Agricultural and Forest Meteorology 112 (2002) 87–102
Energy exchange across a chronosequence of slash
pine forests in Florida
Henry L. Gholz∗ , Kenneth L. Clark
School of Forest Resources and Conservation, University of Florida, P.O. Box 110410, Gainesville, FL 32611, USA
Received 3 January 2002; received in revised form 3 May 2002; accepted 10 May 2002
Abstract
We measured net atmospheric exchanges of energy and water vapor using eddy covariance along a chronosequence of Pinus
elliottii plantations in north Florida: a recent clear-cut, a mid-rotation stand, and a 24-year-old, rotation-aged stand. Reflected
energy averaged 0.26 of incoming solar radiation at the clear-cut and 0.18 at the closed-canopy stands. The sum of sensible
(S), latent (LE) and soil heat fluxes accounted for 89 and 85% of net radiation (Rnet ) at the clear-cut and mid-rotation age sites.
Both S and LE were linearly related to Rnet at all sites. Seasonal differences occurred in the proportion of Rnet attributable to
S and LE. S was a much smaller proportion of Rnet when the clear-cut and the mid-rotation age stands were flooded in the
summer. LE was a greater proportion of Rnet during the summer/fall at all sites when LAI was greatest. Bowen ratios (S/LE)
were 0.34, 0.50 and 0.59 in the summer/fall and 0.71, 0.77 and 1.00 in the winter/spring at the clear-cut, mid-rotation and
rotation-aged stands, respectively. Maximum rates of evapotranspiration (ET) in the summer were 0.6 mm h−1 at all sites.
Mean daily rates averaged 3.3 mm per day in the summer/fall and 2.0 mm per day in the winter/spring. Although, changes
in LAI and canopy structure were large, annual ET estimates were similar and averaged 959, 951 and 1110 mm per year
along the chronosequence. Results suggest that energy partitioning and annual ET in these pine forests are more sensitive to
environmental fluctuations than to management activities.
© 2002 Elsevier Science B.V. All rights reserved.
Keywords: Eddy covariance; Plantation; Sensible heat; Latent heat; Energy balance; Evapotranspiration; Florida
1. Introduction
The forested landscape of the southeastern US
lower Coastal Plain is dominated by a mixture of
upland evergreen pine stands and deciduous cypress
wetlands, in a ratio of about 70:30 (Myers and Ewel,
1990). Most of the current pine stands are second or
third generation, even-aged plantations managed for
wood fiber, while the wetland forests are naturally
regenerated after harvesting at longer return times.
Cyclic forest management activities in the uplands
∗ Corresponding author.
E-mail address: [email protected] (H.L. Gholz).
occur over a spatial scale of 30–100 ha and at a time
scale of 20–25 years, and include harvesting, site
preparation, replanting and fertilization. Seasonal
differences in leaf area and surface characteristics
occur between upland and wetland stands because of
the phenological differences between the dominant
tree species (i.e. evergreen pine versus deciduous cypress). Environmental conditions, particularly of the
soil, and wildfire are both highly variable in space
and time and contribute to variability in leaf area and
surface characteristics across this landscape. Because
environmental conditions and management practices
interact to affect energy partitioning by these surfaces, landscape-level fluxes of energy and water with
0168-1923/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved.
PII: S 0 1 6 8 - 1 9 2 3 ( 0 2 ) 0 0 0 5 9 - X
88
H.L. Gholz, K.L. Clark / Agricultural and Forest Meteorology 112 (2002) 87–102
the atmosphere are likely to be highly dynamic. Both
field and simulation studies have shown that changes
in land surface characteristics affect the partitioning
of water and energy fluxes. Accumulating evidence
suggests that in many cases such changes are large
enough to influence weather and climate at a range of
spatial and temporal scales (Pielke et al., 1998; André
et al., 1989; Gash and Nobre, 1997).
The energy balance of a forest may be expressed as
Rnet = Rg (1 − A)
(1)
where Rnet is absorbed solar radiation, Rg the incident
(above-canopy) solar radiation, and A the albedo of
the surface (or reflectance of incident radiation). Rnet
is partitioned as
Rnet = S + LE + G + B + P
(2)
where S is sensible heat exchange, LE the latent heat
exchange, G the soil heat flux, B the change in the
heat storage within the forest air space and biomass,
and P the energy used in photosynthesis. In this analysis, we ignore B because these stands have short
canopies, low leaf area indices (LAI) and relatively
low biomass (Gholz et al., 1991). We also ignore P,
because net primary production in these ecosystems
utilizes <1.5% of absorbed solar radiation annually
(Gholz et al., 1991; Clark et al., 1999). On the other
hand, G may be a large term, especially where LAI is
low, as when forest harvesting has recently occurred.
A simplified energy balance equation for the Florida
landscape can therefore be expressed as
Rnet = S + LE + G
(3)
Sensible heat exchange and LE usually each average about half of Rnet (Bowen ratio, β = 1) for
well-watered, closed-canopy forests when the canopy
is dry (Landsberg and Gower, 1997). Transpiration
(T) is difficult to measure at the canopy or ecosystem
level, although, it is typically the largest component
of the LE flux. T has been estimated for these ecosystems only a few times using indirect approaches
such as hydrologic mass/volume balances (Heimburg,
1984; Riekerk, 1989; Ewel and Smith, 1992) or models that scale up small sample chamber measurements
(Brown, 1981; Ewel and Gholz, 1991; Liu et al.,
1997). We have little information on how changes
in S and LE correspond to changes in leaf area and
surface characteristics with management practices in
either upland or wetland ecosystems of the region.
Leaf area is the major biotic variable regulating
both the evaporation of intercepted precipitation and
transpiration and is highly dynamic across this landscape. LAI starts at close to zero following the harvest
of plantations and reaches a maximum after about 10
years (Gholz and Fisher, 1982). Slash pine holds two
age classes of needles through the summer and fall,
but only one age class over winter, thus, seasonal variation in LAI over the variously-aged pine stands in
this landscape is also large (Gholz et al., 1991).
