- Email: [email protected]
Evaluation of potential evaporation as a means to infer loblolly pine seedling physiological response to a given microclimate
~"
Forest Ecology and Management
.~
ELSEVIER
Forest Ecology and Management 67 (1994) 241-255
Evaluation of potential evaporation as a means to infer loblolly pine seedling physiological response to a given microclimate R i c h a r d A. Valigura a'*, M i c h a e l G. M e s s i n a b aNational Oceanic and Atmospheric Administration, Air Resources Laboratory, Building SSMC Ili, Room 3229, 1315 East West Highway, Silver Spring, MD 20910, USA bDepartment of Forest Science, Texas A&M University, College Station, TX 77843-2135, USA
Accepted 19 November 1993
Abstract
Foresters understand that there is no single correct way to treat every forest stand to produce optimum results, and that poor treatment can produce adverse effects on most sites. An example of this problem concerns areas that produce poor environmental conditions for regeneration upon clearcutting. Valigura and Messina (Journal of Environmental Management, 1994) found that loblolly shelterwoods substantially influence the temperature, radiation, humidity and windspeed of the seedling-level microclimate relative to those in a clearcut, but they could not predict the effects of these modified understory conditions on seedling performance. The primary question addressed in this study is: on harsh sites, what advantages/disadvantages does a partial overstory impose on seedlings growing underneath relative to seedlings grown in a clearcut? The objective was to evaluate the combination equation for potential evaporation as a relative means to infer the amount of stress imposed on loblolly pine seedlings in a shelterwood or clearcut environment, subsequently inferring the effect of a partial overstory on seedling performance. It was shown that although PE statistically explained some of the variance measured in seedling physiological variables, it was not possible to use PE estimates to infer seedling performance to microclimate, on these sites, during the relatively wet period of 1990/1991. This study qualitatively confirmed the ability of a shelterwood to favorably influence the seedling microclimate relative to a clearcut microclimate. However, there is still a need to explore and develop an inferential technique to evaluate the effect of different environments on seedling performance. Keywords: Shelterwood; Combination equation; Regeneration
I. Introduction
It is estimated that half a million acres in East Texas are difficult to regenerate by traditional clearcut and plant methodologies. In many cases, the main causes for loblolly pine (Pinus taeda L. ) regeneration failure on such sites are high tem*Corresponding author.
perature and moisture stress (Messina, 1991 ). One possible method of reducing these stresses is the shelterwood method of regeneration. This method entails the gradual removal of the overstory in a series of cuts that eventually exposes the understory environment to full sunlight conditions. Regeneration within the shelterwood method can be either natural or artificial. Valigura and Messina (1994) found that loblolly
0378-1127/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSDI 0378-1127 ( 93 ) 03344-I
242
R.A. Valigura, M. G. Messina / Forest Ecology and Management 67 (1994) 241-255
shelterwoods substantially influence temperature, radiation, humidity and windspeed of the seedling-level microclimate relative to those in a clearcut. However, they could not predict the effects of these modified understory conditions on seedling performance (stomatal conductance (qs), leaf water potential (~u~), height and survival), and posed the following question: On harsh sites, what advantages/disadvantages does a partial overstory impose on seedlings growing underneath relative to seedlings grown in a clearcut? Although direct measurements of seedling physiological condition (i.e. stomatal conductance, water potential) quantify the effects of the shelterwood on seedling performance, they require the collection of large amounts of concurrent data at many different sites, and do not include any causal investigation. The approach taken in the current study involves the development of an inferential approach. In theory, an inferential approach would be able to infer seedling performance from measured site factors (e.g. radiation, soil moisture ) and is therefore limited only by our ability to relate physical microclimate variables to effects on seedling physiology. Quantification of these relationships is often a demanding task. For instance, stomatal conductance can be represented as q, ocf(PAR, ~u~,VPD, T,) where PAR is photosynthetically active radiation, VPD is vapor pressure deficit, and T~ is air temperature. Although each of these variables directly effect qs and can be measured independently, they also have interactive effects (e.g. T, and VPD are positively correlated). Microclimate functions have been developed that integrate the effects of the relevant environmental variables to each other. This study evaluates one such equation, the combination equation for potential evaporation (PE), for use as an inferential approach to site evaluation. Van Bavel (1966) stated that it has 'always' been recognized that a site or area should be characterized by the evaporative losses that may
take place when water is available at the surface (i.e. PE). However, in the last 25 years, potential evaporation has received mixed reviews in the literature, especially in regard to its usefulness in classifying sites in terms of the physiological conditions of plants growing there (Rosenberg et al., 1993; Bringfelt, 1985; Dunin et al., 1985; Harding, 1987). Popular opinion may be summarized according to Lee ( 1978 ), who stated that the concept of PE "has, if nothing more, survival value. One must question, however, whether 'value' in this context is to take a positive or negative sign". The combination equation has been shown to be applicable in conditions over most homogeneous canopies. The emphasis is placed here because these conditions do not apply to the sites used in this study. However, the objective of this study was not to quantify PE specifically, but to: evaluate PE, as estimated with the combination equation, as a relative means to infer the amount of stress imposed on loblolly pine seedlings in a shelterwood and clearcut environment, and thereby infer the effect of a partial overstory on seedling performance. When using a relative measure, precision and uniformity in procedure is more important than accuracy of estimate. One of the original definitions of PE is the evaporation that would occur from well-watered short grass (Penman, 1948). Improvements to Penman's original equation have been made by combining sensible heat and water vapor functions with the energy balance concept; this is termed the combination equation. The form of the combination equation for PE used in the present study is PLE= { ( A ( T ) , (Rn - G ) ) +
( (Cp *Pa * V D P ) / r a ) } / { A ( T )
+y}
where PLE is the potential latent heat flux (W m-2), Rn is net radiation (W m-2), G is soil heat flux (W m -2), cp is the specific heat of air (J g K -~ ), pa is the density of air (g m-S), VPD is vapor pressure deficit (Pa), r~ is the neutral
R.A. Valigura, M.G. Messina /Forest Ecology and Management 67 (1994) 241-255
aerodynamic resistance calculated for infinite fetch (s m - l ), L is the latent heat of vaporization of air (J Kg- 1), A (T) is the slope of the saturation vapor pressure (SVP) curve at air temperature (Pa K -1), and 7 is the psychometric constant (Pa K -~ ). All variables are measured at a specified reference height. The three main assumptions made when deriving this form of the combination equation are (i) canopy storage is a relatively small part of the energy balance, (ii) air-surface exchange of momentum, heat and water vapor can be related to a single level within the canopy (the analogy to one big leaf is common ), and (iii) that A (T) is the same at both air and surface temperature (Van Bavel, 1966 ). For a full discourse on the combination equation and its assumptions see Monteith and Unsworth, 1990.
2. Site description At present, there is no guaranteed method to determine, pre-harvest, if a site will present regeneration problems. For this study, the site was chosen as a result of a survey of local foresters. The soil, elevation and region of the study site were the same as sites that had previously been hard to regenerate. The study site was located in Cherokee County, Texas, at 31 °41 ' N and 95 ° 15'W and had an average elevation of 98 m above mean sea level. The average slope of the site was 3 o with an aspect of 152 °. Annual precipitation averages 1140 mm, and is distributed fairly evenly throughout the year. The soils on the site are a mixture of the Boswell and Bowie Series. Boswell fine sandy loam is a Plinthic Paleudult with 0.20 m of friable fine sandy loam over a blocky red clay to 0.40 m. Bowie loamy fine sand is similar to the Boswell series but has 0-0.35 m of friable-loose fine sandy loam over compact yellow or brownish sandy clay loam to 0.66 m. The site indexes (base age 50 years) for loblolly pine are 23.7 and 25.3 m on the Boswell and Bowie Series, respectively. Likewise, for shortleaf pine (Pinus echinata Mill.) the site indexes are 20.7 m and 23.5 m, respectively (Mowery and Oakes, 1959). The
243
stand was naturally regenerated, approximately 50 years old, and had a mean basal area of 13 m 2 ha -1 in pine sawtimber (0.20 m diameter at breast height or greater) before harvesting.
3. Materials and methods The loblolly pine shelterwood chosen for this project was located adjacent to a large clearcut. It was harvested to leave 6.9 m 2 ha-L of residual overstory (predominantly loblolly pine), and measured 305 m X 3 0 5 m (9.3 ha). It was bordered on the southern and eastern sides by clearcuts, and on the northern and western sides by undisturbed forest. Undesired trees of all species that remained after harvest were injected with a systemic herbicide (picloram + 2,4 D). All environmental measurements in the shelterwood were conducted in a centrally located measurement plot (0.15 ha). The clearcut selected for this study was a commercial clearcut over 200 ha in size, adjacent to the shelterwood study. The clearcut site was prepared by drum chopping and burning so that all above-ground vegetation was removed from the site. All environmental measurements in the clearcut were conducted in a centrally located measurement plot judged to be of the same elevation and aspect as the measurement plot in the shelterwood. Herbicide (glyphosphate) was applied to the measurement plots to eliminate herbaceous growth in order to decrease variability in ground-level conditions between overstory treatments. Sampling began on 21 May 1990 and continued to 30 April 1991. All instruments referred to in the following sections were purchased from Campbell Scientific Inc., Logan, Utah. Instrument readings were recorded by a model CR l 0 battery-powered datalogger and connected AM32 multiplexer. Recording was done on a 24-h basis, with all instrument signals read every 10 s and averaged over 30-min intervals.
