Download Postulated Feedbacks of Deciduous Forest Phenology on

VOLUME 13
J O U R N A L OF C L I M A T E
IS DECEMBER 2000
Postulated Feedbacks of Deciduous Forest Phenology on Seasonal
Climate Patterns in the Western Canadian Interior
E. H. HOGG
AND
D. T. PRICE
Canadian Forest Service, Edmonton, Alberta, Canada
T. A. BLACK
Department of Soil Science, University of British Columbia, Vancouver, British Columbia, Canada
(Manuscript received I November 1999, in final form 22 February 2000)
ABSTRACT
A large portion of the western Canadian interior e xh ibits a d istinctive seasonal pattern in long-term mean
surface temperatures characterized by anomalously warmer conditions in spring and autumn than would be
e xpected from a s inusoidal modeL The anomaly is greatest over the southern boreal forest of western Canada,
where trembling aspen (Populus tremuloides M ichx.)-a deciduous, broad-leaved species -is an important
component. In this region, mean temperatures are 2°_3 °C warmer in April and October but nearly rc cooler
in June and July, relative to a best-fitting s inusoidal function. Another feature of the climate in this region is
that average precipitation is low ( 15-30 mm month-I) from October to April but increases sharply during the
summer growing season (50-100 mm month-I from June to August). Eddy correlation and sap flow measurements
in a boreal aspen forest indicate profound seasonal changes in transpiration and energy partition ing assoc iated
w ith the deciduous nature of the forest canopy. Latent heat (water vapor) flux reaches a maximum during the
summer period when leaves are present, while sensible heat flux is h ighest in early spring when the forest is
leafless. Thus , it is postulated that feedbacks of leaf phenology of aspen forests, which occupy a large area of
the western Canadian interior, may contribute significantly to the distinctive seasonal patterns of mean temperature
and precipitation that occur in this region.
1. Introduction
Although there is a long history of speculation that
forests affect rainfall patterns, the idea that terrestrial
vegetation may significantly influence climate at re­
gional to global scales received little attention during
much of the twentieth century (Thompson 1980; Hay­
den 1998). Over the past two decades, however, ad­
vances in technology have enabled studies of vegetation
feedbacks on atmospheric and climatic processes over
a wide range of scales (Pielke et al. 1998) through sim­
ulation modeling and interdisciplinary field experiments
such as the boreal ecosystem-atmosphere study (BO­
REAS), which focused on a large study area in the west­
ern Canadian boreal forest (Sellers et al. 1997). Such
studies have resulted in major changes in perspective
concerning the nature of vegetation-climate interac­
tions. For example, it had been suggested by Bryson
Corresponding author address: E. H. (Ted) Hogg, Research Sci­
entist, Northern Forestry Centre, Canadian Forest Service, Natural
Resources Canada, 5320-122 Street, Edmonton, AB T6H 3S5, Can­
ada.
E-mail: [email protected]
© 2000 American Meteorological Society
(1966) that the distribution of the Canadian boreal forest
is determined by the average positions of the arctic front
in winter and in summer. More recently, however, Pielke
and Vidale (1995) proposed that the boreal forest itself
has a significant influence on the position of the arctic
front. This interpretation was based on BOREAS mea­
surements showing that low albedo of the boreal forest
causes significantly greater heating of the overlying air
mass relative to that over arctic tundra, leading to a
significant warming of regional climate (Otterman et al.
1984; Foley et al. 1994; Bonan et al. 1995). Another
potentially important feedback of vegetation is the re­
cycling of precipitation and soil moisture through
evapotranspiration. Indeed, recent studies indicate that
regional rainfall may decline significantly following de­
forestation and other losses of vegetation cover, espe­
cially in continental interiors such as the Amazon basin
(Shukla et al. 1990), the Sahel region of west Africa
(Savenije 1995), and in northern China and Mongolia
(Xue 1996).
In the continental interior of western Canada, the bo­
real forest occupies an area of more than one million
square kilometers and is composed of a variety of forest
types, including evergreen conifers [e.g., black spruce,
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J O U RN A L O F C L I M A T E
Picea mariana (Mill.) BSp, and jack pine, Pinus bank­
siana Lamb.] and deciduous hardwoods, of which trem­
bling aspen (Populus tremuloides Michx.) is by far the
most abundant (Peterson and Peterson 1992; Hogg
1994). One of the major observations of the BOREAS
experiment (Sellers et al. 1997) was that the midsummer
evaporative fraction (the ratio of latent heat flux to avail­
able energy) was much smaller over the coniferous for­
est (ca. 0.3-0.45) than over trembling aspen forest (ca.
0.6-0.9). Despite the slightly lower albedo of coniferous
forest (Betts and Ball 1997), midsummer transpiration
rates were nearly twice as great from the trembling as­
pen forest as from the coniferous forest ecosystems stud­
ied in BOREAS (Black et al. 1996). For deciduous veg­
etation such as aspen, however, there was strong sea­
sonal variation in energy partitioning associated with
leaf emergence and senescence (Blanken et al. 1997),
as transpiration rates decreased to near zero during the
seasons when the aspen canopy was leafless (typically
early October to mid or late May).
Aspen-dominated deciduous and mixed-wood forests
occupy a significant proportion of the southern boreal
forest of western Canada (Peterson and Peterson 1992),
Major aspen areas
(boreal and cordilleran)
*
BOREAS Old Aspen site
!?;;/a Aspen parkland
I:::: :1 Grassland (prairie)
D
I: : : j
especially in the province of Alberta (Fig. 1), where
these forest types occupy about half of the total forested
area based on maps from the Canadian Forest Inventory
(Lowe et al. 1994). Trembling aspen is also the pre­
dominant tree species in the aspen parkland, a transi­
tional vegetation zone located between the boreal forest
and the prairies to the south (Bird 1961; Hogg and Hur­
dle 1995).