G is always the smallest of the right-hand terms of
Eq. (3), but becomes more significant where LAI has
been reduced by disturbances such as harvesting and
site preparation. Changes in G with management practices are particularly significant in the carbon budgets
of these ecosystems, because soil temperatures drive
rates of auto- and heterotrophic respiration and therefore soil CO2 fluxes.
This paper focuses on the changes in energy partitioning over a management cycle, or rotation, using
measurements at three ages along a chronosequence
of the dominant managed slash pine (Pinus elliottii
var elliottii) ecosystem in north-central Florida. Our
research had two main objectives: (1) to measure net
fluxes of energy and water vapor from these stands
using eddy covariance over a range of meteorological
and phenological conditions; and (2) to evaluate the
importance of environmental and physiological controls over net fluxes at time scales up to 1 year. In contrast to the balance between S and LE observed in older
closed-canopy stands, we hypothesized that: (1) sensible heat fluxes would dominate the energy balance
at the recent clear-cut because LAI is reduced; (2) LE
fluxes are primarily a function of LAI and available energy (Rnet ) and should increase as a linear function of
the accumulated LAI on recently harvested sites; and
(3) seasonal changes in LAI would affect energy partitioning between S and LE at all the sites, particularly
that LE should account for a greater proportion of Rnet
in the summer and fall than in the winter and spring.
2. Materials and methods
2.1. Study sites
Study sites were established 15 km north-east of
Gainesville, FL (29◦ 44 N, 82◦ 9 30 W). Long-term
H.L. Gholz, K.L. Clark / Agricultural and Forest Meteorology 112 (2002) 87–102
(1955–1995) mean January and July temperatures
were 14 and 27 ◦ C, respectively (NOAA, 1996).
Mean monthly minimum and maximum temperatures
throughout the study period (January 1995–December
1999) ranged from 5.9 to 19.5 ◦ C and 21.5 to
32.9 ◦ C for January and July, respectively. Long-term
(1955–1995) mean annual rainfall for the Gainesville
Airport (ca. 5 km south of our sites) was 1287 mm and
annual precipitation in 1996–1999 was 1388, 1391,
1247, and 959 mm, respectively (NOAA, 2001). Soils
of all the sites are ultic alaquods (sandy, siliceous,
thermic) that are poorly drained and low in organic
matter and available nutrients. The distributions of
discontinuous subsurface spodic (organic) and argillic
(clay) horizons range between 30 and 70 cm and
between 100 and 200 cm depth, respectively.
The clear-cut site was previously dominated by a
rotation-aged, 25-year-old (in 1997) industrial plantation of slash pine managed for pulpwood production.
Following a stem-only harvest that occurred between
May 1997 and January 1998, residues were raked into
piles and the site was bedded twice (May and November 1998). Seedlings were then planted at harvest density in December 1998 and January 1999. Minimum
fetch at this ecosystem was ca. 800 m. Above-ground
LAI accretion at the clear-cut site (Table 1) was estimated using clip plots (1.0 or 0.25 m2 ) randomly
located within 250 m of the meteorological tower, at
3, 6, 12 and 24 months following harvest. The water
table depth at this site fluctuated between the surface
and 1.3 m depth during this study.
Two mid-rotation commercial slash pine plantations were also used because one was lost to wildfire
early in this study (June 1998), after which measurements were moved to a second site nearby. In both
89
cases, trees were 10 years old at the beginning of the
study (planted in 1987). The soils and management
histories were indistinguishable between the two sites,
so that all data were pooled for these analyses. Understory vegetation in both consisted of native species
reestablished after site preparation, primarily Serenoa
repens, Ilex glabra and Myrica cerifera. Minimum
fetch from the towers was approximately 800 m and
both were surrounded by other slash pine plantations
between 5 and 25 years old. Tree inventories and measurements of diameter at 1.3 m (dbh, cm) and height
(m) were conducted annually. Tree LAI was estimated
from allometric relationships based on the destructive
harvest of 35 trees (from both sites) applied to the
dbh inventories of the stands (Table 1). Understory
LAI was estimated from census data in each plot
using allometric relationships based on various plant
dimensions (Gholz et al., 1999 and unpublished data).
Both of these mid-rotation stands had closed canopies
near the maximum LAI for these sites (Gholz and
Fisher, 1982). The water table at these sites ranged
from at the surface to 2.7 m depth during the study.
The rotation-aged site (now the clear-cut) was dominated by a 25-year-old (in 1997) slash pine plantation.
Mixed genotype seedlings were planted at harvest
density following stem-only harvest of the previous
stand, chopping and broadcast burning of residues,
and bedding. The stand had not been thinned or fertilized since establishment. Understory vegetation was
dominated by S. repens, I. glabra and M. cerifera.
Minimum fetch at this ecosystem was ca. 800 m and it
was surrounded by similarly managed 9–20-year-old
slash pine plantations. Water table ranged from at
the surface to 1.6 m during this study. This site was
previously described by Clark et al. (1999).
Table 1
Structural attributes of the three slash pine chronosequence sitesa
Parameter
Clear-cut
Mid-rotation
Rotation-aged
Mean maximum stand height (m)
Mean dbh (cm)
Basal area (m2 ha−1 )
Mean stand density (# trees ha−1 )
LAI (m2 m−2 , all-sided); seasonal range
0.3–1.0b
11.0c
NA
0
NA
0.1–3.0c
9.8 ± 0.4c
15.7 ± 0.6c
2075
3.1–5.1
19.2 ± 1.0
17.4 ± 0.2
31.4 ± 1.4
1184
4.0–6.5
a
Sampling occurred over four plots (625 m2 each) at the mid-rotation and rotation-aged sites.