3. I. Meteorological measurements The primary use for the measurements taken during this study was to quantify the input terms
R.A. Valigura, M.G. Messina / Forest Ecology and Management 6 7 (1994) 241-255
244
for the combination equation. The intention was to characterize the microclimate to which a seedling might normally be exposed at chosen points in both the shelterwood and clearcut stands, rather than to provide an average value for each stand condition. A summary of the instrumentation is presented in Table I. Radiation instruments were mounted horizontally at 1.2 m above the soil surface to avoid excessive interference of the surface radiation regime by the instrument and its supporting structures, while still giving a reasonable approximation of the radiation environment of seedlings (Holbo and Childs, 1987). Humidity and windspeed were measured at 0.3 m above the soil surface, the approximate seedling height. Saturation vapor pressure (SVP) was calculated using the humidity probe measurements in Teten's equation SVP = 0.061078 . e x p ( ( 17.269. Ta)/ (237.3 + T a ) ) where Ta is the average air temperature ( ° C ) at 0.30 m. A four-level temperature profile was measured to support the calculation of soil heat flux as well as the quantification of the extreme temperatures that occurred on each site (Valigura, 1991 ). Spatial variability was of concern
due to the small sampling area of each thermocouple. Therefore, averages of five sample points were taken at each depth and height within a cirkel of 3 m diameter. Two circles were sampled in both the shelterwood and the clearcut sites. This provided a total of eight average temperature readings per site type (two averages at each of the four levels above and below the soil surface ). Averages were taken by parallel wiring of the individual thermocouples on an electrical terminal strip before the voltage was sent to the CR10 datalogger.
3.2. Soil water Soil moisture was sampled gravimetrically at three depths, 0-0.05 m, 0.05-0.15 m and 0.150.30 m, in a herbicide-treated plot adjacent to the measurement plot. Due to the distance between the site and laboratory, soil samples were taken every 3-4 days. Soil moisture data were used to calculate soil heat flux (method discussed later). Preliminary trials showed that a 10% error in volumetric water content determination results in an average change of less than 10% in soil heat flux calculations, and this averaged to less than 4% change in estimated potential evaporation. Therefore, two soil samples for each plot per
Table 1 Equipment details System
Height
ModeP
(m) Wind speed Temperature
3-cup anemometer Copper-constantan thermocouples
Relative humidity RH temperature
Surface film PCRC-11 chip Fenwall Electronics UUTS 1J 1 thermistor Polyethylene shielded non-aspirated radiometer Multi-channel micro-processing system
Net radiation Data acquisition
0.30 0.30 -0.001 -0.05 -0.30 0.30 0.30 1.20
R.M. Young 03101 On site
Campbell Scientific Model 207 Campbell Scientific Model 207 Radiation energy balance systems !5.5 Campbell Scientific CR 10
aldentification of specific instruments here or elsewhere should not be interpreted as an endorsement relative to equivalent systems available from other sources.
R.A. Valigura, M.G. Messina / Forest Ecology and Management 6 7 (I 994) 241-255
sampling day were deemed adequate. The two soil samples allowed for both the Boswell and Bowie Series to be represented. Moisture content was determined for each soil type, and then combined for the calculation of soil heat flux.
3.3. Soil heat flux Soil heat flux (W m - 2 ) was estimated using the temperature gradient method (Kimball and Jackson, 1979; Flint and Childs, 1987) which is based on
G= Cs , A Z , (AT~At)
245
C,=Cm(pb)+Cw(Ov)Pw where Cm is the soil particle and rock fragment heat capacities (MJ Mg -1 °C -t ) Pb is soil bulk density, Cw is the soil water heat capacity (MJ Mg- ] ° C - ] ), 0v is the soil water content, and Pw is the density of water (assumed to be 1 Mg m - a). By summing heat fluxes of multiple sections of soil it is possible to estimate soil heat flux. Because of thermocouple positioning, two levels of heat flux were determined: 0-0.05 m and 0.050.30 m.
3.4. Aerodynamic resistance
where G is the soil heat flux, C, is the soil volumetric heat capacity (MJ m - 3 o C - t ). A Z i s t h e thickness of the measurement layer (m), AT is the change in temperature (°C), and At is the time interval (s). Based on studies of soils with similar composition and structure, the soil and water heat capacities were assumed to be 1.47 and 4.18 MJ Mg -t °C -1, respectively (Childs et al., 1985). These values also compared well with other published ranges of values. Cs was calculated by
Aerodynamic resistance (ra) was estimated by two different methods so that actual conditions would most likely occur somewhere within the tested range of values. The same resistance equations were used for both the clearcut and shelterwood (i.e. ignoring the overstory trees). Although initial requirements were not met (homogeneous plant canopy and adequate fetch), the first approximation ofra used the logarithmic profile for a bare soil. Environmental
1000
I
-I
Logarithmic
. . . . . Linear 800
~
600
o
400
"ES
EO0
0
.< 0
0.0
I
I
i
0.5
1.0
1.5
2.0
Average Windspeed (m s-I) Fig. 1. A comparison between the two aerodynamic resistance functions versus average wind speed values. The linear function simulates a crop canopy 1 m in height. The logarithmic function simulates bare soil conditions.
R.A. Valigura, M.G. Messina / Forest Ecology and Management 67 (I 994) 241-255
246
Table 2 Months and days chosen to represent environmental conditions in the clearcut and shelterwood Period
Day number
July and August 1990
207,208,209,210,219,220, 223,224,225,227,229,237, 238,241,242
NovemberandDecember1990 309,310,311,314,315,316, 328,332,333,338,339,341, 342,352,353 83,85,88,89,90,97,99, 100,105,106,110,112,120, 121
March and April 1990
stability correction parameters usually accounted for in this method were not incorporated into the resistance estimates due to errors expected from the inadequate canopy parameters. The logarithmic resistance equation calculated for neutral conditions takes the form (Duell, 1990 )
r. = { l n ( z - d + zo) /Zo}{ln(z-d+ zv ) /zv} (k2*u(z)}
where z is the height in profile (m), dis the displacement height (m), zo is the roughness length for momentum (m), Zv is the roughness length for vapor (m), k is yon Karman's constant, and u(z) is the wind speed at height z ( m s -1 ). The value of zo for bare soil has been experimentally determined to be close to 0.01 m (Fuchs et al., 1969; Van Bavel and Hillel, 1976 ). Using the relationship between momentum and vapor roughness based on the dimensionless parameter B - 1 (Verma and Barfield, 1979 ), zv for bare soil will be taken to be 0.002 m. The second method used to approximate ra involved a simple linear relationship between resistance and windspeed, which gave values comparable with values given in the literature (Jarvis et al., 1976; Grace, 1980; Dixon and Grace, 1984). The equation was adapted to ra = 1 0 0 0 - (u(z)/O.O0202) where all variables are as previously defined. Both methods estimated maximum resistance to be 1000 s m - l. A graphical representation of both of the ra functions is presented in Fig. 1. The large range bordered by the two functions is due to the canopy conditions simulated by each func-
too
80
60
40
20
o
_2
145 165 185 205 225 2 4 5 2 6 5 2 8 5 305 325 3 4 5 3 6 5 20
1990
40
60
80
100 120
1991 D ay N u m b e r
Fig. 2. Rainfall events and magnitude throughout the sample period. Periods 1, 2 and 3 are underlined.
R.A. Valigura, M.G. Messina / Forest Ecology and Management 67 (1994) 241-255
I
5°I
~
I
I
I
f
I
I
A
Shelterwood
....
- -
247
Clearcut
40 o
.o
30 ¢9
0
o
10 o
i o
0
I
q
]
I
50
I
I
B C o
©
40
30
o 20
0 10
? Q
1
I
I
I
I
i
I
L
I
I
i
i I I I
45 165 185 205 225 245 285 285 305 325 345 365
1990
Day N u m b e r
20
I
I
I
I
I
40
60
BO
100
120
1991
Fig. 3. Soil water contents of the (A) 0 to - 0 . 0 5 m, and (B) - 0 . 0 5 to - 0 . 3 0 m soil layers. Measured on the clearcut and the shelterwood sites over the sample period.
tion. The logarithmic profile is based on bare soil, while the linear profile is based on a canopy 1 m in height. The 'true' value of aerodynamic resistance should fall within this range of values. 4. Results and discussions Sampling began on 25 May 1990 (day number
145 ) and concluded on 30 April 1991 (day number 120). Out of the 342 days spanned by the experiment, between 158 and 209 days were successfully measured depending on the variable, thereby facilitating the quantification of differences between the sites. Due to the large amount of data collected, three 2-month periods were chosen to represent the
248
R.A. Valigura, M.G. Messina / Forest Ecology and Management 67 (1994) 241-255
Table 3 Ranges and totals of potential latent heat flux (PLE) and evaporation (PET), calculated using the logarithmic ra equation, for the clearcut and shelterwood sites over 158 days, and the average sunny days (ASD) Clearcut PLE (PET)
Period sums MJ m -e (kg m -2) Total period 1300.0 ( 527.2 ) Period 1 180.0 (74.8) Period 2 85.5 (34.8) Period 3 140.0 (56.9)
840.0 20.0 40.4 92.4
Maximum daily sum MJ m -2 d ~ (kg m -2 d - ~) Total period 16.3 (6.7) Period 1 15.4 (6.4) Period 2 7.9 (3.2) Period 3 12.2 (4.9) Minimum daily sum MJ m -e d t (kg m -2 d-~ ) Total period - 1.1 ( - 0.3 ) Period 1 0.4 (4.3) Period 2 2.5 (1.0) Period 3 3.9 (1.6) ASDdailysumMJm-2d-~ (kgm-Zd Period 1 12.1 Period 2 5.7 Period 3 9.3
Shelterwood PLE (PET)
l) (5.0) (2.3) (3.6)
11.5 9.3 4.1 9.9
(350.7) (48.5) (16.4) (37.8)
(4.8) (3.8) (1.1) (4.1)
-0.6 (-0.3) 6.6 (2.7) 1.6 (0.6) 2.2 (0.9)
Differencea
460.0 60.0 45.4 47.7
(176.5) (26.4) (18.5) (19.0)
5.8 6.1 3.8 2.3
(1.9) (2.6) (2.1) (0.8)
0.5 3.6 0.9 1.7
(0.0) (1.6) (0.4) (0.7)
7.9 (3.3) 2.7 (1.1) 6.2 (2.5)
4.2 (1.7) 3.0 (1.2) 3.1 (1.1)
Maximum 30-min average W m -2 (g m -2 s- l ) Period 1 603.8 (0.25) Period 2 382.2 (0.16) Period 3 607.1 (0.25)
573.0 (0.24) 225.4 (0.09) 452.5 (0.19)
30.8 (0.01) 156.8 (0.06) 317.0 (0.06)
Minimum 30-min average W m -2 (g m -2 s -I ) Period 1 - 3 4 . 9 ( -0.01 ) Period 2 - 121.2 ( - 0 . 0 5 ) Period 3 - 100.7 ( - 0 . 0 5 )
-93.3 (-0.04) -130.5 ( - 0 . 0 5 ) -128.4 ( - 0 . 0 5 )
58.4 (0.30) 9.3 (0.00) 8.7 (0.00)
295.8 (0.12) 126.6 (0.05) 306.1 (0.12)
97.0 (0.04) 114.5 (0.03) 67.1 (0.02)
- 1 . 7 (0.00) - 2 . 6 (0.00) - 3 . 5 (0.00)
1.0 (0.00) - 3 . 1 (0.00) - 11.9 ( - 0 . 0 1 )
ASD maximum 30-min average W m - 2 ( g m - ~ Period 1 392.8 (0.16) Period 2 241.1 (0.10) Period 3 373.2 (0.15) ASD minimum 30-rain average W m -2 Period 1 -0.7 Period 2 -5.7 Period 3 - 15.4
s - ~)
(g m -2 s- i ) (-0.00) (-0.00) (-0.01)
aClearcut minus shelterwood. r a n g e o f c o n d i t i o n s e x p e r i e n c e d o n e a c h site. O u t o f e a c h 2 - m o n t h p e r i o d , 15 d a y s w e r e c h o s e n t o represent the 'sunniest' days measured over that p e r i o d , as j u d g e d b y t h e t o t a l a m o u n t o f P A R a n d
r a d i a t i o n r e c e i v e d t h a t day. T h e s e d a y s w e r e t h e n a v e r a g e d a n d a r e p r e s e n t e d as a n ' a v e r a g e s u n n y day' (ASD) from each period. The months and d a y s c h o s e n a r e l i s t e d i n T a b l e 2. S u n n y d a y s
R.A. Valigura, M.G. Messina / Forest Ecology and Management 6 7 (I 994) 241-255 16
I
l
I
[
~
I
t
~
I
I
I
I
I
I
I
249
W~
...... Shelterwood 14
~
10
t
Clearcut
"
i ~4
20
1990
40
60
80 tO0
120
1991 Day Number
Fig. 4. Daily totals of potential evaporation (PE) calculated for the clearcut and shelterwood sites over the sampling period. The two smooth lines are third order regressions intended to show annual patterns on the clearcut (upper line) and shelterwood (bottom line) sites. Missing values are due to instrumentation problems.
