and leaf stomatal conductance (Krishnan
and others 2006) in these hybrid poplar trees
are very sensitive to soil moisture content. These
results are also consistent with Barr and others
(2007) who reported that for the SOA site, mild-tomoderate drought suppressed Re (mainly through
the suppression of heterotrophic respiration near
the soil surface) but had little effect on GEP,
whereas severe droughts suppressed both Re and
GEP but with greater impact on GEP. The root
systems of these 5-year old trees likely were not
fully developed and confined to upper soil layers, so
they would be relatively sensitive to limited soil
water availability. Reduced metabolic activity would
then greatly affect water and nutrient uptake, hence
limiting photosynthesis quite significantly.
Ecosystem respiration is controlled by multiple
environmental (for example, temperature and
moisture) and biotic (for example, production and
allocation of photosynthate) variables and by the
complex interactions among them (Högberg and
others 2001; Davidson and Janssens 2006; Johnsen
and others 2007). We used the model of Jassal and
others (2005) to make preliminary estimates of soil
respiration rates from the profiles of soil CO2 concentration measured using the Vaisala CO2 sensors.
These estimates could be compared to the above
canopy measurements of NEE. During 2005, and
for much of 2006, heterotrophic soil respiration
was close to 100% of measured NEE because the
trees were only recently established and competing
weed vegetation treated with herbicide and frequent mechanical weeding, while the soil contained large amounts of labile carbon residues from
previous agricultural crops. For 2008 and 2009, the
modeling and other studies (for example, Hanson
and others 2000; Bond-Lamberty and others 2004)
indicated that heterotrophic soil respiration would
account for at least 50% of Re, depending on
Carbon, Water, and Energy Exchanges of a Hybrid Poplar Plantation
aboveground biomass and time of year. Figure 6
shows the seasonal cycle determined from the soil
CO2 concentration measurements. There was a
surprisingly large, transient peak every year around
the time snow melted (mid-April), which is likely
to be partially due to a decrease in substrate diffusivity as the surface layers were temporarily saturated with water from melting snow (Jassal and
others 2005). It could also be due in part to exponential growth and multiplication of ‘‘snow molds’’
which have an optimal temperature range reported
to be -2 to 0°C by Monson and others (2006) and
Schmidt and others (2009). Soil CO2 concentration
then increased exponentially with soil temperature
from late April until late June as indicated by the
slope of the increase, reflecting the intensification
of microbial and plant root activity. In late July and
August, soil CO2 concentration generally decreased
back to the levels of early June, with the magnitude of this decrease largely due to the control of
soil water availability on soil CO2 production. As
senescence and dormancy progressed, soil CO2
concentration ramped down from September to
December, but at a given soil temperature, soil CO2
concentration was higher in September than in
May. This hysteresis probably was caused by
greater availability of labile substrate (litter from
leaves and fine root turnover) in September than in
May. Figure 6 clearly shows the complexity of the
seasonal controls on soil CO2 concentration, with
different dominant variables influencing the concentration at different periods.
Figure 6. Relationship between intra-annual changes in
soil temperature and soil CO2 concentration measured at
100 mm depth at the hybrid poplar site, in the vicinity of
the eddy covariance tower for 2005–2008 (data for 2009
were omitted due to sensor failure in winter 2008/2009).
Large arrows with words in parentheses indicate the causes
of the major changes in CO2 concentration.
ANNUAL CARBON
AND
WATER BALANCES
Table 1 shows the calculated annual GEP increased
rapidly each year, from approximately 20 g C m-2
in 2005 to about 470 g C m-2 in 2009, whereas
annual Re totals also generally increased but much
less dramatically, from about 330 to 450 g C m-2.
Table 1 (column 4) shows the stand became a small
C sink for the first time in 2009, indicated by the
annual NEE of -17 g C m-2 (negative NEE denotes downward CO2 flux and hence net uptake by
the ecosystem). The calculated net loss of C in 2005
was also remarkably close to the 335 ± 8 g C m-2
reported by Saurette and others (2008) for the first
year of another var. Walker plantation also established in central Alberta.