There has been increasing recognition of the impor­
tance of adequately characterizing seasonal changes in
land surface characteristics, such as the leaf area index
of deciduous vegetation, for realistic regional represen­
tation of climate in general circulation model (GCM)
simulations (e.g., Xue et al. 1996). For example,
Schwartz (1992, 1996) showed that the onset of spring
leafing in eastern North America leads to discontinuities
in the time series of meteorological conditions near the
surface, including humidity, wind, and diurnal temper­
ature range. Thus, the seasonal cycle of vegetation leaf­
ing and senescence may exert a strong control over
land-atmosphere interactions (Moulin et al. 1997). De­
spite the abundance of aspen forests and other deciduous
vegetation in the western Canadian interior, the potential
Arctic
Subarctic (forest-tundra)
$=1 Boreal forest
U
1/' ,,"J
Temperate forest
Cordilleran (forest and alpine)
\
FIG. I. Map showing d is tribu tion of major v eg etation zones (adap ted from Ecoregions Working Group 1989) and selected cl imate s tations
in w es tern and c en tral Canada (codes defined in Table I). Major aspen areas are general ized based on Canada's forest inventory ( e.g., Lowe
et al. 1994).
15 DECEMBER 2 000
4231
H O G G E T AL .
TABLE 1 . List of climate s tations shown in Fig. I , with coefficien ts (a, b, and c) for b es t-fi tting s in usoidal model [Eq. ( I ) ] and magnitude
of m ean April/October temperature anomaly ( /,,) for the period 1 96 1 -90.
S ta tion
name
'Fort Vermilion , Alta.
'Fort McMurray, Al ta.
Fort Nelson, B .C.
'Princ e Alb ert, Sask.
*Loon Lake, Sask.
Fort Smith , N.W. T.
*Winnipeg, Man.
*The Pas, Man.
*Dauphin , Man.
Thompson, Man.
Armstrong, Ont.
*Edmonton , Al ta.
*Fairview, Al ta.
B eaverlodge, Al ta.
Regina, Sask.
Val D' Or, Que.
L ethbridg e, Al ta.
Norman Wells, N. W.T.
Kamloops, B .C .
Toronto, Ont.
Baker Lake, Nunavut
Vancouver, B .C .
Map
code
Latitude
(N)
Longitude
(W)
FV
FM
FN
PA
LO
FS
WI
TP
DA
TH
AR
ED
FA
BV
RE
VL
LE
NW
KA
TO
BL
VN
58°23'
56°39'
58°50'
53°1 3 '
54 °03 '
60°0 1 '
49°54'
5 3 °5 8 '
5 1 °06'
55°48'
50°1 7 '
53°1 8 '
56°04'
55°12'
50°26 '
48°04'
49°38'
65°17'
50°42'
43 °40'
64 °1 8 '
49°1 1 '
1 1 6 °02'
I WI 3 '
1 22 °35 '
105°4 1 '
109°06'
1 1 1 °5 7 '
97 °14'
1 0 1 °06'
1 00°03'
97°52'
88°54'
1 1 3 °35'
1 1 8 °23'
1 1 6 °24'
104 °40'
77°47 '
1 1 2 °48'
1 26 °48 '
1 20°27 '
79°24'
96°05 '
1 23 °1 0 '
(,�
a
-0.8
0.3
-1 .0
0.5
0.8
-2.9
2 .5
-0.2
1.8
-3 .3
-1 .2
2. 1
1 .8
2.0
2.5
1.3
5.7
-5.9
8 .7
9.0
-1 2 . 1
9 .9
b
c
1 9 .9
1 8 .3
19.7
1 8.8
1 7 .5
2 1 .1
1 8 .9
1 9. 5
1 8.4
20.3
18.3
1 5 .4
1 5 .9
14.7
1 7. 8
1 7.0
1 3 .3
23.3
1 2.5
1 3 .3
22.4
7.2
1 3.5
1 3 .6
1 1 .0
1 5 .7
1 5.2
1 6.0
1 7.7
1 8.2
18.1
17.1
20.6
1 4 .9
14.3
1 5 .0
16. 1
20.7
16.8
1 5 .3
1 2 .7
23.4
26.8
19.3
IT
2.9
2.9
2.8
2.7
2.7
2 .5
2.4
2.4
2 .3
2.2
2.2
2.1
2.1
2.0
2.0
1 .5
1 .0
0.4
0.4
0.2
-0.3
-0.5
* S tations used in s easonal analyses of daily temperature and precipitation data (Fig . 10).
role of leaf phenology as a cause of feedback on the
climate of this region has received little attention. Based
on model simulations showing that terrestrial evapo­
transpiration promotes rainfall and causes summer cool­
ing (e.g., Shukla and Mintz 1982; Shukla et al. 1990;
Dirmeyer 1994), we postulated that leaf phenology of
deciduous vegetation may exert a significant influence
on seasonal patterns of temperature and precipitation in
the west-central Canadian interior east of the Rocky
Mountains (Fig. 1).
Even if such feedbacks are important, they may be
difficult to detect over one or two growing seasons be­
cause of high temporal and spatial variability in mete­
orological conditions, particularly precipitation. An
analysis of the long-term climate record, however, might
provide evidence of vegetation feedbacks in this region.
The objective of this study was to examine the geo­
graphic variation in long-term averages of seasonal
changes in temperature and precipitation patterns as a
preliminary means of determining the likely importance
of deciduous vegetation phenology on the climate of the
western Canadian interior. We also compared the results
of this analysis with observed seasonal changes in en­
ergy partitioning and transpiration at the BOREAS Old
Aspen tower site, located centrally within the region of
interest near the southern edge of the Canadian boreal
forest.
2. Methods
a. Climate data
Climate data used in the analyses included the 196190 monthly mean temperature and precipitation normals
for Canadian stations from Environment Canada (1994).
Additional climate data for 1961-90 were used to gen­
erate maps of spatial variability across North America
(see below). A more detailed analysis was also con­
ducted using daily temperature and precipitation data
for nine climate stations located in or near the major
commercial zone of trembling aspen in the southern
boreal forest of the Canadian prairie provinces of Al­
berta, Saskatchewan, and Manitoba (Table 1; Fig. 1).