were chosen because they are of the most interest to this study of harsh sites. Furthermore, overcast and rainy days are inherently variable and difficult to interpret as averages. Period 1 spans 35 days of the middle summer months, with 26 days successfully instrumented, and had a relatively constant weather pattern, with a total of 4.5 mm of rainfall. Period 2 spans 45 days in late fall, with 30 days successfully instrumented, and was characterized by passing weather fronts and decreasing radiation loads, with a total of 131 mm of rainfall. Period 3 covers 39 days in early spring, with 30 days successfully instrumented, and had consistent rainfall indicative of one of the wettest springs on record, with a total of 218 mm of rainfall.
4.1. Rainfall and soil moisture An unusually large amount of rainfall, 794 mm, was measured for the 209 days successfully sampled (Fig. 2). Due to its effect on soil moisture
contents, the large rainfall possibly delayed or negated any potential harsh site effects. There are few readily interpretable trends in volumetric water content in the upper soil layer between the clearcut and shelterwood sites for the overall period. Water content in the upper soil layer (0-0.05 m) ranged from 2.5 to 40% over the full sampling period (Fig. 3 (A)). There were three intervals (Days 186-198, 217-252 and 7495 ) when the shelterwood site had considerably higher moisture contents in the upper layer of soil than did the clearcut site. Volumetric soil water contents in the lower layer of soil showed no overall trends between the clearcut and shelterwood sites. Water contents in the lower soil layer (0.05-0.30 m) ranged from 12 to 33% over the full sampling period. Smaller amplitude fluctuations in the lower layers of soil (Figs. 3 (A), ( B ) ) can be attributed to the lower infiltration rates associated with the clay layer in the soils on these sites (Boswell and Bowie series). Several times throughout the period, perched water tables were observed at the interface of the upper sandy horizons with the
250
R.A. Valigura, M.G. Messina / Forest Ecology and Management 67 (1994) 241-255
600
' ' ' ; ' ' ' --- Sheltcrwood~
;
'
'
'
I
'
'
'
A
B
500 400 300 200 100 0 -100 300
i
i
i
I
i
r_l
,
'
,
i
'
'
'
I
i
i
i
I
r
~
J
'
,
,
r
'
'
i
'
C
D
200 100 0 -100 -200 -300 i
-400
i
I
i
i
J
I
i
i
i
I
i
L
,
500
45
'
'
'
I
'
'
'
I
'
,
'
I
'
'
7 1
F
40 400
35 30
300
25 200
20 15
100 10 5 0
0 J
,
i
I
i
600
i
~
I
1200
i
,
i_l
1800
i
i
,
i
2400
0
,
,
I
800
,
,
,
1_J
1200
,
,
I
1800
,
,
,
2400
Hour Fig. 5. Average sunny day (ASD) patterns exhibited by (A) net radiation (W m 2 ) , ( B ) soil heat flux (W m-2), (C) air temperature at 0.30 m (°C), (D) windspeed at 0.30 m ( m s -1 ), (E) vapor pressure deficit at 0.30 m (KPa), and (F) potential latent heat flux ( W m -2) measured on (estimated for) the clearcut and shelterwood sites during Period 1.
lower clay horizons. Much of the variation in Figs. 3(A), (B) can be attributed to the highly variable nature ofgravimetric moisture sampling.
4.2. Potential evaporation (PE) Although all PE calculations were performed using both the linear and logarithmic aerodynamic equations, there were no significant differences in results. Therefore, all values included in this report were calculated using the logarithmic equation.
Daily PE totals (integrated from the 30 min meteorological averages) followed the same basic trends, but were greater in the clearcut than in the shelterwood (Table 3; Fig. 4). Net radiation and soil heat flux accounted for between 92 and 98% of the estimated nighttime PE, and between 65 and 80% of estimated daytime PE. VPD was shown to consistently increase PE estimates by 2-8%, both day and night, thus lending credibility to the hypothesis that PE is affected by, but cannot be directly predicted by, VPD (Rosenberg et al., 1983). Through its effects on Ra,
R.A. Valigura, M.G. Messina / Forest Ecology and Management 67 (I 994) 241-255 600
'
'
'
i
'
'
'
i
'
'
'
I
'
'
Shelterwood
---
500
A '
Clearcut
251
1.0 B 0.8
400 0.6
300 200
0.20"4~_j
100
,'
,,,,--
-,,.,.
',
0 _
-100 300
,
,
i
I
,
,
,
I
J
,
L
I
,
i
,
'
,
I
'
,
'
i
,
,
'
I
'
'
,
O. 0
,
~ ,
'
'
i
,
,
~
I
,
"?
'
l
,
,
t
,
,
,
D
C 200 100 0 -100 -200 -300 ,
-400 45
,
,
I
,
,
,
I
I
[
,
i
'
I
'
'
'
T
'
L
'
'
i
'
500
.
.
.
I
'
'
'
I
~
'
'
'
I
'
'
'
I
E
40
'
'
' F
400
35 30
300
25
200
20 15
lOO
.
10 5 0 600
1200
1800
2400
0
000
1200
1800
2400
Hour Fig. 6. Average sunny day (ASD) patterns exhibited by (A) net radiation ( W m 2), (B) soil heat flux ( W m - Z ) , ( C ) air temperature at 0.30 m ( ° C ), ( D ) windspeed at 0.30 m (m s - ' ), ( E ) vapor pressure deficit at 0.30 m ( K P a ) , and (F) potential latent heat flux ( W m -2) measured on (estimated for) the clearcut and shelterwood sites during Period 2.
wind was shown to increase FE estimates consistently, accounting for 10-15, 20-40 and 10-20% of estimated PE for ASDs in Periods 1-3, respectively. Diurnal patterns of calculated PE for each of the three ASDs are shown in Figs. 5-7. Trends were very similar across all three ASD periods. During the early morning interval (between 06:00 and 10:00 h) PE estimations were highly variable, becoming close, to and even less than, zero (condensation). This is a direct result of calculated soil heat flux being greater than incoming radiation, a condition which is improb-
able, and one that is likely associated with errors in the soil heat flux calculations. During this interval, calculated PE reached higher levels in the shelterwood than in the clearcut (Figs. 5-7), a condition which is hard to evaluate due to the probable error in calculating soil heat flux. However, PE being greater in the shelterwood during the morning is not unreasonable: more incoming radiation may be used for soil heating in the clearcut relative to that in the shelterwood, thereby decreasing PE in the clearcut. The fact that actual vapor pressure (AVP) is also greater in the shelterwood during this interval (Figs. 5-
252
R.A. Valigura, M.G. Messina / Forest Ecology and Management 6 7 (1994) 241-255
'
800
'
' l
'
'
'
I
'
'
'
1
'
1.0
'
.... S h e l t e r w o o d ~
500
A
i
'
'
'
i
'
'
'
i
, ~
r
]E]
0,8 / ) ~ , ' ' / I ~
400
0.6
300
200
0.4
100 0.2
O --IO0
, l i l , J + l , i , l , i ,
300
'
'
'
+
'
'
'
I
'
'
O.O
'
I
'
'
6
' C
5
200
100
4i
0 -100 -200 - 3 0 0
- 4 0 0
45 40 35 30 25 20 15 10 5 0
'
'
'
'
I
'
'
'
'
'
'
[
'
'
'
'
'
'
I
'
'
'
'
'
500
'
I
'
'
'
I
'
'
'
I
'
'
' P
400
300
200
100
0 _
0
600
1200
1800
2400
0
i
r
+ 600
I
i
+
i 1200
L
,
r
,
f
i
1800
i
i
2400
Hour Fig. 7. Average sunny day (ASD) patterns exhibited by (A) net radiation (W m-2), (B) soil heat flux (W m-2), (C) air temperature at 0.30 m ( ° C), (D) windspeed at 0.30 m (m s-~ ), (E) vapor pressure deficit at 0.30 m (KPa), and (F) potential latent heat flux (W m -2) measured on (estimated for) the clearcut and shelterwood sites during Period 3.