Above-ground biomass averaged 45.2, 83.3, 62.0 and 184.8 g C m−2 over the first 2 years following the clear-cut; trees were planted
as seedlings in the second year.
c Fall, 2000.
b
90
H.L. Gholz, K.L. Clark / Agricultural and Forest Meteorology 112 (2002) 87–102
2.2. Eddy covariance measurements
Net ecosystem fluxes of S and LE were measured using a closed-path eddy covariance system at each site
(Moncrieff et al., 1997; Clark et al., 1999). The systems were composed of: (1) a three-dimensional sonic
anemometer (R3A, Gill Instruments Ltd.) mounted at
the top of a 4 m meteorological tower at the clear-cut
site, a 15 m tower at the mid-rotation sites and a 24 m
tower at the rotation-aged site; (2) a fast-response,
closed-path infrared gas analyzer (Li-Cor LI-6262);
(3) a 30 m long, 0.4 cm ID Teflon coated (clear-cut,
mid-rotation sites) or nylon (rotation-aged site) tube
connected to a small air pump; and (4) a lap-top PC
running either EdiSol or RCOM software. The inlet
of the tube was placed between the upper and lower
sensors of the sonic anemometer and air was drawn
through the LI-6262 at a rate of 5.0–6.0 l min−1 . Airflow was regulated with a Tylan General mass flow
controller or calibrated rotometers. The mean lag
time from the tube inlet to the LI-6262 was 5–7 s for
all sites. The LI-6262 was calibrated every 1–5 days
using an Li-610 dew point generator.
Flux calculation software carried out coordinate
rotation of the horizontal wind velocities to obtain
turbulence statistics perpendicular to the local streamline. The covariance between turbulence and scalar
concentrations was maximized through the examination of 0.1 s intervals on both sides of a fixed lagtime
(in this case, ca. 6 s). Fluxes were estimated from the
deviations between instantaneous measures of vertical
wind speed (w ) and a low pass filter (w̄). We used
two different methods of calculating this filter, a 200 s
running mean at the rotation-aged stand (EdiSol,
Moncrieff et al., 1997) and Reynolds detrending using a 200 s constant at the younger stands (Katul
et al., 1999). Because of the relatively short scalar
roughness lengths and uniform canopy structure at
these sites, we assumed that the influence of coherent
structures in the Reynolds detrending were minimized. Fluxes were calculated in discrete half-hour
intervals and then corrected for the frequency attenuation of turbulent structure in the sampling tube
and non-ideal frequency response of the LI-6262
using transfer functions (Moncrieff et al., 1997).
Barometric pressure data were used to calculate
fluxes at ambient atmospheric pressure. Latent energy
fluxes were rejected during periods with measur-
able rainfall, incomplete half-hourly sampling periods, or when condensation occurred in the sampling
lines.
2.3. Meteorological and soil heat flux
measurements
Continuous meteorological measurements were
made at all sites. Incoming short-wave radiation
(LI-200), PPFD (LI-190), net radiation (#Q7, Radiation and Energy Balance Systems Inc.), air temperature and relative humidity (#HMP 23 UT, Vaisala
Inc.), wind speed and direction (#12–002, R. M.
Young Co.) and precipitation were measured at
2–4 m at the clear-cut, at 14 m on the towers at the
mid-rotation sites and at 24 m at the rotation-aged site
(2–4 m above mean canopy height at all sites). Soil
heat flux was measured using heat flux transducers
(#HFT-3.1, Radiation and Energy Balance Systems
Inc.) buried at 10 cm depth at the clear-cut and the
mid-rotation sites; soil heat flux was not measured
at the rotation-aged site. Meteorological data were
recorded with Omnidata data loggers. Water table
depth was measured with a Stevens water depth gauge
at each site. Barometric pressure data were obtained
from the Gainesville Regional Airport.
To estimate mean energy exchanges from the sites
during times when we were not measuring eddy
fluxes, we developed seasonal linear regression equations to predict exchanges using continuous meteorological data. “Summer and fall” was defined as May
through October and “winter and spring” as November through April, which roughly correspond to the
“growing season” and “dormant season,” although,
significant leaf net photosynthesis and net ecosystem
carbon gain occur throughout the year (Clark et al.,
1999; Teskey et al., 1994).
The partitioning of Rnet defines how solar energy
reaching the forest canopy or soil surface is balanced by the various opposing fluxes of energy out
of the ecosystem. However, foresters and other land
managers are principally interested in how clearcutting and regrowth affect ecosystem water balances.
Therefore, energy units were translated into fluxes of
water vapor (ET) by dividing by the latent heat of
vaporization, here in units of mm H2 O, and annual
ET estimated using the relationships between measured ET and Rnet . Annual totals were calculated for
H.L. Gholz, K.L. Clark / Agricultural and Forest Meteorology 112 (2002) 87–102
91
1996–1997 for the rotation-aged site and 1998–1999
for the clear-cut and mid-rotation sites.
Table 3
Net radiation (W m−2 ) as a function of incident solar radiation
(W m−2 ) along the chronosequence of slash pine stands
2.4. Precipitation, throughfall and interception loss
Site
Year
Equation
r2
Clear-cut
1998
1999
Rnet = −22.38 + 0.75Rg
Rnet = −22.58 + 0.72Rg
0.97
0.98
Mid-rotation
1997–1998
1999
Rnet = −27.49 + 0.82Rg
Rnet = −26.40 + 0.82Rg
0.99
0.99
Rotation-aged
1996–1997
Rnet = −20.25 + 0.82Rg
0.99
Precipitation and throughfall depths were measured
at the rotation-aged site for 23 rain events from 15
June 1996 to 9 November 1996. Only events for which
there was no rain for 12 h preceding the event (to insure initial canopy dryness) and for which the rain fell
continuously during the event were used. All collectors
were cleared of water and litter before each rain event.