These stations were selected based on having a relatively
long period of record (30-107 years) and proximity to
grid points of GCM outputs from the Canadian Climate
Centre (see below).
h.
Pattern of seasonal temperature variation
The seasonal cycle of long-term, mean temperature
closely resembles a simple sinusoidal function at most
midlatitude locations, primarily owing to the sinusoidal
nature of the annual variation in solar forcing resulting
from the earth's orbit around the sun and its tilted axis
of rotation (List 1958; Jones 1992). However, in a pre­
liminary analysis, we noticed a distinctive pattern of
seasonal variation in mean monthly temperature at sev­
eral climate stations in the western Canadian interior.
This pattern is characterized by long-term mean tem­
peratures that are anomalously warmer during April and
October, relative to what would be expected from a
sinusoidal function, with correspondingly cooler-than­
expected mean temperatures in midsummer (example
shown for Prince Albert, Saskatchewan, in Fig. 2).
In the present study, this feature was examined for
4232
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VOLUME 1 3
J O U RN A L O F C L I M A T E
0
western Canadian interior, it was found that the two
largest positive deviations in mean monthly temperature
consistently occurred during April (aTApr) and October
(aToct)' Thus, an appropriate index (IT) of the observed
pattern of anomalous warmth in spring and autumn
could be calculated as follows:
(2)
0
E -10
c:
'"
Q)
�
•
Alternatively, / T can be estimated using a much simpler
method, both conceptually and computationally:
JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC
Month
FIG. 2. M ean monthly temperatures at P rinc e Albert, Saskatchewan ,
fo r th e p eriod 1961-90 ( indicated as symbols; data from Environmen t
Canada). Observed m ean temperatures in Apr and Oct are 2.60 and
3. 1°C, respec tiv ely, warmer than those obtained from th e bes t-fi tting
s inusoidal equation ( indicated as solid l ine). Equa tion ( 1) parameters
are giv en in Table 1.
Canadian climate stations by first calculating the de­
viation of observed mean monthly temperature from that
obtained from a best-fitting sinusoidal equation of the
following form:
T,
=
a - b cos[21T(t - c)/365],
(1)
where T, is modeled mean temperature for each day of
the year (t
1 to 365 day, corresponding to the dates
1 January to 31 December) as estimated by the equation,
a is the modeled mean annual temperature ('C), b is the
modeled seasonal amplitude of mean temperature, and
c gives the modeled seasonal phase lag in days. Modeled
monthly temperatures were derived from Eq. (1) through
averaging the modeled daily values belonging to each
month.
The best-fitting sinusoidal function was determined
for each climate station through an iterative, nonlinear
regression procedure in which the values of the three
coefficients were sequentially varied to obtain the min­
imum sum of squares of the deviations between ob­
served and modeled mean temperatures for each month.
Coefficients a and b were initially set to the correspond­
ing values of observed mean temperature and its sea­
sonal amplitude, while the lag coefficient (c) was varied
in steps of 0.1 day from 0 to 35 day to obtain the smallest
sum of squares of the deviations. Using the optimum
value of c, the amplitude (b) was then varied in steps
of O.I°C across all values within ±5°C from its initial
value. The optimum value of b was then used and the
procedure was repeated until the sum of squares of the
deviations reached a constant minimum value. Coeffi­
cients b and c were then set to their optimum values
while varying the value of a in 0.05°C steps, but the
optimum value of a was always within ±O. loC of its
initial (observed) value. The deviations (aT) of ob­
served minus modeled values of mean monthly tem­
perature were then determined for each station.
After applying the above analysis for stations in the
=
(3)
where TApr and Tact are mean observed monthly tem­
peratures for April and October, respectively, and TAnnual
is the mean observed annual temperature. Based on an
analysis of 582 major climate stations across western
and central Canada, estimates of / T using the two meth­
ods were similar within expected statistical and mea­
surement errors [values using Eq. (3) were smaller by
0.057°C, with std dev
0.046°C].
=
c. Seasonal variation in solar radiation
Seasonal changes in observed values of monthly
mean global solar radiation (Environment Canada 1982)
were also examined from available data for the region
of interest. The effect of latitude on the seasonal pattern
of potential (extraterrestrial) solar radiation incident on
a horizontal surface was also modeled using a simple
sinusoidal function for solar declination (annual ampli­
tude of 23.44°) with an assumed seasonally constant
solar flux density of 1370 W m-2• Values of either ob­
served or potential solar radiation were fitted to a si­
nusoidal function using the same method as that used
for temperature [Eq. (1)].
d. Seasonal pattern of precipitation
It was postulated that large-scale seasonal changes in
transpiration of aspen forests, associated with leaf phe­
nology, might lead to a feedback that would lead to an
enhancement of precipitation during the summer period
when canopies are usually fully leafed (June to August),
relative to spring and fall periods when canopies are
generally leafless (April and October). As a first step
toward examining this hypothesis, an index of the sum­
mer enhancement of precipitation (/p) was calculated
from the 1961-90 climate normals for each station as
follows:
where PJun' PJul, and P Aug are mean monthly precipitation
(mm) for June, July, and August (summer), respectively,
while P A pr and POct are mean monthly precipitation for
April (spring) and October (autumn), respectively.
H O G G ET AL .
1 5 DECEMBER 2000
4233
.fiG. 3. Dis tribu tion of climate s tations used in th e spatial analysis of m ean monthly temperature
and precipitation ( 1 96 1 -90). Addi tional climate s ta tions repor ting precipita tion only, and also used
in the spatial analysis, are not shown.
e. Mapping of spatial variation in climatic
characteristics
Monthly normals for the period 1961-90 were ob­
tained (Environment Canada 1994) for 2831 climate sta­
tions for temperature and for 3154 climate stations for
precipitation, supplemented by data for 1726 British Co­
lumbia stations obtained from C. Daly at Oregon State
University (http://www.ocs.orst.edulpub/dalyibc..p2.1st).