7 ) may also strengthen the validity of the calculated PE trends. Furthermore, Dalton (1991) measured seedling stomatal conductance and porometer transpiration (PT) rates (see below) on these sites, including Period 1, and found that they were higher during these morning hours from seedlings on the shelterwood site than from those on the clearcut. For all three periods, calculated PE levels in the clearcut became higher than those in the shelterwood at 11:00 h and remained higher throughout the rest of the day and night (Figs. 5-7).
4.3. Seedling performance Final seedling survival and heights were measured on both sites (Dalton, 1991). One year after planting, survival was highest on the clearcut site (50%) as compared to the shelterwood site (42%), although this difference was not statistically significant. There were also no significant differences in seedling heights or understory leaf area, indicating that the overstory did not significantly reduce seedling growth. These results may be related to the large amount of rainfall during 1990/1991, which may have off-
R.A. Valigura, M.G. Messina / Forest Ecology and Management 67 (1994) 241-255
4.4. Evaluation of potential evaporation
0.0 u EL
-0.2
0.0 Q
----
Clearcut
....
Shelterwood
0.2
-0.4
-)
-0.4
0.6
0.6
-0.8
-0.8
r. I
b ~o
0.20
0.20
0.15
0.15
0.10
0.10
0.05
0,05
E o
>
le~
E
c
18
18
16
16
14
14 12
10
Q L_
253
10
8
8 6
4 6
8
10
12
14
16
18
20
Time of Day (ho~O Fig. 8. Seasonal average (A) water potential (MPa), (B) stomatal conductance (cm s -1), and (C) porometer transpiration (rag m -2 s -~) as affected by regeneration technique (adapted from Dalton, 1991 ).
set any survival advantages provided by the shelterwood. Dalton (1991 ) also measured ~ul, qs and PT rates throughout the growing season of 1990 with a whole seedling porometer (four times during each measurement day). The seasonal average diurnal trend in each variable is shown in Fig. 8. PT and q~ were lower in the shelterwood, while ~1 was higher. Coupled with the apparent lack of overstory influence on growth rates, these data suggest that the shelterwood seedlings maintained more favorable physiological conditions than the clearcut seedlings. Data from five contiguous days (i.e. 20 data points) that occurred during measurement Period I were used in the evaluation of PE.
Higher PE estimates on the clearcut site suggest that there should be higher survival and growth in the shelterwood due to the decreased stress levels. This was not the case. Out of the five primary comparative data categories (survival, growth, ~q, qs and PT), only two showed significant trends between sites, indicating an inability of PE to predict seedling performance in any given year. However, a few general inferences can be drawn from the PE results. Although a moist soil dries at the potential rate for only a short time, higher evaporative demands in the clearcut (Table 3) should have caused consistently lower water contents in the clearcut relative to those in the shelterwood. This was not apparent (Fig. 3 ). Given the similarities in understory leaf area and qs, it may be concluded that the difference in evaporation rates between the two sites was made equal by the water usage of the shelterwood canopy. During this study there were three intervals, mentioned above, where soil water content in the shelterwood was greater than in the clearcut. It is possible to speculate that water usage by the overstory, at the given level of spacing, was less influential on soil moisture trends in the shelterwood than was the reduction in evaporative demand between the clearcut and shelterwood, a potential advantage supplied by the partial overstory. Simple linear regression and general linear model ( G L M ) procedures were used to investigate any specific relationship between PE and measured ~1, qs and PT. All attempts to predict seedling variables by regression showed little to no statistical significance (i.e, no predictive power). A GLM was used to determine to what extent the variance in stomatal conductance, water potential and porometer transpiration (the dependant variables) could be explained by the variability in day, time (fixed independent variables) and PE (continuous,independent variable). The results showed that a significant amount of the variability (*P<0.05 to ***P< 0.001 ) in the three seedling variables was explained on both sites. However, the amount of
254
R.A. Valigura, M.G. Messina / Forest Ecology and Management 6 7 (1994) 241-255
variability accounted for by PE within the GLM models was significant for the clearcut site only, and only for ~ul and PT. That is, day and time of day explained a major percentage of the variation in seedling water potential, stomatal conductance and transpiration. This result, in itself, is expected. The point of interest in the GLM results is that PE was significantly correlated with seedling physiological conditions only in the clearcut. This is likely a result of the seedlings measured by Dalton ( 1991 ) being outside of the meteorological measurement plot (i.e. within 50 m ). Microclimate can be extremely variable in a shelterwood, and it is possible for the measurement plot to have been in shade while the seedlings measured were in full sunlight, thereby masking the effects of PE. Therefore, PE may have had the same effect on seedlings in the shelterwood as it had in the clearcut, but this was not discernable because measurements were not taken within the same plot as were the PE measurements.
5. Conclusion The near-surface environment of an East Texas clearcut and shelterwood site were instrumented to evaluate PE, as estimated with the combination equation, as a relative means to infer loblolly pine seedling physiological response to a given microclimate. It was shown that although PE statistically explained some of the variance measured in seedling physiological variables, it was not possible to use PE estimates to infer seedling response to microclimate, on these sites, during the relatively wet period of 1990/1991.
The current study qualitatively confirmed the ability of a shelterwood to favorably influence the seedling microclimate relative to a clearcut microclimate. However, there is still a need to explore and develop an inferential technique to evaluate the effect of different environments on seedling performance.
Acknowledgments Land was made available for and the research funded by Temple-Inland Corporation, Diboll, TX, USA. We thank Jim Heilman, Craig McKinley and Allan Tiarks for their guidance and advice throughout the research process.
References Bringfelt, B., 1985. A forest evapotranspiration model using synoptic scale weather data. In: B.A. Hutchinson and B.B. Hicks (Editors), The Forest-Atmosphere Interaction. Proc. Forest Environmental Measurements Conf., Oak Ridge, TN. 23-28 October 1983, pp. 25-37. Childs, S.W., Holbo, H.R. and Miller, E.L., 1985. Shadecard and shelterwood modification of the soil temperature environment. Soil Sci. Soc. Am. J., 49:1018-1023. Dalton, C., 199 I. Water relations ofloblolly pine (Pinus taeda L. ) seedlings planted under a shelterwood and in a clearcut. Masters Thesis, Texas A&M University. Dixon, M. and Grace, J., 1984. Effect of wind on the transpiration of young trees. Ann. Bot., 53:811-819. Duell, L.F.W., 1990. Estimates of evapotranspiration in alkaline scrub and meadow communities of Owens Valley, California, using the Bowen-ratio, eddy correlation, and Penman-combination methods. US Geol. Survey WaterSupply Paper: 2370-E. Dunin, F.X., McIlroy, I.C. and O'Loughlin, E.M., 1985. A lysimeter characterization of evaporation by eucalypt forest and its representativeness for the local environment. In: B.A. Hutchinson and B.B. Hicks (Editors), The Forest-Atmosphere Interaction. Proc. Forest Environmental Measurements Conf., Oak Ridge, TN, 23-28 October 1983, pp. 25-37. Flint, A.L. and Childs, S.W., 1987. Field procedure for estimating soil thermal environments. Soil Sci. Soc. Am. J., 51: 1326-1331. Fuchs, M., Tanner, C.B., Thurtell, G.W. and Black, T.A., 1969. Evaporation from drying surfaces by the combination method. Agron. J., 61: 22-26. Grace, J., 1980. Some effects of wind on plants. In: J. Grace, E.D. Ford and P.G. Jarvis (Editors), Plants and their Environment. Blackwell, Oxford, pp. 175-204. Harding, L., 1987. Relationships of environmental variables with first-year survival and growth ofloblolly pine in East Texas. Masters Thesis, Texas A&M University. Holbo, H.R. and Childs, S.W., 1987. Summertime radiation balance of clearcut and shelterwood slopes in southwest Oregon. For. Sci., 33:504-516. Jarvis, P.G., James, G.B. and Landsberg, J.J., 1976. Coniferous forest. In: J.L. Monteith (Editor), Vegetation and tile Atmosphere, Vol. 2. Academic Press, New York, pp. 171240.
R.A. Valigura, M. G. Messina / Forest Ecology and Management 67 (1994) 241-255 Kimball, B.A. and Jackson, R.D., 1979. Soil heat flux. In: B.J. Barfield and J.F. Gerber (Editors), Modification of the Aerial Environment of Plants. ASAE Monograph, pp. 211229. Lee, R., 1978. Forest Microclimatology. Columbia University Press, New York, 276 pp. Messina, M.G., 1991. Herbicide, fertilizer, and shade influence loblolly pine (Pinus taeda L.) growth and survival on harsh Texas sites. Presented at the Sixth Biennial Southern Silvicultural Research Conf., Memphis, TN, 30 October- 1 November 1990. Monteith, J.L. and Unsworth, M.H., 1990. Principles of Environmental Physics. 2nd edn. Edward Arnold, London. 291 pp. MoweR', I.C. and Oakes, H., 1959. Soil survey of Cherokee County, Texas. US Soil Conservation Service and Texas Agricultural Experiment Station. US Government Printing Office, Washington, DC. Penman, H.L., 1948. Natural evaporation from open water, bare soil and grass. Proc. R. Soc. London, Set. A, 193: 120-145.
255
Rosenberg, N.J., Blad, B.L. and Verma, S.B., 1983. Microclimate: The Biological Environment. John Wiley, New York, 495 pp. Valigura, R.A., 1991. Quantification of seedling environment in an East Texas loblolly pine (Pinus taeda L. ) shelterwood and clearcut. Ph.D. Dissertation, Texas A&M University. Valigura, R.A. and Messina, M.G., 1994. Modification of Texas clearcut environments with loblolly pine shelterwoods. J. Environ. Manage., in press. Van Bavel, C.H.M., 1966. Potential evaporation: the combination concept and its experimental verification. Water Resour. Res., 2: 455-467. Van Bavel, C.H.M. and Hillel, D.I., 1976. Calculating potential and actual evaporation from a bare soil surface by simulation of concurrent flow of water and heat. Agric. Meteorol., 17: 453-476. Verma, S,B. and Barfield, B.J., 1979. Aerial and crop resistances affecting energy transport. In: B.J. Barfield and J.F.Gerber (Editors) Modification of the Aerial Environment of Plants. ASAE Monograph, pp. 230-248.