Mean throughfall depth was regressed on precipitation
depth to estimate canopy interception. Stemflow had
been measured previously at a similar nearby stand
as <1% of annual precipitation by Allen and Gholz
(1996); we used their results to estimate stemflow
as a fraction of precipitation in this study. The same
interception model was used at the mid-rotation site,
given the similar LAI and stand structure to the older
stand. Interception was assumed to be negligible at the
clear-cut site.
3. Results
The relationship between friction velocity (u∗ ,
m s−1 ) and horizontal wind speed (u, m s−1 ) was linear for all sites (Table 2). The R2 values were high
considering that all data from all conditions were
included. At the clear-cut, the slope of this relationship was 0.08 in the first year following harvest,
and increased to 0.11 by the second year as LAI
and biomass accumulated. Friction velocity averaged
0.26 of horizontal wind speed at the mid-rotation and
rotation-aged closed-canopy sites.
Table 2
Friction velocity (u∗ , m s−1 ) as a function of horizontal wind speed
(u, m s−1 ) along the chronosequence of slash pine stands
Site
Year
Equation
r2
Clear-cut
1998
1999
u∗ = 0.079u + 0.007
u∗ = 0.106u + 0.003
0.84
0.91
Mid-rotation
1998
1999
u∗ = 0.266u − 0.092
u∗ = 0.247u − 0.071
0.87
0.85
Rotation-aged
1995–1997
u∗ = 0.260u − 0.013
0.83
Net radiation (Rnet ) was linearly related to incoming
global radiation (Rg ) for all sites and years (Table 3).
From these relationships, mean albedos (A) were
calculated as 0.26, 0.18 and 0.18 for the clear-cut,
mid-rotation and rotation-aged sites, respectively.
Comparisons of Rnet with the sum of sensible, latent
and soil heat fluxes indicate that energy balances were
within 12–15% of closure for half-hourly measurements at the clear-cut and mid-rotation sites (slopes =
0.883 and 0.852, respectively; Table 4, Fig. 1). At the
rotation-aged stand, S plus LE accounted for 74% of
Rnet . If we assume that mid-day soil heat fluxes were
the same as those at the mid-rotation stands with similar canopies (ca. 30 W m−2 ), then the balance here
approached those of the other sites.
Sensible heat fluxes were a linear function of Rnet at
all sites (Table 4). Seasonal differences were observed
in the relationship between S and Rnet at the clear-cut
and the mid-rotation sites. At the clear-cut, S was the
smallest proportion of Rnet during the wet summer season in the first year following harvest, when standing
water was present in the interbeds. However, as LAI
continued to accumulate and the depth of the water table dropped below the surface, this strong seasonality
was no longer observed, and the relationship between
S and Rnet became indistinguishable from those at the
older sites (Fig. 2, Table 4). Summer flooding also occurred to a lesser extent at the mid-rotation site following large storms, and the proportion of Rnet attributed
to S was similarly reduced at this time (Table 4). Differences in the relationship between S and Rnet among
seasons were not significant at the rotation-aged site.
Significant (t-tests, P < 0.01) seasonal differences
were observed in the slopes of the relationships between LE and Rnet at all sites (Table 4, Fig. 3). During
the summer and fall, LE was consistently a greater
92
H.L. Gholz, K.L. Clark / Agricultural and Forest Meteorology 112 (2002) 87–102
Table 4
Energy balance, sensible heat and latent heat equations for the chronosequence sites
Site
Equation
r2
na
Season
Clear-cut
[S + LE + G] = 0.883 Rnet + 4.327
S = 0.209 Rnet − 2.694
S = 0.352 Rnet + 3.915
S = 0.297 Rnet + 2.606
LE = 0.624 Rnet + 20.650
LE = 0.395 Rnet + 16.081
LE = 0.476 Rnet + 16.599
0.93
0.89
0.89
0.86
0.85
0.78
0.77
4868
1087
5027
6114
1146
3823
4969
Pooled
Summer/fallb
Winter/spring
Pooled
Summer/fallb
Winter/spring
Pooled
Mid-rotation
[S + LE + G] = 0.852 Rnet + 11.043
S = 0.307 Rnet − 0.925
S = 0.408 Rnet − 7.354
S = 0.380 Rnet − 7.054
LE = 0.520 Rnet + 23.212
LE = 0.395 Rnet + 15.737
LE = 0.408 Rnet + 16.655
0.92
0.83
0.87
0.86
0.87
0.85
0.84
8568
491
8671
9162
1052
7516
8568
Pooled
1998 Summerb
Fall/winter/spring
Pooled
Summer/fall
Winter/spring
Pooled
Rotation-aged
[LE + S] = 0.744 Rnet − 2.164
S = 0.391 Rnet − 14.210
S = 0.436 Rnet − 14.388
S = 0.405 Rnet − 14.218
LE = 0.359 Rnet + 11.875
LE = 0.253 Rnet + 10.053
LE = 0.312 Rnet + 7.855
0.92
0.86
0.83
0.85
0.89
0.82
0.81
990
551
439
990
551
439
990
Pooled
Summer/fall
Winter
Pooled
Summer/fall
Winter
Pooled
a
b
Sample sizes are lower for LE than S fluxes because data were excluded when water vapor condensation was observed in the lines.
The soil surface was often flooded following large storms during this period.
proportion of Rnet when compared to the winter and
spring. For example, at an Rnet of 500 W m−2 (near
the maximum observed in the winter), mean LE rates
during the summer and fall were 1.6, 1.3 and 1.4 times
greater than those measured during the winter and
spring at the clear-cut, mid-rotation and rotation-aged
sites, respectively. These proportional differences were
maintained across the range of Rnet values. Seasonal
differences in the slopes of the relationships between
LE and Rnet indicate that changes in LAI are important for controlling LE in these ecosystems, even at
the clear-cut.