In addition, mean monthly values for the 1961-90 pe­
riod for the United States and Mexico were calculated
using adjusted data for 2009 temperature and 2675 pre­
cipitation stations extracted from the Global Historical
Climate Network database (http://www.ncdc.noaa.gov/
cgi-binlres40.pl). The combined dataset provided good
spatial coverage for most of North America, especially
over most southern portions of Canada (Fig. 3). Indices
of anomalous warmth in April and October [IT; Eq. (3)]
and of summer precipitation patterns [/p; Eq. (4)] were
calculated for each station location. Elevation data were
obtained from the hydrologically corrected USGS GTO­
P030 digital elevation model (http://edc.usgs.gov/
landdaac/gtop030/hydro/nulem.html) and resampled us­
ing the ARCIINFO GRID Program (Environmental Sys­
tems Research Institute, Inc., Redlands, CA) to a lO-km
square grid on the Lambert Conformal Conic projection.
Values of IT and Ip were then interpolated to this lO-km
grid by applying the gradient plus inverse distance squared
(GIDS) weighting interpolation method (Nalder and Wein
1998; Price et al. 1998; Price et al. 2000), treating latitude,
longitude, and elevation as independent variables.
f Comparison with GCM outputs
The average observed value of IT was estimated for
the commercial zone of trembling aspen in the southern
boreal forest of western Canada, based on the 1961-90
monthly climate normals from the nine representative
stations (Table 1) used in the more detailed analyses
indicated above. This was compared with the corre­
sponding average value of IT calculated at the nine ad­
jacent grid points from the 1 X CO2 outputs of the
Canadian Climate Centre GCM2 (McFarlane et al. 1992;
http://www.cccma.bc.ec.gc.ca).
g. Field measurements at a boreal aspen site
A large international field experiment (BOREAS) was
conducted in the region, primarily during 1994 and
1996. BOREAS has provided a wealth of information
on mass and energy exchange between the boreal forest
4234
J O U RN AL O F C L I M AT E
and the atmosphere (Sellers et al. 1997). Measurements
included tower-based monitoring of sensible and latent
heat (water vapor) fluxes at sites in several forest types,
including the Old Aspen site (53°38'N, 106°12'W), lo­
cated about 70 km northeast of Prince Albert, Saskatch­
ewan (Fig. 1). This site is almost exclusively dominated
by a pure stand of trembling aspen about 70 years old
and 18-22 m tall, with an understory shrub layer dom­
inated by 2-m-tall beaked hazelnut (Corylus cornuta
Marsh.). Eddy correlation measurements of sensible and
latent heat fluxes were made at the site in 1993-94
(Black et al. 1996; Blanken et al. 1997), while water
fluxes were also estimated from sap flow measurements
within individual aspen stems at the same site (Hogg et
al. 1997). However, these measurements did not fully
cover the autumn leaf-fall period; thus in this paper, we
report similar results for 1996, when both sap flow and
eddy correlation measurements were made continuously
from mid-April to late October. For each day, 24-h av­
erages of sensible and latent heat flux were calculated
from half-hourly measurements made at the top of a
39-m tower above the aspen canopy using the methods
described by Blanken et al. (1997). Sap flow was mon­
itored hourly at the I.3-m height on two aspen tret<s -at
each of two locations using the heat pulse method (Hogg
and Hurdle 1997; Hogg et al. 1997). Results from the
four trees were averaged and reported as daily averages
of sap flow per unit area of conducting sapwood (Qs;
mm3 h-I m-2) (Edwards et al. 1996; Hogg et al. 2000).
I.5°C). At the selected Canadian climate stations outside
the range of the boreal forest, however, the seasonal
patterns in f1T were inconsistent, with little or no ten­
dency for positive temperature anomalies in April and
October (IT from - 0.4° to +1.0°C) (Fig. 4c).
Mapping of spatial variation in IT (Fig. 5) shows a
zone of elevated values (+1.4° to +3.0°C) that extends
>4000 km from the Yukon-Alaska boundary to north­
ern Quebec, encompassing most of the Canadian boreal
forest, aspen parkland, and northern prairies. The areas
with largest IT (>2.2°C) include most of the major areas
with significant aspen-dominated forest cover in the por­
tion of the Canadian boreal forest west of 95°W lon­
gitude (Fig. 1), although high values also occurred in
the Yukon and the northern third of Saskatchewan,
where aspen is common but not a regionally dominant
component of the vegetation. In east-central North
America, values of IT appeared to be reduced by about
1°C in the Great Lakes region, including the major aspen
areas in Ontario and Quebec. Slightly positive values
of IT (0° to +1.4°C) occurred over most of the other
forested areas in Canada and also across much of the
central and eastern United States. Values of IT were
slightly negative (-1° to O°C) along the Atlantic and
Pacific coasts and across most of the western United
States and strongly negative over western Alaska and
the high arctic islands, especially Ellesmere Island (IT
< - 4°C).
h.
3. Results and discussion
a. Seasonal patterns in monthly mean temperature
Long-term monthly mean temperatures showed a dis­
tinctive seasonal pattern over the southern boreal forest
of the west-central Canadian interior, where trembling
aspen form a major component of the vegetation (Fig.