Forest Ecology and Management
.~
ELSEVIER
Forest Ecology and Management 67 (1994) 241-255
Evaluation of potential evaporation as a means to infer loblolly pine seedling physiological response to a given microclimate R i c h a r d A. Valigura a'*, M i c h a e l G. M e s s i n a b aNational Oceanic and Atmospheric Administration, Air Resources Laboratory, Building SSMC Ili, Room 3229, 1315 East West Highway, Silver Spring, MD 20910, USA bDepartment of Forest Science, Texas A&M University, College Station, TX 77843-2135, USA
Accepted 19 November 1993
Abstract
Foresters understand that there is no single correct way to treat every forest stand to produce optimum results, and that poor treatment can produce adverse effects on most sites. An example of this problem concerns areas that produce poor environmental conditions for regeneration upon clearcutting. Valigura and Messina (Journal of Environmental Management, 1994) found that loblolly shelterwoods substantially influence the temperature, radiation, humidity and windspeed of the seedling-level microclimate relative to those in a clearcut, but they could not predict the effects of these modified understory conditions on seedling performance. The primary question addressed in this study is: on harsh sites, what advantages/disadvantages does a partial overstory impose on seedlings growing underneath relative to seedlings grown in a clearcut? The objective was to evaluate the combination equation for potential evaporation as a relative means to infer the amount of stress imposed on loblolly pine seedlings in a shelterwood or clearcut environment, subsequently inferring the effect of a partial overstory on seedling performance. It was shown that although PE statistically explained some of the variance measured in seedling physiological variables, it was not possible to use PE estimates to infer seedling performance to microclimate, on these sites, during the relatively wet period of 1990/1991. This study qualitatively confirmed the ability of a shelterwood to favorably influence the seedling microclimate relative to a clearcut microclimate. However, there is still a need to explore and develop an inferential technique to evaluate the effect of different environments on seedling performance. Keywords: Shelterwood; Combination equation; Regeneration
I. Introduction
It is estimated that half a million acres in East Texas are difficult to regenerate by traditional clearcut and plant methodologies. In many cases, the main causes for loblolly pine (Pinus taeda L. ) regeneration failure on such sites are high tem*Corresponding author.
perature and moisture stress (Messina, 1991 ). One possible method of reducing these stresses is the shelterwood method of regeneration. This method entails the gradual removal of the overstory in a series of cuts that eventually exposes the understory environment to full sunlight conditions. Regeneration within the shelterwood method can be either natural or artificial. Valigura and Messina (1994) found that loblolly
0378-1127/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSDI 0378-1127 ( 93 ) 03344-I
242
R.A. Valigura, M. G. Messina / Forest Ecology and Management 67 (1994) 241-255
shelterwoods substantially influence temperature, radiation, humidity and windspeed of the seedling-level microclimate relative to those in a clearcut. However, they could not predict the effects of these modified understory conditions on seedling performance (stomatal conductance (qs), leaf water potential (~u~), height and survival), and posed the following question: On harsh sites, what advantages/disadvantages does a partial overstory impose on seedlings growing underneath relative to seedlings grown in a clearcut? Although direct measurements of seedling physiological condition (i.e. stomatal conductance, water potential) quantify the effects of the shelterwood on seedling performance, they require the collection of large amounts of concurrent data at many different sites, and do not include any causal investigation. The approach taken in the current study involves the development of an inferential approach. In theory, an inferential approach would be able to infer seedling performance from measured site factors (e.g. radiation, soil moisture ) and is therefore limited only by our ability to relate physical microclimate variables to effects on seedling physiology. Quantification of these relationships is often a demanding task. For instance, stomatal conductance can be represented as q, ocf(PAR, ~u~,VPD, T,) where PAR is photosynthetically active radiation, VPD is vapor pressure deficit, and T~ is air temperature. Although each of these variables directly effect qs and can be measured independently, they also have interactive effects (e.g. T, and VPD are positively correlated). Microclimate functions have been developed that integrate the effects of the relevant environmental variables to each other. This study evaluates one such equation, the combination equation for potential evaporation (PE), for use as an inferential approach to site evaluation. Van Bavel (1966) stated that it has 'always' been recognized that a site or area should be characterized by the evaporative losses that may
take place when water is available at the surface (i.e. PE). However, in the last 25 years, potential evaporation has received mixed reviews in the literature, especially in regard to its usefulness in classifying sites in terms of the physiological conditions of plants growing there (Rosenberg et al., 1993; Bringfelt, 1985; Dunin et al., 1985; Harding, 1987). Popular opinion may be summarized according to Lee ( 1978 ), who stated that the concept of PE "has, if nothing more, survival value. One must question, however, whether 'value' in this context is to take a positive or negative sign". The combination equation has been shown to be applicable in conditions over most homogeneous canopies. The emphasis is placed here because these conditions do not apply to the sites used in this study. However, the objective of this study was not to quantify PE specifically, but to: evaluate PE, as estimated with the combination equation, as a relative means to infer the amount of stress imposed on loblolly pine seedlings in a shelterwood and clearcut environment, and thereby infer the effect of a partial overstory on seedling performance. When using a relative measure, precision and uniformity in procedure is more important than accuracy of estimate. One of the original definitions of PE is the evaporation that would occur from well-watered short grass (Penman, 1948). Improvements to Penman's original equation have been made by combining sensible heat and water vapor functions with the energy balance concept; this is termed the combination equation. The form of the combination equation for PE used in the present study is PLE= { ( A ( T ) , (Rn - G ) ) +
( (Cp *Pa * V D P ) / r a ) } / { A ( T )
+y}
where PLE is the potential latent heat flux (W m-2), Rn is net radiation (W m-2), G is soil heat flux (W m -2), cp is the specific heat of air (J g K -~ ), pa is the density of air (g m-S), VPD is vapor pressure deficit (Pa), r~ is the neutral
R.A. Valigura, M.G. Messina /Forest Ecology and Management 67 (1994) 241-255
aerodynamic resistance calculated for infinite fetch (s m - l ), L is the latent heat of vaporization of air (J Kg- 1), A (T) is the slope of the saturation vapor pressure (SVP) curve at air temperature (Pa K -1), and 7 is the psychometric constant (Pa K -~ ). All variables are measured at a specified reference height. The three main assumptions made when deriving this form of the combination equation are (i) canopy storage is a relatively small part of the energy balance, (ii) air-surface exchange of momentum, heat and water vapor can be related to a single level within the canopy (the analogy to one big leaf is common ), and (iii) that A (T) is the same at both air and surface temperature (Van Bavel, 1966 ). For a full discourse on the combination equation and its assumptions see Monteith and Unsworth, 1990.
2. Site description At present, there is no guaranteed method to determine, pre-harvest, if a site will present regeneration problems. For this study, the site was chosen as a result of a survey of local foresters. The soil, elevation and region of the study site were the same as sites that had previously been hard to regenerate. The study site was located in Cherokee County, Texas, at 31 °41 ' N and 95 ° 15'W and had an average elevation of 98 m above mean sea level. The average slope of the site was 3 o with an aspect of 152 °. Annual precipitation averages 1140 mm, and is distributed fairly evenly throughout the year. The soils on the site are a mixture of the Boswell and Bowie Series. Boswell fine sandy loam is a Plinthic Paleudult with 0.20 m of friable fine sandy loam over a blocky red clay to 0.40 m. Bowie loamy fine sand is similar to the Boswell series but has 0-0.35 m of friable-loose fine sandy loam over compact yellow or brownish sandy clay loam to 0.66 m. The site indexes (base age 50 years) for loblolly pine are 23.7 and 25.3 m on the Boswell and Bowie Series, respectively. Likewise, for shortleaf pine (Pinus echinata Mill.) the site indexes are 20.7 m and 23.5 m, respectively (Mowery and Oakes, 1959). The
243
stand was naturally regenerated, approximately 50 years old, and had a mean basal area of 13 m 2 ha -1 in pine sawtimber (0.20 m diameter at breast height or greater) before harvesting.
3. Materials and methods The loblolly pine shelterwood chosen for this project was located adjacent to a large clearcut. It was harvested to leave 6.9 m 2 ha-L of residual overstory (predominantly loblolly pine), and measured 305 m X 3 0 5 m (9.3 ha). It was bordered on the southern and eastern sides by clearcuts, and on the northern and western sides by undisturbed forest. Undesired trees of all species that remained after harvest were injected with a systemic herbicide (picloram + 2,4 D). All environmental measurements in the shelterwood were conducted in a centrally located measurement plot (0.15 ha). The clearcut selected for this study was a commercial clearcut over 200 ha in size, adjacent to the shelterwood study. The clearcut site was prepared by drum chopping and burning so that all above-ground vegetation was removed from the site. All environmental measurements in the clearcut were conducted in a centrally located measurement plot judged to be of the same elevation and aspect as the measurement plot in the shelterwood. Herbicide (glyphosphate) was applied to the measurement plots to eliminate herbaceous growth in order to decrease variability in ground-level conditions between overstory treatments. Sampling began on 21 May 1990 and continued to 30 April 1991. All instruments referred to in the following sections were purchased from Campbell Scientific Inc., Logan, Utah. Instrument readings were recorded by a model CR l 0 battery-powered datalogger and connected AM32 multiplexer. Recording was done on a 24-h basis, with all instrument signals read every 10 s and averaged over 30-min intervals.
3. I. Meteorological measurements The primary use for the measurements taken during this study was to quantify the input terms
R.A. Valigura, M.G. Messina / Forest Ecology and Management 6 7 (1994) 241-255
244
for the combination equation. The intention was to characterize the microclimate to which a seedling might normally be exposed at chosen points in both the shelterwood and clearcut stands, rather than to provide an average value for each stand condition. A summary of the instrumentation is presented in Table I. Radiation instruments were mounted horizontally at 1.2 m above the soil surface to avoid excessive interference of the surface radiation regime by the instrument and its supporting structures, while still giving a reasonable approximation of the radiation environment of seedlings (Holbo and Childs, 1987). Humidity and windspeed were measured at 0.3 m above the soil surface, the approximate seedling height. Saturation vapor pressure (SVP) was calculated using the humidity probe measurements in Teten's equation SVP = 0.061078 . e x p ( ( 17.269. Ta)/ (237.3 + T a ) ) where Ta is the average air temperature ( ° C ) at 0.30 m. A four-level temperature profile was measured to support the calculation of soil heat flux as well as the quantification of the extreme temperatures that occurred on each site (Valigura, 1991 ). Spatial variability was of concern
due to the small sampling area of each thermocouple. Therefore, averages of five sample points were taken at each depth and height within a cirkel of 3 m diameter. Two circles were sampled in both the shelterwood and the clearcut sites. This provided a total of eight average temperature readings per site type (two averages at each of the four levels above and below the soil surface ). Averages were taken by parallel wiring of the individual thermocouples on an electrical terminal strip before the voltage was sent to the CR10 datalogger.