The slopes of the relationship between Rnet and LE
were steeper at the mid-rotation site when compared to
the rotation-aged site during both seasons (P < 0.01
for both comparisons, Table 4). Latent heat fluxes were
slightly larger than sensible heat fluxes at the clear-cut
and the mid-rotation age sites during periods without
flooding (mean β = 0.8 in the winter/spring, Table 5)
and similar at the rotation-aged site (mean β = 1.0).
In contrast, when the clear-cut was flooded, much
lower rates of S and higher LE resulted in values of
β < 0.33. When flooding occurred at the mid-rotation
site in the summer, β were <0.5 (Table 5). The proportional decrease in β between summer/fall and
winter/spring sampling periods was similar for the
mid-rotation (0.67) and rotation-aged sites (0.71).
The clear-cut had much higher soil heat fluxes at
all depths when compared to the mid-rotation site.
For example, maximum values of half-hourly mid-day
G at 10 cm depth during clear sky conditions in the
Table 5
Bowen ratios (β = S/LE; mean±1 S.D.) for daytime measurements
along the chronosequencea
Site
Season
β
Clear-cut
Summer/fall
Winter/spring
0.35 ± 0.19
0.72 ± 0.47
Mid-rotation
Summer/fall
Winter/spring
0.49 ± 0.32
0.75 ± 0.49
Rotation-aged
Summer/fall
Winter/spring
0.70 ± 0.41
0.96 ± 0.79
a
Data are pooled over years.
H.L. Gholz, K.L. Clark / Agricultural and Forest Meteorology 112 (2002) 87–102
93
Fig. 1. The sum of sensible, latent and soil heat fluxes (S + LE + G, W m−2 ) versus net radiation (Rnet , W m−2 ) along the chronosequence
of slash pine stands (clear-cut, mid-rotation, rotation-aged). See Table 4 for the corresponding equations and statistics.
summer approached 100 and 60 W m−2 at the clear-cut
during the first and second years following harvesting,
respectively, whereas values at the mid-rotation site
rarely exceeded 30 W m−2 . Daytime peaks in soil heat
fluxes at 10 cm depth typically lagged the peak in Rnet
by 2–3 h.
The mean maximum ET rate across the sites was
0.6 mm H2 O h−1 , which occurred in the late morning and early afternoon in midsummer, despite the
fact that VPD was often relatively low (<1.5 kPa)
at this time (Table 6). Mean maximum rates corresponded with the highest Rnet values and flooded
soils at the clear-cut, or maximum seasonal LAI
at the mid-rotation and rotation-aged sites. Maximum ET rates during the winter and spring were
0.3 mm H2 O h−1 at the clear-cut and 0.4 mm H2 O h−1
at the mid-rotation and rotation-aged sites. Lower
hourly (and daily) ET rates at the clear-cut in winter
94
H.L. Gholz, K.L. Clark / Agricultural and Forest Meteorology 112 (2002) 87–102
Fig. 2. The relationship between sensible heat flux (S, W m−2 ) and net radiation (Rnet , W m−2 ) along the chronosequence of slash pine
stands (clear-cut, mid-rotation, rotation-aged).
Table 6
Hourly and daily rates of evapotranspiration (ET) along the chronosequence of slash pine stands
Site
Clear-cut
Mid-rotation
Rotation-aged
Year
1998
1999
1998–1999
1996–1997
Maximum hourly
ET (mm h−1 )
Mean daily ET
(mm per day ± 1 S.D.)
Maximum daily ET
(mm per day)
Summer
Winter
Summer
Summer
Winter
0.6
0.5
0.6
0.6
0.3
0.3
0.4
0.4
3.6
2.8
3.1
3.6
5.1
4.0
4.9
5.8
3.6
3.0
4.0
4.0
±
±
±
±
0.9
0.6
0.8
0.4
Winter
1.9
1.9
2.1
2.0
±
±
±
±
0.7
0.5
0.7
0.4
H.L. Gholz, K.L. Clark / Agricultural and Forest Meteorology 112 (2002) 87–102
95
Fig. 3. The relationship between latent heat flux (LE, W m−2 ) and net radiation (Rnet , W m−2 ) along the chronosequence of slash pine
stands (clear-cut, mid-rotation, rotation-aged).
and spring were due primarily to the dieback of
perennial herbaceous plants following the first hard
frosts, which greatly reduced LAI at this time.
At the mid-rotation and rotation-aged sites, ET increased rapidly with increasing VPD until about 1 kPa,
after which rates leveled off. Only at the highest VPD
values were ET rates reduced and this occurred primarily in the mid-afternoon during hot, cloudless days
in the spring and early summer. When the relationship
between ET and Rnet was separated into three VPD
classes (<1.0, >1.0 to 2.0, >2.0), no significant differences in the slopes of the regression lines occurred
and none were consistent site to site (Fig. 4). Lack of
differences indicates that there was little stomatal response to changes in VPD in slash pine and that Rnet
was the major factor controlling short-term ET rates.
96
H.L. Gholz, K.L. Clark / Agricultural and Forest Meteorology 112 (2002) 87–102
Fig. 4. The relationship between evapotranspiration (ET, mm h−1 ) and vapor pressure deficit (VPD, kPa) along the chronosequence
(clear-cut, mid-rotation, rotation-aged).