1). The general nature of this pattern is shown in the
climate of Prince Albert, Saskatchewan, where spring
and autumn temperatures are warmer and summer and
winter temperatures are cooler than expected from the
best-fitting sinusoidal equation (Fig. 2). The seasonal
pattern and magnitude of monthly deviations (f1T) were
similar for representative climate stations located in or
near major areas of aspen forest cover over a geographic
area extending at least 1500 km, from northwestern Al­
berta to southern Manitoba (Fig. 4a). The months with
the greatest positive values of f1T were in April (+1.7°
to +2.6°C) and October (+2.4° to +3SC), giving av­
erage April and October temperature anomalies (IT) of
2.1° to 2.9°C (Table 1). Values of f1T were consistently
negative during the summer months of June, July, and
August at each of these stations. Seasonal patterns in
f1T were also similar at other stations located elsewhere
in the Canadian boreal forest across a distance of >3000
km (Fig. 4b), although the trend was less pronounced
at the easternmost location in western Quebec (IT
=
VOLUME 13
Solar radiation patterns
As might be expected, seasonal change in observed
global solar radiation closely approximates a sinusoidal
relationship based on the long-term monthly average of
four climate stations with available data in the boreal
forest and aspen parkland of western Canada (latitudes
ranging from about 50° to 55°N) (Fig. 6a) Thus, ob­
served solar radiation patterns cannot explain the dis­
tinctive seasonal pattern for mean temperature in this
region, where conditions in spring and autumn are
warmer and summer temperatures are correspondingly
cooler than would be expected from a sinusoidal rela­
tionship.
If the effects of cloud and other atmospheric absorp­
tion influences are neglected, the theoretical seasonal
pattern of potential solar radiation incident to a hori­
zontal surface is nearly sinusoidal at a latitude of 45°N,
but for more northerly latitudes, potential solar radiation
exhibits negative anomalies from a best-fitting sinusoi­
dal equation during spring and autumn (as shown for
600N in Fig. 6b). Thus, it appears that the expected
latitudinal influences on solar radiation patterns have a
relatively minor influence on the observed spatial var­
iation in IT across Canada and the northern United States
(Fig. 5) except in areas north of the Arctic Circle, where
the observed strongly negative values of IT may be ex­
plained, at least partially, by the prolonged winter period
of polar night.
4235
HOGG ET AL.
15 DECEMBER 2000
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JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC
FiG. 4. Devia tions (dT) of monthly m ean temperature from best-fi tting s inusoidal equa tions for selec ted
Canadian cl imate s tations located (a) n ear major areas of aspen forest cover in th e Canadian prairie provinces
show ing a common s easonal pattern of monthly temperature anomalies ( IT = 2. 1 ° to 2.9°C); (b) o th er areas
showing a similar s easonal pattern ( IT = 1 .5 ° to 2.8°C); and (c) areas lacking this s easonal pattern' ( IT = -0.5°
to 1 .0°C). Equation (I) parameters for each s ta tion are g iv en in Tabl e 1 .
4236
VOLUME 13
JOU RNAL OF CL I MATE
60
-2.6
-2.2
-1.8
-1.4
-1.0
-0.6
-0.4
50
-0.2
0.0
0.2
0.4
0.6
40
1.0
1.4
1.8
2.2
2.6
30
120
110
100
90
80
70
FIG. 5. Map showing spatial variation in the average Apr and Oct temperature anomaly (IT) [see Eq. (3) in text] in Canada and adja­
cent areas of the United States based on the interpolation of values from climate stations for the period 1961-90.
c. Seasonal patterns in monthly mean precipitation
Climate stations in the southern boreal forest and as­
pen parkland of western Canada show strong seasonal
variation in precipitation (Fig. 7). Mean monthly pre­
cipitation generally peaks at 60-100 mm during the
summer months (June-August) but is typically low (1530 mm) from October to April. The spatial distribution
of Ip (Fig. 8) shows that the area of the Canadian boreal
forest tends to exhibit greater summer enhancement of
precipitation than the areas located either to the north
(subarctic) or south (prairie or temperate forest). Over
most of the boreal forest and aspen parkland, there is a
significant summer enhancement of precipitation (Ip >
40 mm) except in the more northerly areas (e.g., North­
west Territories) or in areas adjacent to large bodies of
water (e.g., the Great Lakes, Lake Winnipeg, and along
the Atlantic coast). Values of Ip are particularly high
(>60 mm) over north-central Alberta (Fig. 1), where
areal coverage of trembling aspen is highest based on
the Canadian Forest Inventory (e.g., Lowe et al. 1994).
It should be noted, however, that other areas of North
America showed values of Ip that were nearly as great
(e.g., northern Quebec) or even greater (e.g., southern
Alaska and along the Atlantic coast of the southeastern
United States) than those recorded in Alberta.
d. Observed seasonal changes in a boreal aspen
forest
Tower-based, eddy correlation measurements made at
the BOREAS Old Aspen site (location shown in Fig.
1) during 1996 showed dramatic seasonal changes in
energy partitioning and transpiration associated with
spring leafing and autumn leaf fall of the aspen canopy
(Fig. 9). The flux of sensible heat greatly exceeded that
of latent heat (water vapor) during the spring (April and
May) when the aspen was leafless. However, sensible
heat flux decreased significantly during the period of
increasing leaf area from late May to early June, when
there was a corresponding increase in latent heat (water
vapor) flux that continued to dominate over sensible heat
until the beginning of leaf senescence in early Septem­
ber. Measurements of upward movement of water (sap
flow) within individual aspen stems showed a similar
1 5 DECEMBER 2 000
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JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC
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4237
H O G G E T AL .
20
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JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC
Month
FIG. 6. (a) S easonal chang e in observ ed monthly solar radiation
for aspen-dominated areas of the w estern Canadian interior based on
av erag e values of four climate stations (symbols) from Environm ent
Canada (1982). The climate stations used correspond to the codes
TP, WI, ED, and BV in Fig. 1 . B est-fitting sinusoidal equation is also
shown (solid line). (b) Theoretical values of s easonal change in po­
tential ( extraterrestrial) global solar radiation, as would be m easured
incident to a plane parallel to the earth's surfac e at three latitudes
(thick solid lines). B est-fitting sinusoidal functions are also shown
(thin solid lines; note that at 45°N, the two lines coincide).
seasonal pattern to that of latent heat flux from the eco­
system. Both sets of measurements support the hypoth­
esis that leaf phenology is a major determinant of sea­
sonal changes in evapotranspiration from the aspen­
dominated landscape. Average daily evapotranspiration
rates from the forest were more than five times greater
(2.6 mm day-I) during the period with significant sap
flow when the aspen canopy was in leaf (29 May-28
September) than during the leafless periods in spring
and autumn (average of 0.46 mm day-I for 19 April28 May and 29 September-30 October). Similar results
were reported at this site for 1993-94 (Blanken et al.