In this study, our primary data quality control
was the u* criterion for both daytime and nighttime
EC measurements. For the period June to September, the u* criterion was used to remove about
14%, 11%, 17%, 12%, and 18% of the data for
each of 2005 to 2009, respectively. As shown in
Figure 7, ecosystem respiration in winter months
was entirely estimated. Hence, Re was estimated for
periods when the eddy covariance system was not
operating, assuming a linear response of R10 to
temperature and seasonality. Figure 7 provides an
example of the smoothing approach described
previously which was used to estimate annual
carbon balances. It should be noted that the
smoothing did not account for the very transient
peaks in soil CO2 concentration seen in April 2006–
2009, which would imply a significant increase in
measureable Re for a few days each year. It is
therefore likely that the annual estimates of positive NEE are conservative and that in 2009, the
small C sink may actually have been a small source.
With these caveats in mind, the total loss of C from
the stand over the 5 years was approximately
630 g m-2 (6.3 mg C ha-1).
The time for a stand of trees to reach C-neutrality
(that is, zero annual total NEE) depends on the
history of land use, silvicultural practices, ecophysiological traits and growth rates of the vegetation, and climate and soil characteristics. Carbonneutrality was estimated to be 4 years for a slash
pine plantation in Florida (Clark and others 2004),
4–5 years for a white pine plantation in southern
Ontario (Peichl and others 2010), 13 years for Scots
pine naturally regenerated after logging in Siberia
(Schulze and others 1999), 19 years for boreal
black spruce regenerated after fires in Manitoba
(Litvak and others 2003), and 20 years for a
coastal Douglas-fir plantation in British Columbia
(Humphreys and others 2006). All of these earlier
T. Cai and others
Table 1.
Summary of Annual Carbon and Water Balances at the Hybrid Poplar Plantation, 2005 to 2009
Year
GEP (g C m-2)
Re (g C m-2)
NEE (g C m-2)
P (mm)
ET (mm)
2005
2006
2007
2008
2009
21
110
237
392
469
333
297
351
428
452
312
187
114
36
-17
352
358
395
414
226
281
304
304
323
277
Values for January to March 2005 and for November to December 2009 were estimated due to lack of local measurement data. Winter precipitation data were estimated from
Environment Canada climate stations within a 90 km radius of the study site. Symbols are defined in the text.
Figure 7. Calculated 5-day mean values of the normalized respiration rate at 10°C (R10), derived from nighttime half-hourly NEE measurements made when u* >
0.1 m s-1 (open circles) compared to the smoothed values
(solid squares) used to estimate ecosystem respiration as a
function of half-hourly air temperature. A minimum
value of 0.5 was assumed for the smoothed value of R10
during winter.
studies ‘‘traded space for time’’ by studying chronosequences, that is, by comparing the annual C
fluxes for multiple stands of distinctive developmental stages and ages (Amiro 2001; Goulden and
others 2006). Although such studies are extremely
useful, they suffer from concerns about differences
in soil conditions and microclimate which may
confound the long-term trends inferred from multiple sites over short periods. In contrast, the present study followed the stand’s C dynamics directly
and continuously from the date of planting. This
unique feature was possible because we were able
to locate a plantation extensive enough to enable
excellent eddy covariance measurements to be
made over multiple years. Furthermore, the rapid
achievement of C-neutrality reflects the intrinsically fast growth of this hybrid poplar variety,
which may be comparable to pine plantations
reported by Clark and others (2004) and Peichl and
others (2010). The planned rotation age for man-
Figure 8. Seasonal water balances from June 2005 until
November 2009. For illustrative purposes, the annual
totals of precipitation (P) and evapotranspiration (ET)
were split into active (June–September) and dormant
(October–May) periods.
aged hybrid poplar stands in central Alberta is only
20 years.