Using the relationships in Table 4 and measured
Rnet , mean daily transpiration rates at the mid-rotation
and rotation-aged sites during dry or nearly dry canopy
conditions were estimated at 3.1 and 3.6 mm per day
under a range of meteorological conditions in summer and fall (Table 6). When evaporation from wet
canopies during and immediately following rain events
(i.e. interception loss) was added to this estimate, total mean daily ET in the summer was estimated at 3.6
and 4.0 mm per day, respectively. Maximum daily ET
rates during cloudless conditions in the summer were
calculated at 4.9 and 5.8 mm per day. When standing
water occurred in the interbeds during the first year
following harvest, mean daily and maximum rates of
H.L. Gholz, K.L. Clark / Agricultural and Forest Meteorology 112 (2002) 87–102
97
Table 7
Annual water budgets for the chronosequence of slash pine stands
(ET + I)/P (%)
Site
Year
Precipitation (mm per year)
ETa (mm per year)
Clear-cut
1998
1999
1246.8
1008.0
1048.4
869.2
0
0
84.1
86.2
Mid-rotation
1998
1999
1246.8
876.8
1014.1
887.1
108.4
106.5
90.0
113.3
Rotation-aged
1996
1997
1185.6
1390.6
1001.0
1171.9
101.7
113.4
93.0
92.4
a
Interception (mm per year)
Dry canopy conditions (essentially transpiration).
ET at the clear-cut were similar to those at the older
sites. Mean daily transpiration rates during dry canopy
conditions under a range of incident light levels in the
winter ranged from 1.9 ± 0.5 to 2.1 ± 0.7 and maximum daily ET ranged from 3–4 mm h−1 at all sites.
Annual water budgets are summarized in Table 7
and are surprisingly consistent across sites. Evapotranspiration plus interception ranged from 84 to 113%
of precipitation, indicating that the majority of annual
precipitation was returned to the atmosphere in vapor form, as suggested previously through modeling
(Ewel and Gholz, 1991). When the water table was
above a depth of 2.7 m, the unsaturated surface soil
water supply apparently had little effect on ET during either summer or winter at the mid-rotation and
rotation-aged sites. ET rates were maintained through
prolonged drought conditions in the spring of 1999 at
the mid-rotation site and resulted in substantial drawdown of the water table to well below 2.7 m depth.
Soil water remained more abundant at the clear-cut
even during the drought periods, with the water table
remaining above 1.3 m depth throughout the measurement period. Significant drought did not occur during
the measurement period at the rotation-aged site. The
largest differences among the sites and years were:
(1) significant evaporation through interception at the
older sites (8.2–10.6% of precipitation); and (2) high
evaporative losses at the clear-cut and mid-rotation
sites during flooded conditions.
4. Discussion
Canopy structure and soil surface characteristics varied greatly along this chronosequence. Mean
canopy height ranged from 0.1 to 19 m and LAI
ranged from <1 at the clear-cut following harvest to
a seasonal maximum of 6.5 at the rotation-aged site.
Peak LAI was higher at the rotation-aged site when
compared to the mid-rotation sites, although, both
values were within the range of other nearby mature
slash pine stands, with seasonal minimum and maximum of 3.7 and 6.5 (Gholz et al., 1991); both sites
should be considered “closed-canopy”. Reduced LAI
resulted in greater reflectance of Rg and differences
in energy partitioning when the clear-cut is compared
to the other sites. However, large increases in S were
not observed following harvesting and LE rates were
maintained because of flooding of the site. Significant seasonal differences in energy partitioning were
driven by changes in LAI at all the sites.
4.1. Friction velocity
The slope of the relationship between u∗ and u for
two older stands (mean = 0.26) was greater than that
reported for other sites (Baldocchi and Vogel, 1996;
Shuttleworth et al., 1985; Grace et al., 1995). But, absolute values of these slopes may not be as meaningful
as the relative scatter of the data around the regression
lines, because the slopes are a function of instrument
height as well as the aerodynamic characteristics of
the canopy. Our data show relatively little scatter,
particularly at higher wind velocities, indicating that
turbulence is uniformly absorbed by these relatively
homogeneous canopies. Results from forests with aerodynamically rougher canopies (e.g. natural tropical
or old-growth forests) show much greater scatter. Also,
LAI is primarily distributed in the upper 5 m of the
canopy, with understory leaf area concentrated within
1 m of the forest floor. Slash pine trees have very
shallow canopies that are “lolli-popped” at the top of
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H.L. Gholz, K.L. Clark / Agricultural and Forest Meteorology 112 (2002) 87–102
very flexible tree stems and the needles are relatively
long. With increasing wind speeds, the needles and
stems tend to bend leeward, becoming more streamlined and absorbing some momentum in the process.
Therefore, the relationships in Table 2 may become
nonlinear at higher wind speeds, perhaps illustrated at
the clear-cut (i.e. essentially a “hard canopy”), where
slopes were more similar to those from other studies.
4.2. Net energy exchange
Leaf area has a dominant effect on energy partitioning among these ecosystems. Lower LAI accounts for
the increased albedo at the clear-cut during the first and
second years following harvest when compared to the
older stands. Calculated albedos were similar among
the older stands, which are at or beyond canopy closure (Gholz and Fisher, 1982). The zero intercepts and
slopes of the relationship between Rnet and Rg for these
ecosystems are within the range of values reported
from other temperate coniferous forests and plantations (−6 to −126 W m−2 and 0.71–0.91, respectively;
Jarvis et al., 1997; Landsberg and Gower, 1997).
The relationships between Rnet and the sum of
sensible, latent and soil heat fluxes indicate a high degree of closure in the energy balances for these sites
and methodological approaches (Table 4). Deviations
from closure may be due to a number of possible
measurement errors. For example, our fixed-position
net radiometers were mounted relatively close to the
top of the canopies and may not have adequately
represented average Rnet for the entire flux footprint.