1997) except that the increase in latent heat flux started
earlier, as warmer temperatures in May evidently re­
sulted in earlier spring leafing of the aspen canopy (Chen
et al. 1999).
Five years of measurements at this aspen site, in­
cluding studies of seasonal change in leaf area index
(A. G. Barr et al. unpublished), have shown that the
spring increase in water vapor flux was closely linked
to the emergence and expansion of leaves, which started
as early as the end of April during the unusually warm
spring of 1998. The range of dates observed for the first
emergence of aspen leaves was comparable to that re­
corded for this species by Ahlgren (1957) over a 5-yr
period in northeastern Minnesota (27 April-21 May). It
should be noted, however, that there is often a 2-3-week
variation in leaf phenology among aspen clones (Witter
and Waisenen 1978) such that leaf emergence can be
delayed until early June in late-leafing clones during
cool years.
e. Seasonal change in average daily climate at the
regional scale
Mean seasonal changes in temperature and precipi­
tation for each day of the year, shown in Fig. 10, based
120
-6-
E
.s
c:
0
:;:;
S
'0..
'u
�
100
Fort Vermilion, AS
-e-
Edmonton, AS
...... Fort McMurray, AS ..... Prince Albert, SK
-9-...
The Pas, MS
Winnipeg, MS
80
60
a.
>-
::c:
40
�
20
c:
0
-
0
JAN FEB MAR APR MAY JUN
JUL AUG SEP OCT NOV DEC
FIG. 7. S easonal change in m ean monthly precipitation for selected climate stations representing
major areas of aspen cover in the Canadian prairie provinces (196 1-90 normals from Environment
Canada).
4238
VOLUME 13
JOURNAL OF CLIMATE
-260
60
-220
-180
-140
-100
-60
-40
50
-20
o
20
40
60
40
100
140
180
220
260
30
120
110
100
90
80
70
FIG. 8. Map showing spatial variation in the index of summer-enhanced precipitation (lp) [see Eq. (4 ) in text] in Canada and adjacent
areas of the United States based on the interpolation of climate station data for the period 1961-90.
200
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� 150
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APR
91
MAY
122
152
JUN
183
JUL
213
AUG
244
SEP
274
OCT
304
Day of year 1996
FIG. 9. Seasonal change in daily averages of (a) sensible and latent
heat fluxes at the 39-m height above an aspen forest and (b) upward
water movement (sap flow) in four aspen trees. Both sets of mea­
surements were made at the BOREAS Old Aspen site in 1996.
on averages of the long-term climate records from the
nine stations located in or near major areas of aspen
cover in the Canadian prairie provinces of Alberta, Sas­
katchewan, and Manitoba (Fig. 1). These seasonal pat­
terns have a higher temporal resolution than those
shown in Figs. 4 and 7, but are otherwise similar. The
positive anomalies in spring and autumn mean temper­
ature (f).T) are clearly evident, reaching their maximum
values in mid-April and mid- to late October (Fig. 10).
These events coincide approximately with the start and
end of the growing season in the region, when snow is
generally absent from the landscape. One aspect of these
positive temperature anomalies in spring and fall is that
the length of the growing season was significantly great­
er than that obtained from a best-fitting sinusoidal func­
tion with the same mean annual temperature. For ex­
ample, the seasonal period when long-term, mean daily
temperature exceeds 5°C is 18 days longer (23 April13 October) than that obtained from the sinusoidal mod­
el ( l May-3 October). Despite the longer growing sea­
son, however, observed temperatures during the summer
are cooler than those estimated by the same sinusoidal
model (mean f).T
-1.4°C during June and July).
=
HOGG ET AL.
15 DECEMBER 2000
��
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- Long-tenn mean
10
- Sinusoidal equation
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MAR APR MAY JUN
Snow cover
.--4
JUL AUG SEP OCT NOV DEC
Leaves present
�.--.
c-------
,
Snow cover
FIG. 1 0 . Average s easonal variation in characteristics of climate
for nine climate stations (see Tabl e I ) r epresenting major areas of
aspen cover in the Canadian prairi e provinces: (top) m ean observed
daily t emperature (thick line) in relation to b est-fitting sinusoidal
equation ; (middle) m ean daily differenc e (ilT) betw een observed tem­
p erature and the sinusoidal equation; and (bottom) m ean observ ed
daily precipitation. The typical s easonal p eriods with snow cover
(Fisheries and Environm ent Canada 1978) and presence of green
aspen l eaves (Hogg personal observations) in the region are also
shown.
The summer period with negative values of I1T (late
May to mid-September) coincides with the typical pe­
riod when green leaves are present in the forest canopies
of aspen and other deciduous species in the region (Ahl­
gren 1957). This period also coincides with the season
of maximum precipitation, with daily values of 1.5-3.0
mm day-I . In contrast, mean daily precipitation is re­
markably constant at a much lower value, near 0.7 mm
day-I , from early October to mid-April, which includes
the typical I-month period in autumn between leaf fall
and the development of continuous snow cover (Fig.
10).
f Postulated feedbacks of vegetation phenology on
regional climate
Our results show some intriguing coincidences, both
spatially and temporally, between seasonal climate pat­
terns and the distribution and leaf phenology of trem-
4239
bling aspen forests in the west-central Canadian interior.
The area occupied by aspen is difficult to quantify pre­
cisely, but an analysis of the Canadian Forest Inventory
(Lowe et al. 1994; Penner et al. 1997) indicates that in
the region west of Ontario, aspen forests occupy an area
of at least 200 000 km2• If aspen-dominated mixed
woods are also included, the total area may exceed
500 000 km2• Such areas are at least comparable to the
total area occupied by the Great Lakes (240 000 km 2),
which have been recognized as having a significant ef­
fect on the climate in that region (e.g., Changnon and
Jones 1972).