3.2. Soil water Soil moisture was sampled gravimetrically at three depths, 0-0.05 m, 0.05-0.15 m and 0.150.30 m, in a herbicide-treated plot adjacent to the measurement plot. Due to the distance between the site and laboratory, soil samples were taken every 3-4 days. Soil moisture data were used to calculate soil heat flux (method discussed later). Preliminary trials showed that a 10% error in volumetric water content determination results in an average change of less than 10% in soil heat flux calculations, and this averaged to less than 4% change in estimated potential evaporation. Therefore, two soil samples for each plot per
Table 1 Equipment details System
Height
ModeP
(m) Wind speed Temperature
3-cup anemometer Copper-constantan thermocouples
Relative humidity RH temperature
Surface film PCRC-11 chip Fenwall Electronics UUTS 1J 1 thermistor Polyethylene shielded non-aspirated radiometer Multi-channel micro-processing system
Net radiation Data acquisition
0.30 0.30 -0.001 -0.05 -0.30 0.30 0.30 1.20
R.M. Young 03101 On site
Campbell Scientific Model 207 Campbell Scientific Model 207 Radiation energy balance systems !5.5 Campbell Scientific CR 10
aldentification of specific instruments here or elsewhere should not be interpreted as an endorsement relative to equivalent systems available from other sources.
R.A. Valigura, M.G. Messina / Forest Ecology and Management 6 7 (I 994) 241-255
sampling day were deemed adequate. The two soil samples allowed for both the Boswell and Bowie Series to be represented. Moisture content was determined for each soil type, and then combined for the calculation of soil heat flux.
3.3. Soil heat flux Soil heat flux (W m - 2 ) was estimated using the temperature gradient method (Kimball and Jackson, 1979; Flint and Childs, 1987) which is based on
G= Cs , A Z , (AT~At)
245
C,=Cm(pb)+Cw(Ov)Pw where Cm is the soil particle and rock fragment heat capacities (MJ Mg -1 °C -t ) Pb is soil bulk density, Cw is the soil water heat capacity (MJ Mg- ] ° C - ] ), 0v is the soil water content, and Pw is the density of water (assumed to be 1 Mg m - a). By summing heat fluxes of multiple sections of soil it is possible to estimate soil heat flux. Because of thermocouple positioning, two levels of heat flux were determined: 0-0.05 m and 0.050.30 m.
3.4. Aerodynamic resistance
where G is the soil heat flux, C, is the soil volumetric heat capacity (MJ m - 3 o C - t ). A Z i s t h e thickness of the measurement layer (m), AT is the change in temperature (°C), and At is the time interval (s). Based on studies of soils with similar composition and structure, the soil and water heat capacities were assumed to be 1.47 and 4.18 MJ Mg -t °C -1, respectively (Childs et al., 1985). These values also compared well with other published ranges of values. Cs was calculated by
Aerodynamic resistance (ra) was estimated by two different methods so that actual conditions would most likely occur somewhere within the tested range of values. The same resistance equations were used for both the clearcut and shelterwood (i.e. ignoring the overstory trees). Although initial requirements were not met (homogeneous plant canopy and adequate fetch), the first approximation ofra used the logarithmic profile for a bare soil. Environmental
1000
I
-I
Logarithmic
. . . . . Linear 800
~
600
o
400
"ES
EO0
0
.< 0
0.0
I
I
i
0.5
1.0
1.5
2.0
Average Windspeed (m s-I) Fig. 1. A comparison between the two aerodynamic resistance functions versus average wind speed values. The linear function simulates a crop canopy 1 m in height. The logarithmic function simulates bare soil conditions.
R.A. Valigura, M.G. Messina / Forest Ecology and Management 67 (I 994) 241-255
246
Table 2 Months and days chosen to represent environmental conditions in the clearcut and shelterwood Period
Day number
July and August 1990
207,208,209,210,219,220, 223,224,225,227,229,237, 238,241,242
NovemberandDecember1990 309,310,311,314,315,316, 328,332,333,338,339,341, 342,352,353 83,85,88,89,90,97,99, 100,105,106,110,112,120, 121
March and April 1990
stability correction parameters usually accounted for in this method were not incorporated into the resistance estimates due to errors expected from the inadequate canopy parameters. The logarithmic resistance equation calculated for neutral conditions takes the form (Duell, 1990 )
r. = { l n ( z - d + zo) /Zo}{ln(z-d+ zv ) /zv} (k2*u(z)}
where z is the height in profile (m), dis the displacement height (m), zo is the roughness length for momentum (m), Zv is the roughness length for vapor (m), k is yon Karman's constant, and u(z) is the wind speed at height z ( m s -1 ). The value of zo for bare soil has been experimentally determined to be close to 0.01 m (Fuchs et al., 1969; Van Bavel and Hillel, 1976 ). Using the relationship between momentum and vapor roughness based on the dimensionless parameter B - 1 (Verma and Barfield, 1979 ), zv for bare soil will be taken to be 0.002 m. The second method used to approximate ra involved a simple linear relationship between resistance and windspeed, which gave values comparable with values given in the literature (Jarvis et al., 1976; Grace, 1980; Dixon and Grace, 1984). The equation was adapted to ra = 1 0 0 0 - (u(z)/O.O0202) where all variables are as previously defined. Both methods estimated maximum resistance to be 1000 s m - l. A graphical representation of both of the ra functions is presented in Fig. 1. The large range bordered by the two functions is due to the canopy conditions simulated by each func-
too
80
60
40
20
o
_2
145 165 185 205 225 2 4 5 2 6 5 2 8 5 305 325 3 4 5 3 6 5 20
1990
40
60
80
100 120
1991 D ay N u m b e r
Fig. 2. Rainfall events and magnitude throughout the sample period. Periods 1, 2 and 3 are underlined.
R.A. Valigura, M.G. Messina / Forest Ecology and Management 67 (1994) 241-255
I
5°I
~
I
I
I
f
I
I
A
Shelterwood
....
- -
247
Clearcut
40 o
.o
30 ¢9
0
o
10 o
i o
0
I
q
]
I
50
I
I
B C o
©
40
30
o 20
0 10
? Q
1
I
I
I
I
i
I
L
I
I
i
i I I I
45 165 185 205 225 245 285 285 305 325 345 365
1990
Day N u m b e r
20
I
I
I
I
I
40
60
BO
100
120
1991
Fig. 3. Soil water contents of the (A) 0 to - 0 . 0 5 m, and (B) - 0 . 0 5 to - 0 . 3 0 m soil layers. Measured on the clearcut and the shelterwood sites over the sample period.
tion. The logarithmic profile is based on bare soil, while the linear profile is based on a canopy 1 m in height. The 'true' value of aerodynamic resistance should fall within this range of values. 4. Results and discussions Sampling began on 25 May 1990 (day number
145 ) and concluded on 30 April 1991 (day number 120). Out of the 342 days spanned by the experiment, between 158 and 209 days were successfully measured depending on the variable, thereby facilitating the quantification of differences between the sites. Due to the large amount of data collected, three 2-month periods were chosen to represent the
248
R.A. Valigura, M.G. Messina / Forest Ecology and Management 67 (1994) 241-255
Table 3 Ranges and totals of potential latent heat flux (PLE) and evaporation (PET), calculated using the logarithmic ra equation, for the clearcut and shelterwood sites over 158 days, and the average sunny days (ASD) Clearcut PLE (PET)
Period sums MJ m -e (kg m -2) Total period 1300.0 ( 527.2 ) Period 1 180.0 (74.8) Period 2 85.5 (34.8) Period 3 140.0 (56.9)
840.0 20.0 40.4 92.4
Maximum daily sum MJ m -2 d ~ (kg m -2 d - ~) Total period 16.3 (6.7) Period 1 15.4 (6.4) Period 2 7.9 (3.2) Period 3 12.2 (4.9) Minimum daily sum MJ m -e d t (kg m -2 d-~ ) Total period - 1.1 ( - 0.3 ) Period 1 0.4 (4.3) Period 2 2.5 (1.0) Period 3 3.9 (1.6) ASDdailysumMJm-2d-~ (kgm-Zd Period 1 12.1 Period 2 5.7 Period 3 9.3
Shelterwood PLE (PET)
l) (5.0) (2.3) (3.6)
11.5 9.3 4.1 9.9
(350.7) (48.5) (16.4) (37.8)
(4.8) (3.8) (1.1) (4.1)
-0.6 (-0.3) 6.6 (2.7) 1.6 (0.6) 2.2 (0.9)
Differencea
460.0 60.0 45.4 47.7
(176.5) (26.4) (18.5) (19.0)
5.8 6.1 3.8 2.3
(1.9) (2.6) (2.1) (0.8)
0.5 3.6 0.9 1.7
(0.0) (1.6) (0.4) (0.7)
7.9 (3.3) 2.7 (1.1) 6.2 (2.5)
4.2 (1.7) 3.0 (1.2) 3.1 (1.1)
Maximum 30-min average W m -2 (g m -2 s- l ) Period 1 603.8 (0.25) Period 2 382.2 (0.16) Period 3 607.1 (0.25)
573.0 (0.24) 225.4 (0.09) 452.5 (0.19)
30.8 (0.01) 156.8 (0.06) 317.0 (0.06)
Minimum 30-min average W m -2 (g m -2 s -I ) Period 1 - 3 4 . 9 ( -0.01 ) Period 2 - 121.2 ( - 0 . 0 5 ) Period 3 - 100.7 ( - 0 . 0 5 )
-93.3 (-0.04) -130.5 ( - 0 . 0 5 ) -128.4 ( - 0 . 0 5 )
58.4 (0.30) 9.3 (0.00) 8.7 (0.00)
295.8 (0.12) 126.6 (0.05) 306.1 (0.12)
97.0 (0.04) 114.5 (0.03) 67.1 (0.02)
- 1 . 7 (0.00) - 2 . 6 (0.00) - 3 . 5 (0.00)
1.0 (0.00) - 3 . 1 (0.00) - 11.9 ( - 0 . 0 1 )
ASD maximum 30-min average W m - 2 ( g m - ~ Period 1 392.8 (0.16) Period 2 241.1 (0.10) Period 3 373.2 (0.15) ASD minimum 30-rain average W m -2 Period 1 -0.7 Period 2 -5.7 Period 3 - 15.4
s - ~)
(g m -2 s- i ) (-0.00) (-0.00) (-0.01)
aClearcut minus shelterwood. r a n g e o f c o n d i t i o n s e x p e r i e n c e d o n e a c h site. O u t o f e a c h 2 - m o n t h p e r i o d , 15 d a y s w e r e c h o s e n t o represent the 'sunniest' days measured over that p e r i o d , as j u d g e d b y t h e t o t a l a m o u n t o f P A R a n d
r a d i a t i o n r e c e i v e d t h a t day. T h e s e d a y s w e r e t h e n a v e r a g e d a n d a r e p r e s e n t e d as a n ' a v e r a g e s u n n y day' (ASD) from each period. The months and d a y s c h o s e n a r e l i s t e d i n T a b l e 2. S u n n y d a y s
R.A. Valigura, M.G. Messina / Forest Ecology and Management 6 7 (I 994) 241-255 16
I
l
I
[
~
I
t
~
I
I
I
I
I
I
I
249
W~
...... Shelterwood 14
~
10
t
Clearcut
"
i ~4
20
1990
40
60
80 tO0
120
1991 Day Number
Fig. 4. Daily totals of potential evaporation (PE) calculated for the clearcut and shelterwood sites over the sampling period. The two smooth lines are third order regressions intended to show annual patterns on the clearcut (upper line) and shelterwood (bottom line) sites. Missing values are due to instrumentation problems.