Smith et al. (1997) noted that commercially available
net radiometers varied significantly (up to almost
20%) in their readings when compared side-by-side,
so that we may have overestimated Rnet at any of
the sites. Errors in the estimation of S and LE using
the EdiSol system (used at the rotation-aged site) are
thought to be <10% and are likely to be similar for the
systems used at the clear-cut and mid-rotation sites
(Moncrieff et al., 1997). Unmeasured energy fluxes
included heat storage in air within and below the
canopy, heat storage in biomass, and that consumed
by photosynthesis. All these terms are likely to be
minimal at the clear-cut, where the greatest degree of
energy balance was observed. Heat storage in canopy
air and biomass in closed-canopy forests is negligible
over a 24 h period, although, instantaneous values can
reach 100 W m−2 during midmorning in the summer
(Baldocchi and Vogel, 1996; Grace et al., 1996).
Departures may also be due to subtle methodological differences among sites. Both the shallower slopes
of the relationship between Rnet and LE (Table 4) and
higher values of ␤ (Table 5) noted for the rotation-aged
site when compared to the mid-rotation sites suggest
that LE fluxes were relatively low. At the rotation-aged
site, we used nylon tubing and no inlet filter, while
Teflon tubing and inlet filters were used elsewhere.
While there have been no empirical effects of these
differences reported in the literature, nylon does have
a greater permeability to water vapor than Teflon and
the lack of an inlet filter could have resulted in dirtier
interior walls of the sampling tube (Moncrieff and
Clement, personal communication). Both of these
factors could have led to greater attenuation of high
frequency fluctuations in water vapor concentrations,
an underestimation of LE, and thus, a greater departure from energy balance at the rotation-aged site.
Our closure results are similar to a number of
other sites (e.g. Kelliher et al., 1992; Lee and Black,
1993; Goulden et al., 1996; Blanken et al., 1997;
McCaughey et al., 1997; Baldocchi et al., 1997). The
near ideal physical conditions at the Florida sites (e.g.
flat topography and mono-specific, closed-canopy,
even-aged stands) should enable relatively accurate
measurements using eddy covariance. Because the
same measurement systems were used (except as
noted above), whatever biases were introduced were
consistent across the sites and years.
We expected that the initially very low LAI at the
clear-cut would lead to reduced Rnet and latent heat
fluxes, and that there would be greater sensible and
soil heat fluxes relative to the other sites. The clear-cut
did have a greater albedo, lower Rnet flux and greater
G during the first and second years when compared
to the older stands. However, the effect of clearcutting
on LE was surprisingly small and could be accounted
for primarily by greater evaporation from the soil, especially during times when water was ponded on the
surface. Consequently, sensible heat fluxes were much
lower than expected at the clear-cut.
4.3. Evapotranspiration rates
Our ET estimates are similar to those obtained using models and hydrologic mass balances for slash
H.L. Gholz, K.L. Clark / Agricultural and Forest Meteorology 112 (2002) 87–102
pine ecosystems and within the range of those obtained using a variety of techniques in other coniferous forests and plantations (e.g. Liu et al., 1997).
Linear relationships between ET and Rnet at the midand rotation-aged sites (derived from Table 4) are
similar to those obtained at other sites using eddy
covariance when soil water supply is non-limiting
(Baldocchi and Vogel, 1996; Dolman et al., 1998).
Maximum hourly ET rates from 0.4 (winter) to
0.6 (summer) mm h−1 are similar to those from dry
canopies reported for a number of coniferous forests
and plantations (0.46 ± 0.08 mm h−1 , mean ± 1 S.D.;
Kelliher et al., 1993; Dolman et al., 1998). Maximum
ET rates from hardwood forests are generally greater
than those reported from conifer forests. For example,
Baldocchi and Vogel (1996) reported a maximum rate
of 0.75 mm h−1 for a temperate broad-leaved forest
and a maximum rate of 0.7 mm h−1 was reported for
a tropical forest in Rhondonia (Grace et al., 1996).
A suite of environmental factors, particularly VPD,
affect stomatal conductance, the major component of
canopy conductance in coniferous forests (Landsberg
and Gower, 1997). However, VPD in our study apparently had only a minor effect until values exceeded
ca. 2.0 kPa, similar to results at the leaf-level for slash
pine and pond cypress (Bongarten and Teskey, 1986;
Liu, 1996; Teskey et al., 1994). These species are apparently adapted to high soil water potentials during
the growing season and show relatively little sensitivity to changes in VPD compared to coniferous species
which experience regular droughts and are typically
much more sensitive to VPD (e.g. Law and Waring,
1994; Runyon et al., 1994; Sanford and Jarvis, 1986).
Instantaneous rates of ET during the summer were
greater than those during the winter for similar values of Rnet and VPD under all conditions, pointing
to lower LAIs as the differentiating factor. However,
the large cumulative differences between winter and
summer resulted from much lower maximum winter
values of Rnet , primarily caused by lower incident
solar radiation.
Mean daily ET rates at our older sites were not
significantly different from the mean in the review by
Kelliher et al. (1993) of 3.8 ± 0.83 mm per day or
others based on eddy covariance over closed-canopy
coniferous ecosystems during the growing season
(Baldocchi et al., 1997; Baldocchi and Vogel, 1996;
Black et al., 1996; Grace et al., 1996; Shuttleworth,
99
1989). Evapotranspiration from a 12-year-old loblolly
pine (P. taeda) plantation (LAI = 3.5) in North Carolina during periods of non-limiting soil water supply
was estimated at 3.6 mm per day (Oren et al., 1998).
Differences among these sites not only reflect climatic
conditions, but also that coniferous forests in warm
temperate environments typically have lower LAI
and relatively lower stomatal conductance, and thus,
lower canopy conductance, compared to hardwood
forests. In contrast, an open-canopied (LAI = 1.6) P.
ponderosa ecosystem in Oregon had average daily ET
rates of 1.6–1.7 mm per day, with absolute maximum
values after rain events of 4 mm per day (Anthoni
et al., 1999).