Aspen-dominated areas in the west-central Canadian
interior (Fig. 1) generally exhibited larger April and
October temperature anomalies (Fig. 5) than those re­
corded anywhere else in North America, including Alas­
ka, the continental United States, and Mexico. It should
be noted, however, that the anomalies w�re nearly as
great in adjacent areas of the Canadian prairies to the
south and in conifer-dominated portions of the boreal
forest to the north.
Although less striking at the continental scale (Fig.
8), there was also a tendency for a large summer en­
hancement of mean precipitation in aspen-dominated
areas, especially in central Alberta. The analysis of mean
daily climate for the nine stations representing aspen­
dominated areas in western Canada revealed more than
a doubling of mean precipitation (from < 1 to 2 mm
day-I ) (Fig. 10) during the typical period, when spring
leafing of aspen occurs (early May to early June), and
a halving of mean precipitation (from 2 to < 1 mm
day-I ) during the autumn period, when aspen leaves
turn color and abscise (early September to early Oc­
tober). Spring leaf phenology of aspen and other de­
ciduous tree species is determined primarily by cumu­
lative thermal sums (Lechowicz 1984); thus, the onset
of spring rainfall may be expected to contribute little
to leaf phenology, as forest soils in the region are nor­
mally wet following snow melt (typically during April).
Although aspen forests are a major component of the
landscape in this region, it is important to recognize that
there are other deciduous vegetation types that may be
expected to exert similar feedbacks on the climate of
this region. These would include other deciduous tree
species with similar leaf phenology (e.g., see Ahlgren
1957) as well as boreal fens with a significant com­
ponent of deciduous vegetation, where seasonal changes
in evaporative fraction (Lafleur et al. 1997) appear to
be similar to those over aspen forests. Another important
component of the landscape of western Canada is rep­
resented by the aspen parkland and prairie grassland
zones, where a large proportion of the area is under
cultivation for summer wheat and other agricultural
crops. During the period of winter snow cover, typically
from November to March or April, high albedos of these
open landscapes (e.g., Betts and Ball 1997) may lead
to seasonally cooler temperatures than would be ex­
pected if forest cover was present (e.g., Bonan et al.
4240
JOURNAL OF CLIMATE
1992). A t other times o f year, micrometeorological mea­
surements of evapotranspiration and energy partitioning
over natural grassland in southern Saskatchewan (Rip­
ley and Saugier 1978), and even as far south as Kansas
(Verma et al. 1992), show seasonal patterns that are
somewhat similar to those observed for aspen (Fig. 9);
however, strong decreases in grassland transpiration
were noted in both areas during summer periods with
low soil moisture. Even when irrigated, water use by
agricultural crops varies strongly with the stage of de­
velopment, and transpiration rates typically reach their
seasonal maximum for only a few weeks after full cover
is achieved (see Jensen et al. 1990). Thus, although leaf
phenology of grassland and agricultural crops may con­
tribute to the observed seasonal climate patterns in the
west-central Canadian interior, the resultant vegetation
feedbacks are presumably smaller, especially over the
climatically drier regions of the Canadian prairies (Hogg
1994).
Evergreen, coniferous forests predominate over large
areas of the western Canadian interior, especially in cor­
dilleran regions and in the mid- and northern boreal
forests of Saskatchewan and Manitoba (Fig. 1). These
forest types have previously been shown to exert a sig­
nificant feedback through the warming of the regional
climate due to low albedo and high sensible heat fluxes
(e.g., Bonan et al. 1995; Pielke and Vidale 1995). Their
potential contribution to the observed seasonal climate
anomalies is less clear, however, because leaf area and
albedo (Betts and Ball 1997) of these forest types show
relatively little seasonal variation compared with aspen­
dominated forests. As previously noted, midsummer wa­
ter vapor fluxes from landscapes dominated by boreal
conifers are apparently much lower than those domi­
nated by deciduous vegetation such as aspen. Never­
theless, frozen soils during the spring can delay the onset
of transpiration in the boreal coniferous forest, leading
to large sensible heat fluxes (Betts et al. 1998, 1999)
and thus potentially contributing to the high values of
IT that extend into the more northerly, conifer-dominated
areas of this region.
If deciduous forest phenology is a major contributor
to the April and October temperature anomalies in the
southern boreal forest of western Canada, then similar
effects might also be anticipated in other continental
areas dominated by deciduous trees, notably the tem­
perate forests of eastern North America. Differences in
the seasonal timing of such anomalies might be ex­
pected, however, because of earlier spring leafing and
later autumn leaf fall under the milder climatic condi­
tions. In a preliminary analysis (not shown), we have
noted that positive temperature anomalies occur in
March and November across much of the northeastern
United States and adjacent southern Canada, where
these temperate deciduous forests are abundant. Further
examination of relationships between leaf phenology
and seasonal temperature would therefore be useful in
revealing evidence of vegetation feedbacks in eastern
VOLUME 1 3
North America and elsewhere. The index (I,) used in
the present study would need to be modified, however,
before it would be appropriate for investigating postu­
lated temperature feedbacks of vegetation phenology in
temperate regions.
In the absence of model simulations specifically tai­
lored to address questions of how seasonal changes in
transpiration may affect atmospheric processes in this
region (e.g., Xue et al. 1996), it is difficult to assess the
degree to which vegetation feedbacks, including sea­
sonal leafing of deciduous forests, may contribute to the
large summer enhancement of rainfall in the western
Canadian interior. Based on a global analysis by Tren­
berth ( 1998), the proportion of precipitation recycled by
vegetation may be less than 10% when measured over
distances of 500 km. Estimates by Brubaker et al. ( 1993)
indicated a recycling rate of 34% for July rainfall over
a 100Iatitude by 200 10ngitude region of the central Unit­
ed States, an area comparable to that of the Canadian
prairie provinces. Part of the difference in these esti­
mates, however, results from the spatial scale being con­
sidered. When considered collectively over large areas
within continental interiors, terrestrial vegetation may
be found to exert a dominant influence over rainfall
patterns; for example, in the upper Amazon basin (Let­
tau et al. 1979) and in the Sahel region of west Africa
(Savenije 1995), where it was estimated that evapo­
transpiration by the regional vegetation contributes
about 90% of local rainfall. Although comparable es­
timates are not available for the western Canadian in­
terior, global analyses by Charles et al. ( 1994) indicate
that moisture recycling at the continental scale might
account for more than 50% of the annual precipitation
in this region.