were chosen because they are of the most interest to this study of harsh sites. Furthermore, overcast and rainy days are inherently variable and difficult to interpret as averages. Period 1 spans 35 days of the middle summer months, with 26 days successfully instrumented, and had a relatively constant weather pattern, with a total of 4.5 mm of rainfall. Period 2 spans 45 days in late fall, with 30 days successfully instrumented, and was characterized by passing weather fronts and decreasing radiation loads, with a total of 131 mm of rainfall. Period 3 covers 39 days in early spring, with 30 days successfully instrumented, and had consistent rainfall indicative of one of the wettest springs on record, with a total of 218 mm of rainfall.
4.1. Rainfall and soil moisture An unusually large amount of rainfall, 794 mm, was measured for the 209 days successfully sampled (Fig. 2). Due to its effect on soil moisture
contents, the large rainfall possibly delayed or negated any potential harsh site effects. There are few readily interpretable trends in volumetric water content in the upper soil layer between the clearcut and shelterwood sites for the overall period. Water content in the upper soil layer (0-0.05 m) ranged from 2.5 to 40% over the full sampling period (Fig. 3 (A)). There were three intervals (Days 186-198, 217-252 and 7495 ) when the shelterwood site had considerably higher moisture contents in the upper layer of soil than did the clearcut site. Volumetric soil water contents in the lower layer of soil showed no overall trends between the clearcut and shelterwood sites. Water contents in the lower soil layer (0.05-0.30 m) ranged from 12 to 33% over the full sampling period. Smaller amplitude fluctuations in the lower layers of soil (Figs. 3 (A), ( B ) ) can be attributed to the lower infiltration rates associated with the clay layer in the soils on these sites (Boswell and Bowie series). Several times throughout the period, perched water tables were observed at the interface of the upper sandy horizons with the
250
R.A. Valigura, M.G. Messina / Forest Ecology and Management 67 (1994) 241-255
600
' ' ' ; ' ' ' --- Sheltcrwood~
;
'
'
'
I
'
'
'
A
B
500 400 300 200 100 0 -100 300
i
i
i
I
i
r_l
,
'
,
i
'
'
'
I
i
i
i
I
r
~
J
'
,
,
r
'
'
i
'
C
D
200 100 0 -100 -200 -300 i
-400
i
I
i
i
J
I
i
i
i
I
i
L
,
500
45
'
'
'
I
'
'
'
I
'
,
'
I
'
'
7 1
F
40 400
35 30
300
25 200
20 15
100 10 5 0
0 J
,
i
I
i
600
i
~
I
1200
i
,
i_l
1800
i
i
,
i
2400
0
,
,
I
800
,
,
,
1_J
1200
,
,
I
1800
,
,
,
2400
Hour Fig. 5. Average sunny day (ASD) patterns exhibited by (A) net radiation (W m 2 ) , ( B ) soil heat flux (W m-2), (C) air temperature at 0.30 m (°C), (D) windspeed at 0.30 m ( m s -1 ), (E) vapor pressure deficit at 0.30 m (KPa), and (F) potential latent heat flux ( W m -2) measured on (estimated for) the clearcut and shelterwood sites during Period 1.
lower clay horizons. Much of the variation in Figs. 3(A), (B) can be attributed to the highly variable nature ofgravimetric moisture sampling.
4.2. Potential evaporation (PE) Although all PE calculations were performed using both the linear and logarithmic aerodynamic equations, there were no significant differences in results. Therefore, all values included in this report were calculated using the logarithmic equation.
Daily PE totals (integrated from the 30 min meteorological averages) followed the same basic trends, but were greater in the clearcut than in the shelterwood (Table 3; Fig. 4). Net radiation and soil heat flux accounted for between 92 and 98% of the estimated nighttime PE, and between 65 and 80% of estimated daytime PE. VPD was shown to consistently increase PE estimates by 2-8%, both day and night, thus lending credibility to the hypothesis that PE is affected by, but cannot be directly predicted by, VPD (Rosenberg et al., 1983). Through its effects on Ra,
R.A. Valigura, M.G. Messina / Forest Ecology and Management 67 (I 994) 241-255 600
'
'
'
i
'
'
'
i
'
'
'
I
'
'
Shelterwood
---
500
A '
Clearcut
251
1.0 B 0.8
400 0.6
300 200
0.20"4~_j
100
,'
,,,,--
-,,.,.
',
0 _
-100 300
,
,
i
I
,
,
,
I
J
,
L
I
,
i
,
'
,
I
'
,
'
i
,
,
'
I
'
'
,
O. 0
,
~ ,
'
'
i
,
,
~
I
,
"?
'
l
,
,
t
,
,
,
D
C 200 100 0 -100 -200 -300 ,
-400 45
,
,
I
,
,
,
I
I
[
,
i
'
I
'
'
'
T
'
L
'
'
i
'
500
.
.
.
I
'
'
'
I
~
'
'
'
I
'
'
'
I
E
40
'
'
' F
400
35 30
300
25
200
20 15
lOO
.
10 5 0 600
1200
1800
2400
0
000
1200
1800
2400
Hour Fig. 6. Average sunny day (ASD) patterns exhibited by (A) net radiation ( W m 2), (B) soil heat flux ( W m - Z ) , ( C ) air temperature at 0.30 m ( ° C ), ( D ) windspeed at 0.30 m (m s - ' ), ( E ) vapor pressure deficit at 0.30 m ( K P a ) , and (F) potential latent heat flux ( W m -2) measured on (estimated for) the clearcut and shelterwood sites during Period 2.
wind was shown to increase FE estimates consistently, accounting for 10-15, 20-40 and 10-20% of estimated PE for ASDs in Periods 1-3, respectively. Diurnal patterns of calculated PE for each of the three ASDs are shown in Figs. 5-7. Trends were very similar across all three ASD periods. During the early morning interval (between 06:00 and 10:00 h) PE estimations were highly variable, becoming close, to and even less than, zero (condensation). This is a direct result of calculated soil heat flux being greater than incoming radiation, a condition which is improb-
able, and one that is likely associated with errors in the soil heat flux calculations. During this interval, calculated PE reached higher levels in the shelterwood than in the clearcut (Figs. 5-7), a condition which is hard to evaluate due to the probable error in calculating soil heat flux. However, PE being greater in the shelterwood during the morning is not unreasonable: more incoming radiation may be used for soil heating in the clearcut relative to that in the shelterwood, thereby decreasing PE in the clearcut. The fact that actual vapor pressure (AVP) is also greater in the shelterwood during this interval (Figs. 5-
252
R.A. Valigura, M.G. Messina / Forest Ecology and Management 6 7 (1994) 241-255
'
800
'
' l
'
'
'
I
'
'
'
1
'
1.0
'
.... S h e l t e r w o o d ~
500
A
i
'
'
'
i
'
'
'
i
, ~
r
]E]
0,8 / ) ~ , ' ' / I ~
400
0.6
300
200
0.4
100 0.2
O --IO0
, l i l , J + l , i , l , i ,
300
'
'
'
+
'
'
'
I
'
'
O.O
'
I
'
'
6
' C
5
200
100
4i
0 -100 -200 - 3 0 0
- 4 0 0
45 40 35 30 25 20 15 10 5 0
'
'
'
'
I
'
'
'
'
'
'
[
'
'
'
'
'
'
I
'
'
'
'
'
500
'
I
'
'
'
I
'
'
'
I
'
'
' P
400
300
200
100
0 _
0
600
1200
1800
2400
0
i
r
+ 600
I
i
+
i 1200
L
,
r
,
f
i
1800
i
i
2400
Hour Fig. 7. Average sunny day (ASD) patterns exhibited by (A) net radiation (W m-2), (B) soil heat flux (W m-2), (C) air temperature at 0.30 m ( ° C), (D) windspeed at 0.30 m (m s-~ ), (E) vapor pressure deficit at 0.30 m (KPa), and (F) potential latent heat flux (W m -2) measured on (estimated for) the clearcut and shelterwood sites during Period 3.