Annual totals of evapotranspiration (ET) varied
from year to year (Table 1, column 5) and were
compared to annual precipitation in Figure 8. Total
ET and P were partitioned into two periods for each
year: ‘‘dormant’’ (October–May) and ‘‘active’’
(June–September). This was necessary because the
EC system was not operational during the winter
months, and it is important to recognize that the ET
data presented for winter periods are estimates that
should be treated with caution. Dormant period ET
(mainly soil evaporation and snow sublimation)
generally accounted for approximately 20% of the
annual total except for 2006, when it accounted for
almost 45%, likely because the dormant period
during 2006 was warmer and wetter than normal
(Figure 1), leading to higher evaporation from the
largely exposed soil surface. ET during the active
period generally exceeded or barely balanced the
corresponding P, demonstrating the importance of
Carbon, Water, and Energy Exchanges of a Hybrid Poplar Plantation
winter precipitation (mainly snowfall) to meet
summer evaporative demand. Although leaf area
index (LAI) of the trees achieved during 2009 was
certainly the highest of the 5 years, annual ET was
the lowest in that year and still exceeded annual P.
In particular, P during the active period of 2009 was
only about 50% of the corresponding ET.
Our estimates of winter ET do not account for the
possible effects of stand structure on water vapor
sublimation from the snow surface. It is possible
that the sheltering effect provided by the trees from
the end of the 2007 growing season onwards would
reduce sublimation compared to a bare field,
helping to conserve moisture for the following
growing season. The importance of such an effect
remains conjectural, but could be investigated if
year round EC measurements could be accomplished.
Annual ET at the hybrid poplar stand during the
first 5 years was lower than that at SOA, where the
average was approximately 420 mm during 1994–
2003 (Griffis and others 2003; Barr and others
2007; Krishnan and others 2006). The stand density for SOA is approximately 830 stems ha-1,
compared to 1,100 stems ha-1 for the much
younger poplar stand with regular 3 m spacing.
Given that annual precipitation in 2005–2009 has
generally been at or below normal and that the
stand has yet to achieve maximum LAI, it is still
possible that ET at the hybrid poplar stand could
match or exceed that at SOA. At present, the
hybrid poplar stand does not have a significant
understory, unlike the dense hazelnut cover at
SOA, due mainly to aggressive weed control practices. However, as the trees are now tall enough to
shade out competing vegetation, weed control will
no longer be needed, and it is to be expected that
understory evapotranspiration may become more
significant, particularly as the fastigiate growth habit
of the var. Walker poplar clone will prevent complete canopy closure from occurring during the
20-year rotation. In comparison with coniferous
stands, annual ET was similar to that for a southern
boreal old black spruce stand (300 mm) and
higher than that of a southern boreal old jack pine
stand (237 mm), both reported by Kljun and
others (2006). It was also markedly lower than that
of a mature coastal Douglas-fir stand (400 mm)
(Jassal and others 2010). The low P in 2009 evidently limited NEE, demonstrating that annual
precipitation at this site can already limit growth as
well as ET. It remains to be seen whether a return
to ‘‘normal’’ annual precipitation will occur in
coming years to stimulate more rapid growth and
annual ET comparable to or exceeding other boreal
region forest stands, or whether the recent series of
below-normal years is more typical of the future,
imposing limitations on growth rates. Increased
summer continental drying and associated drought
at mid-latitudes have been cited as a likely consequence of global climate change (IPCC 2007). The
continuation of our study will help address how
deciduous plantations at mid-latitudes will respond
to severe and persistent droughts (Hogg and others
2005; Krishnan and others 2006), and their effect
on regional climate and hydrology (Hogg and others 2000; Jackson and others 2005).