Earlier generations of GCMs, which lack sophisti­
cated algorithms for vegetation-atmosphere interac­
tions, may be expected to show limitations in their abil­
ity to characterize seasonal changes in regional climate,
even under present conditions. For example, an exam­
ination of the 1 X CO2 output from the Canadian Cli­
mate Centre's GCM2 (Boer et al. 1992; McFarlane et
al. 1992) shows no evidence of the distinctive positive
temperature anomalies that we detected in the observed
climate record for April and October in the western
Canadian interior. In fact, the value of IT was found to
be slightly negative (-0.6°C) for the average modeled
climate of the GCM2 for the nine grid nodes (see Meth­
ods) in aspen-dominated areas of the Canadian prairie
provinces, whereas the long-term climate data indicated
a strongly positive value of +2SC (based on data in
Fig. 10 for the nine climate stations located near these
same grid nodes). A comparable analysis for precipi­
tation patterns indicates that observed seasonal variation
in monthly precipitation is larger than that modeled by
the GCM primarily because the modeled winter snowfall
(43-50 mm water equivalent per month) is much greater
than observed ( 16--2 1 mm month-I). This difference is
significant even when allowing for a possible under-
15 DECEMBER 2 000
4241
HOGG ET AL.
estimation of about 30% in the historical measurements
of snowfall amounts (A. G. Barr, Environment Canada,
1999, personal communication). The errors in winter
precipitation are smaller in the more recent Canadian
Regional Climate Model (CRCM) (Laprise et al. 1998)
presumably because the higher spatial resolution of the
CRCM more accurately characterizes the topographic
influence of the mountain ranges that block the passage
of moisture from the Pacific into the western Canadian
interior. Based on the results of the present analysis,
significant further improvements in the simulation of
seasonal variation in both temperature and precipitation
may be expected through the implementation of more
sophisticated schemes for representing vegetation-at­
mosphere interactions (e.g., Verseghy 1996; Xue et al.
1996; Bonan 1997), especially the inclusion of decid­
uous forest phenology (e.g., Foley et al. 1996).
4. Conclusions
The analysis of long-term climate records shows dis­
tinctive seasonal patterns in both temperature and pre­
cipitation that broadly coincide with the major areas of
aspen-dominated forest in the west-central Canadian in­
terior. Observed seasonal patterns of energy partitioning
and forest transpiration at the BOREAS Old Aspen site
have led us to the hypothesis that these observed patterns
of regional climate may be significantly influenced by
leaf phenology of aspen and associated deciduous veg­
etation. Although the magnitude and significance of the
postulated feedbacks cannot be established conclusively
from the results of this study, the analysis of seasonal
climate patterns appears to be consistent with our hy­
pothesis. Nevertheless, further investigation is warranted
in view of the potentially major influences of other factors
on the observed seasonal climate patterns, notably the
seasonal movements of air masses and associated fronts
as well as regional feedbacks of other vegetation types,
including boreal coniferous forest and cropland.
To our knowledge, the distinctive seasonal pattern of
mean temperature in this region, characterized by anom­
alously warm temperatures in April and October, has not
been previously reported. In the present study, this sea­
sonal pattern was quantified, and its spatial and temporal
distributions were examined through analyses of devia­
tions from a best-fitting sinusoidal equation. The indices
developed from these deviations should provide a useful
means of testing the ability of models to simulate the
seasonal features of climate in this region and elsewhere.
A preliminary analysis indicated that the distinctive April
and October temperature anomalies observed in western
Canada are not represented by at least one of the earlier­
generation GeMs. Similar tests on more recent models
would be useful in determining their ability to simulate
these seasonal temperature anomalies; however, it ap­
pears unlikely that any GeM will reproduce these anom­
alies unless its internal representation of surface char-
acteristics explicitly considers the seasonal changes in
the leaf area of deciduous forests in this region.
One of the implications of the observed seasonal tem­
perature anomalies is that the length of the growing
season across much of the western Canadian boreal for­
est is about 10% greater than what would be expected
from a best-fitting sinusoidal equation with the same
mean annual temperature. During the summer, however,
temperatures are cooler than what would be expected
from the same sinusoidal equation, while mean precip­
itation is much greater than at other times of year. Both
of these characteristics would be expected to favor forest
growth because the high vapor pressure deficits that
typically occur during hot weather lead to reductions in
stomatal conductance and CO2 uptake (e.g., Dang et al.
1997) while higher midsummer temperatures would·
tend to increase plant respiration rates. Thus, if the pos­
tulated feedbacks on regional climate are significant,
then the presence of trembling aspen and other decid­
uous vegetation could play a role in maintaining the
current distribution of the boreal forest in western Can­
ada (cf. Hogg 1994, 1997). Determining the importance
of these postulated effects, however, will likely await
the application of regional land surface models that in­
clude phenological changes of deciduous vegetation
within global models of the earth's climate system.
Acknowledgments. This work was supported by fund­
ing from the Climate Change Network of the Canadian
Forest Service and the Natural Sciences and Engineer­
ing Research Council of Canada (NSERC), with logis­
tical support for the BOREAS field measurements from
Atmospheric Environment Service, Parks Canada, and
the National Aeronautics and Space Administration. Sap
flow measurements were implemented by P. A. Hurdle,
and analyses of tower-based flux measurements were
conducted with the support of A. G. Barr and Z. Chen.
M. Siltanen provided assistance with the spatial map­
ping, and helpful comments on the manuscript were
provided by M. D. Flannigan, B. D. Amiro, I. D. Camp­
bell, A. G. Barr, and two anonymous reviewers.
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