7 ) may also strengthen the validity of the calculated PE trends. Furthermore, Dalton (1991) measured seedling stomatal conductance and porometer transpiration (PT) rates (see below) on these sites, including Period 1, and found that they were higher during these morning hours from seedlings on the shelterwood site than from those on the clearcut. For all three periods, calculated PE levels in the clearcut became higher than those in the shelterwood at 11:00 h and remained higher throughout the rest of the day and night (Figs. 5-7).
4.3. Seedling performance Final seedling survival and heights were measured on both sites (Dalton, 1991). One year after planting, survival was highest on the clearcut site (50%) as compared to the shelterwood site (42%), although this difference was not statistically significant. There were also no significant differences in seedling heights or understory leaf area, indicating that the overstory did not significantly reduce seedling growth. These results may be related to the large amount of rainfall during 1990/1991, which may have off-
R.A. Valigura, M.G. Messina / Forest Ecology and Management 67 (1994) 241-255
4.4. Evaluation of potential evaporation
0.0 u EL
-0.2
0.0 Q
----
Clearcut
....
Shelterwood
0.2
-0.4
-)
-0.4
0.6
0.6
-0.8
-0.8
r. I
b ~o
0.20
0.20
0.15
0.15
0.10
0.10
0.05
0,05
E o
>
le~
E
c
18
18
16
16
14
14 12
10
Q L_
253
10
8
8 6
4 6
8
10
12
14
16
18
20
Time of Day (ho~O Fig. 8. Seasonal average (A) water potential (MPa), (B) stomatal conductance (cm s -1), and (C) porometer transpiration (rag m -2 s -~) as affected by regeneration technique (adapted from Dalton, 1991 ).
set any survival advantages provided by the shelterwood. Dalton (1991 ) also measured ~ul, qs and PT rates throughout the growing season of 1990 with a whole seedling porometer (four times during each measurement day). The seasonal average diurnal trend in each variable is shown in Fig. 8. PT and q~ were lower in the shelterwood, while ~1 was higher. Coupled with the apparent lack of overstory influence on growth rates, these data suggest that the shelterwood seedlings maintained more favorable physiological conditions than the clearcut seedlings. Data from five contiguous days (i.e. 20 data points) that occurred during measurement Period I were used in the evaluation of PE.
Higher PE estimates on the clearcut site suggest that there should be higher survival and growth in the shelterwood due to the decreased stress levels. This was not the case. Out of the five primary comparative data categories (survival, growth, ~q, qs and PT), only two showed significant trends between sites, indicating an inability of PE to predict seedling performance in any given year. However, a few general inferences can be drawn from the PE results. Although a moist soil dries at the potential rate for only a short time, higher evaporative demands in the clearcut (Table 3) should have caused consistently lower water contents in the clearcut relative to those in the shelterwood. This was not apparent (Fig. 3 ). Given the similarities in understory leaf area and qs, it may be concluded that the difference in evaporation rates between the two sites was made equal by the water usage of the shelterwood canopy. During this study there were three intervals, mentioned above, where soil water content in the shelterwood was greater than in the clearcut. It is possible to speculate that water usage by the overstory, at the given level of spacing, was less influential on soil moisture trends in the shelterwood than was the reduction in evaporative demand between the clearcut and shelterwood, a potential advantage supplied by the partial overstory. Simple linear regression and general linear model ( G L M ) procedures were used to investigate any specific relationship between PE and measured ~1, qs and PT. All attempts to predict seedling variables by regression showed little to no statistical significance (i.e, no predictive power). A GLM was used to determine to what extent the variance in stomatal conductance, water potential and porometer transpiration (the dependant variables) could be explained by the variability in day, time (fixed independent variables) and PE (continuous,independent variable). The results showed that a significant amount of the variability (*P<0.05 to ***P< 0.001 ) in the three seedling variables was explained on both sites. However, the amount of
254
R.A. Valigura, M.G. Messina / Forest Ecology and Management 6 7 (1994) 241-255
variability accounted for by PE within the GLM models was significant for the clearcut site only, and only for ~ul and PT. That is, day and time of day explained a major percentage of the variation in seedling water potential, stomatal conductance and transpiration. This result, in itself, is expected. The point of interest in the GLM results is that PE was significantly correlated with seedling physiological conditions only in the clearcut. This is likely a result of the seedlings measured by Dalton ( 1991 ) being outside of the meteorological measurement plot (i.e. within 50 m ). Microclimate can be extremely variable in a shelterwood, and it is possible for the measurement plot to have been in shade while the seedlings measured were in full sunlight, thereby masking the effects of PE. Therefore, PE may have had the same effect on seedlings in the shelterwood as it had in the clearcut, but this was not discernable because measurements were not taken within the same plot as were the PE measurements.
5. Conclusion The near-surface environment of an East Texas clearcut and shelterwood site were instrumented to evaluate PE, as estimated with the combination equation, as a relative means to infer loblolly pine seedling physiological response to a given microclimate. It was shown that although PE statistically explained some of the variance measured in seedling physiological variables, it was not possible to use PE estimates to infer seedling response to microclimate, on these sites, during the relatively wet period of 1990/1991.
The current study qualitatively confirmed the ability of a shelterwood to favorably influence the seedling microclimate relative to a clearcut microclimate. However, there is still a need to explore and develop an inferential technique to evaluate the effect of different environments on seedling performance.
Acknowledgments Land was made available for and the research funded by Temple-Inland Corporation, Diboll, TX, USA. We thank Jim Heilman, Craig McKinley and Allan Tiarks for their guidance and advice throughout the research process.
References Bringfelt, B., 1985. A forest evapotranspiration model using synoptic scale weather data. In: B.A. Hutchinson and B.B. Hicks (Editors), The Forest-Atmosphere Interaction. Proc. Forest Environmental Measurements Conf., Oak Ridge, TN. 23-28 October 1983, pp. 25-37. Childs, S.W., Holbo, H.R. and Miller, E.L., 1985. Shadecard and shelterwood modification of the soil temperature environment. Soil Sci. Soc. Am. J., 49:1018-1023. Dalton, C., 199 I. Water relations ofloblolly pine (Pinus taeda L. ) seedlings planted under a shelterwood and in a clearcut. Masters Thesis, Texas A&M University. Dixon, M. and Grace, J., 1984. Effect of wind on the transpiration of young trees. Ann. Bot., 53:811-819. Duell, L.F.W., 1990. Estimates of evapotranspiration in alkaline scrub and meadow communities of Owens Valley, California, using the Bowen-ratio, eddy correlation, and Penman-combination methods. US Geol. Survey WaterSupply Paper: 2370-E. Dunin, F.X., McIlroy, I.C. and O'Loughlin, E.M., 1985. A lysimeter characterization of evaporation by eucalypt forest and its representativeness for the local environment. In: B.A. Hutchinson and B.B. Hicks (Editors), The Forest-Atmosphere Interaction. Proc. Forest Environmental Measurements Conf., Oak Ridge, TN, 23-28 October 1983, pp. 25-37. Flint, A.L. and Childs, S.W., 1987. Field procedure for estimating soil thermal environments. Soil Sci. Soc. Am. J., 51: 1326-1331. Fuchs, M., Tanner, C.B., Thurtell, G.W. and Black, T.A., 1969. Evaporation from drying surfaces by the combination method. Agron. J., 61: 22-26. Grace, J., 1980. Some effects of wind on plants. In: J. Grace, E.D. Ford and P.G. Jarvis (Editors), Plants and their Environment. Blackwell, Oxford, pp. 175-204. Harding, L., 1987. Relationships of environmental variables with first-year survival and growth ofloblolly pine in East Texas. Masters Thesis, Texas A&M University. Holbo, H.R. and Childs, S.W., 1987. Summertime radiation balance of clearcut and shelterwood slopes in southwest Oregon. For. Sci., 33:504-516. Jarvis, P.G., James, G.B. and Landsberg, J.J., 1976. Coniferous forest. In: J.L. Monteith (Editor), Vegetation and tile Atmosphere, Vol. 2. Academic Press, New York, pp. 171240.
R.A. Valigura, M. G. Messina / Forest Ecology and Management 67 (1994) 241-255 Kimball, B.A. and Jackson, R.D., 1979. Soil heat flux. In: B.J. Barfield and J.F. Gerber (Editors), Modification of the Aerial Environment of Plants. ASAE Monograph, pp. 211229. Lee, R., 1978. Forest Microclimatology. Columbia University Press, New York, 276 pp. Messina, M.G., 1991. Herbicide, fertilizer, and shade influence loblolly pine (Pinus taeda L.) growth and survival on harsh Texas sites. Presented at the Sixth Biennial Southern Silvicultural Research Conf., Memphis, TN, 30 October- 1 November 1990. Monteith, J.L. and Unsworth, M.H., 1990. Principles of Environmental Physics. 2nd edn. Edward Arnold, London. 291 pp. MoweR', I.C. and Oakes, H., 1959. Soil survey of Cherokee County, Texas. US Soil Conservation Service and Texas Agricultural Experiment Station. US Government Printing Office, Washington, DC. Penman, H.L., 1948. Natural evaporation from open water, bare soil and grass. Proc. R. Soc. London, Set. A, 193: 120-145.
255
Rosenberg, N.J., Blad, B.L. and Verma, S.B., 1983. Microclimate: The Biological Environment. John Wiley, New York, 495 pp. Valigura, R.A., 1991. Quantification of seedling environment in an East Texas loblolly pine (Pinus taeda L. ) shelterwood and clearcut. Ph.D. Dissertation, Texas A&M University. Valigura, R.A. and Messina, M.G., 1994. Modification of Texas clearcut environments with loblolly pine shelterwoods. J. Environ. Manage., in press. Van Bavel, C.H.M., 1966. Potential evaporation: the combination concept and its experimental verification. Water Resour. Res., 2: 455-467. Van Bavel, C.H.M. and Hillel, D.I., 1976. Calculating potential and actual evaporation from a bare soil surface by simulation of concurrent flow of water and heat. Agric. Meteorol., 17: 453-476. Verma, S,B. and Barfield, B.J., 1979. Aerial and crop resistances affecting energy transport. In: B.J. Barfield and J.F.Gerber (Editors) Modification of the Aerial Environment of Plants. ASAE Monograph, pp. 230-248.