Limited comparisons can be made between our
closed-canopy pine systems and adjacent cypress
wetlands at two levels. At the leaf-level during the
summer, there is little difference between transpiration rates of slash pine and pond cypress, nor between canopy transpiration rates when scaled using
a model (Liu, 1996). In the winter, ET rates measured using eddy covariance for the wetlands were
less than half of rates for the slash pine sites (Clark
and Gholz, unpublished data). Cypress ponds have
very low LAIs at this time, since cypress is deciduous, although, there are numerous smaller evergreen
species of trees and shrubs in the understory. Maximum winter ET rates for the wetland were only
0.15 mm H2 O h−1 , even though there was a free-water
surface. Maximum winter daily ET for cypress ecosystem was 1.0 mm per day. In this case, seasonal phenological and site hydrological differences together
controlled landscape-level variation in ET, while
leaf-level variations in stomatal conductance were
unimportant.
4.4. Annual evapotranspiration
Annual ET for the older slash pine sites estimated
with eddy covariance are within the range of those
reported using models and hydrologic balances. Maximum rates of ET for plantations in northern Florida
were estimated at 1200 mm per year by Liu et al.
(1997), compared to our average annual value of
1218 mm per year. Ewel and Gholz (1991) used a
model that extrapolated leaf-level measurements to the
canopy and incorporated a simple feedback relationship between soil water and transpiration to obtain a
100
H.L. Gholz, K.L. Clark / Agricultural and Forest Meteorology 112 (2002) 87–102
simulated maximum ET in a wet year of 1280 mm per
year for a plantation similar to our rotation-aged site.
Their estimates of ET simulated using meteorological data for Gainesville from dry (840 mm rainfall),
average (1320 mm) and wet (1411 mm) years, ranged
from 829 to 1280 mm per year, virtually identical to
our range (869–1285 mm ET).
Estimated transpiration for three cypress wetlands
adjacent to our study sites obtained using Liu’s (1996)
ETM model ranged from 446 to 587 mm per year,
with total ET substantially higher, ranging from 934
to 1044 mm for 1991–1993 (Liu et al., 1997). Higher
ET in this case reflects greater evaporation from
the free-water surface, the area of which fluctuated
greatly in some of Liu’s modeled wetlands. Modeled
ET estimates for our pine stands are comparable to
long-term water budget estimates of about 1000 mm
per year from large watersheds in the vicinity of our
study site, comprised of the same land-use mixture of
pine flatwoods and cypress wetlands (Riekerk, 1989).
Differences between the two types of ecosystems in
terms of average annual energy fluxes are not large, in
spite of the wetland being dominated by a deciduous
overstory.
The similarities in annual latent and sensible heat
balances belie very large differences in the fixation of
energy into carbon along the chronosequence. There
are large differences in the estimated net C fluxes
between the clear-cut and closed-canopy pine sites.
Net ecosystem production (NEP) was −1269 and
−882 g C m−2 per year at the clear-cut in 1998 and
1999, and averaged 590 and 674 g C m−2 per year at
the mid-rotation and rotation-aged sites (Clark et al.,
in review). However, calculated ecosystem respiration
was similar in magnitude at all three sites, ranging
from 1907 to 2387 g C m−2 per year. Large differences also occur in net carbon fluxes between the two
Florida ecosystem types. Clark et al. (1999) found
a 14-fold difference in their NEP, with the mature
pine plantation a much greater carbon sink than the
cypress wetland, mainly because of much larger increments in tree stems and accumulation of carbon
in litter. However, in contrast to energy fluxes, little
seasonality was observed in the instantaneous rates
of net CO2 exchange at the same light levels during
the daytime at the older sites, because lower LAIs
in winter are apparently compensated for by lower
ecosystem respiration rates (Clark et al., 1999).
5. Conclusions
Our results suggest that (1) changes in albedo as
a result of management activities are relatively small
and short lived (non-detectable after 8–10 years) and
(2) ET does not change much over the 70% of the
landscape dominated by managed pine forests in relation to clearcutting, but rather is altered primarily
by drought effects. When (ET + I ) = 113% of precipitation and the water table is below 2.7 m, LE rates
will obviously be reduced eventually. In addition,
although, substantial phenological differences exist
between the dominant trees in the two types of ecosystems, the 30% of the landscape in wetlands apparently
has similar annual ET as the uplands, at least during
wetter years when they have surface water. In contrast
to our expectations, fluctuations in precipitation and
incident radiation appear to be the real controllers
of the multi-year dynamics of energy partitioning in
this landscape.
In retrospect, it became clear that this study was
carried out at the end of an extended period of relatively wet conditions for north-central Florida. Conditions after our study concluded became unusually dry,
with cumulative rainfall from 1998 to 2001 totaling
over 1700 mm less than the 3 year average (NOAA,
2001). Droughts in this region occur with a frequency of about every 15 years, with a second year
of dry weather usually following the first (Gholz
and Boring, 1991). Clearly, models for predicting long-term energy and carbon fluxes for these
ecosystems must include appropriate feedbacks and
constraints through water stress effects on LAI and
stomatal conductance. Energy and carbon balances
obtained using eddy covariance in such environments
must be made over very long periods in order to be
able to account for the full range of expected climate
conditions.
Acknowledgements
We thank Ian Beverland, Ford Cropley, John Moncrieff, Henry Loescher, Caijun Sun, Shuguan Lui,
Chang Ming Fang, Suzy Brock, Jose Luis Hierro,
Nate Warford, Amy Konopacky, Ryan Harris and
Steven Smitherman for assistance in the field. We
thank the Jefferson-Smurfit Corporation, the Rayonier
H.L. Gholz, K.L. Clark / Agricultural and Forest Meteorology 112 (2002) 87–102
Timber Corporation and the Donaldson Family for
allowing access to the slash pine ecosystems. This research was funded by Department of Energy, National
Institute of Environmental Change (NIGEC), Southeastern Regional Center. This is Florida Agricultural
Experiment Station Journal Series # R-08855.
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