LARGE-SCALE IMPLICATIONS
OF THIS
STUDY
Several studies have indicated that changes in land
surface cover, particularly those affecting leaf area
index (LAI), can affect precipitation patterns, and
may already have had significant impacts on global
climate (for example, Chase and others 1996;
Pielke Sr 2001). In Canada’s Prairie Provinces,
where much of the 9 million ha considered suitable
for afforestation exists (Natural Resources Canada
2006), droughts are frequent and likely to increase
according to many global climate model scenarios
(IPCC 2007). Given the potential for increased ET
as hybrid poplar stands reach maturity, large-scale
afforestation might cause local water tables to be
lowered (compared to grassland or dryland crops
such as wheat). If these plantations do use more
water, however, this does not necessarily make less
water available elsewhere. Hogg and others (2000)
found that a significant fraction of rainfall in the
western boreal forests is likely recycled, having
been evaporated from the grassland and agricultural regions further south.
CONCLUSIONS
The carbon (C) budget of a fast-growing hybrid
poplar plantation established in a water-limited
region of central Alberta, Canada, was measured
using eddy covariance for the five growing seasons
following planting in June 2005. Gross ecosystem
production (GEP) increased every year, causing the
annual C balance to shift from being a source of at
least 3.3 mg C ha-1 in the first year to approximately C neutral in 2009. There was significant
interannual variation in growing season duration,
heat sum, and water balance over the 5 years,
which greatly affected annual C balance. The
analysis showed that GEP and ecosystem respiration were particularly sensitive to temperature
during spring and to soil water availability in
summer. In particular, 2009 was noteworthy for
T. Cai and others
being exceptionally cold in spring and for receiving
only 50% of normal precipitation. The low spring
temperatures delayed leaf emergence and development significantly, whereas the very dry conditions reduced growth rates throughout July and
August—both contributing to annual growth and
carbon uptake well below their potential that year.
In fact annual rainfall was lower than the 30-year
normal in 4 out of the 5 years, which raised
questions about regional climate trends, the potential responses of the plantation in wetter than
normal years, and the general viability of afforestation in this region with a warming climate. It is
probable that as the plantation increases in size and
leaf area index (LAI), growth will become even
more water-limited in some years. Even if precipitation increases in the future (as expected from
climate scenario projections), it is unlikely this increase would generally be sufficient to offset the
increased evaporative demand.
Albedo measured at the eddy covariance tower
did not change appreciably over three winters,
even as the trees and LAI grew rapidly, (except in
February 2006 when early snowmelt was almost
certainly not limited to the plantation). These results suggest that large scale modeling estimates of
increased radiative forcing due to lower albedo
resulting from afforestation in high latitudes could
be significantly exaggerated in regions where fastgrowing deciduous plantations can be established
over large areas. However, as with the trends
observed to date in the annual energy balances, it is
important to remember that the plantation has not
yet reached maturity, so assertions for future
changes in albedo remain conjectural.
Because there are multiple feedbacks resulting
from changes in land use, affecting both water and
energy balances as well as C dynamics, the climatic
consequences of replacing present-day agricultural
land with extensive forest plantations in a region
that is already water-limited cannot easily be
determined. Resolving such questions will require
process models, adequately calibrated using data
from comprehensive multi-year studies such as the
one reported here, to realistically couple vegetation
physiological responses to regional climate.
ACKNOWLEDGEMENTS
We would like to thank management and staff of
Alberta-Pacific Forest Industries Inc. for their continued interest and financial support. Derek Sidders, Jag Bhatti, Carmela Arevalo and Marty
Siltanen of Natural Resources Canada, Northern
Forestry Centre in Edmonton, have provided much
input throughout the course of the study. Scott
Chang and Robert Grant of the University of Alberta, and Rachhpal (Paul) Jassal and Andy Black
of UBC have all contributed help and ideas. Don
Huber, owner of the Huber Farm near Ashmont,
Alberta, has allowed us unhindered access to the
site. We greatly appreciate the valuable comments
and suggestions from Altaf Arain of McMaster
University who reviewed an early version of the
manuscript, as well as the positive and helpful
suggestions from two anonymous reviewers.
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