Energy exchange across a chronosequence of slash pine forests in Florida

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

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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

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(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

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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

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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 (w). ¯ 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

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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

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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.

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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

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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

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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.

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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

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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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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

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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,

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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

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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

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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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