l 

United States 

Department of 

Agriculture 

Forest Service 

Intermountain 

Research Station 

Research Paper 

INT-377 

Overstory Removal 

and Residue 

Treatments Affect 

Soil Surface, Air, 

and Soil 

Temperature: 

Implications for 

Seedling Survival 

Roger D. Hungerford 

Ronald E. Babbitt 

This file was created by scanning the printed publication.

Errors identified by the software have been corrected;

however, some errors may remain.
l 

THE AUTHORS 

ROGER D. HUNGERFORD is a research forester 

at the Intermountain Research Station's Forestry Sciences 

Laboratory in Missoula, MT. His current research includes 

forest micrometeorology and forest ecology. 

RONALD E. BABBITT is an electronics technician at the 

Intermountain Station's Forestry Sciences Laboratory in 

Bozeman, MT. His duties included development of instrumentation for environmental monitoring. He is currently 

working on systems to monitor logging operations and to 

measure ground water. 

RESEARCH SUMMARY 

Timber harvesting and residue reduction practices that 

alter shade, surface thermal properties, and moisture influence energy balance and heat transfer on the site, significantly influencing temperatures. Because the problems of 

mortality to seedlings due to high temperature and insufficient moisture are potentially widespread and expensive, it 

is crucial to be able to identify problem sites during the 

planning process. 

Following alternative overstory removal and residue 

reduction treatments at three locations, ground surface 

temperature maximums increased and minimums declined 

significantly as the amount of overstory removal increased. 

Potentially lethal high temperatures (>133 °F) occurred frequently on all sites, while potentially lethal minimum temperatures (<25 °F) occurred on two sites. Potentially lethal 

temperatures occurred more frequently under treatments 

involving greater overstory removal. Similar relationships 

between treatment intensity and maximum and minimum 

temperatures were observed among alternative surface 

conditions resulting from residue treatments. In general, 

maximum and minimum temperatures of burned and litter 

surfaces in clearcuts were not different, but mineral soil 

surfaces were significantly different from the litter surfaces 

under a shelterwood cutting. Temperatures of a chip surface included maximums significantly cooler than that of 

burned and litter surfaces. Observed mortality of planted 

seedlings on study sites was consistent with the pattern of 

potentially lethal temperatures measured on the units. 

Clearcutting significantly increased soil temperatures at 

depths to 16 inches compared to the uncut treatment. 

Humus temperatures in a shelterwood were intermediate 

between the clearcut and uncut treatments. Surface condition also significantly altered temperatures at 2 to 

16 inches deep. Average temperatures under burned, litter, 

and chip surfaces were all different; those under burned 

surfaces were warmest and those under chips were 

coolest. In general, the pattern of treatment differences in 

the soil is the same as at the surface. Overstory treatment 

did not influence air temperature. 

If high temperatures or frosts are expected on a site, 

varying the amount of overstory removal or providing 

shade by leaving more residues on the surface may reduce the potential for seedling mortality. Scarification of 

the surface to provide for a mineral soil seedbed will also 

reduce the potential for high temperatures and, thus, seedling mortality. It is important to be aware of the thermal 

properties of the surface materials and understand the 

consequences of leaving plant litter, branches, charcoal, 

loosened soil, and other materials on the surface. Where 

the potential for frost pockets exists, cutting units should 

be laid out so cold drainage is not blocked. Overstory 

canopies can be effective in reducing the occurrence of 

midsummer frosts. 

The use of trade or firm names in this publication is for 

reader information and does not imply endorsement by the 

U.S. Department of Agriculture of any product or service. 

March 1987 

Intermountain Research Station 

324 25th Street 

Ogden, UT 84401 
Overstory Removal and 

Residue Treatments Affect 

Soil Surface, Air, and Soil 

Temperature: Implications 

for Seedling Survival 

INTRODUCTION 

Roger D. Hungerford 

Ronald E. Babbitt 

Logging results in microclimate changes that influence 

seedling regeneration and subsequent forest development. 

The most significant and immediate microclimate changes 

are temperature, light, and moisture. Of the many plant 

processes that are influenced by temperature, light, and 

moisture (Kramer and Kozlowski 1979; Gates 1980), seedling establishment is one of the most crucial in the early 

life of a forest stand. 

Temperatures from 120 to 140 °F (Hare 1961) may be 

lethal for first-year conifer seedlings, depending on species 

(Baker 1929; Silen 1960) and exposure time (Baker 1929; 

Levitt 1980). Condition of the protoplasm, internal water 

relations, and tissue mass also influence mortality (Levitt 

1980). Baker (1929) stated that the limit of safety was 

122 °F for 2 hours. Maximum surface temperatures on 

south-facing and west-facing slopes in western Montana 

frequently exceed 138 °F and often 149 °F (Shearer 1967), 

with maximums reaching 174 °F (Shearer 1981). Lotan 

(1964) found soil surface temperatures often reached 149 

to 167 °F on gentle slopes at high-elevation sites in southwestern Montana and eastern Idaho. Surface temperatures 

on slash-burned clearcuts in Oregon exceed lethal levels on 

69 to 97 percent of the south slopes and nearly 50 percent 

of the north slopes (Silen 1960; Hallin 1968). The extent 

(spatially and temporally) of temperatures exceeding lethal 

levels on sites in the Northern Rockies is not known other 

than for point-in-time measurements. 

High temperatures on exposed soil surfaces cause significant mortality of conifer seedlings (Baker 1929; Silen 

1960; Cochran 1969). Shearer (1967) reported that high 

temperature was the major cause of mortality of first-year 

western larch (Larix occidentalis Nutt.) seedlings on 

south-facing and west-facing slopes in Montana. In some 

cases, however, seedlings of lodgepole pine (Pinus contorta 

Dougl.) have survived temperatures of 149 to 167 °F 

(Lotan 1964). High heat loads created by opening the 

forest also cause drying of the surface through increased 

evaporation. As the surface dries, maximum temperatures 

increase (van Wijk and DeVries 1966; Cochran 1969). 

Low temperatures also cause significant mortality of 

conifer seedlings during the growing season (Cochran and 

Bernsten 1973; Lotan and Perry 1983). Topography, moisture conditions, and surface thermal properties contribute 

to the occurrence of frosts (Cochran 1969; Fowler 1974). 

Lethal low temperatures vary by species from 30 to 14 °F 

(Cochran and Bernsten 1973; Levitt 1980). 

Temperature variation at the surface and in the air layer 

surrounding seedlings is a function of the heat flux density 

at the surface and the thermal properties of the seedbed 

material. Expected temperature variations are described 

by the amount of heat energy incident upon the surface 

and how the heat energy is distributed at the surface. The 

components of heat amount and distribution are described 

by the equation (Rose 1966; Cochran 1969): 

G = Rn- H- LE 

where 

G = the heat flux density at the surface 

Rn = the net radiation flux density (measure of energy 

available at the surface) 

H = the sensible heat flux density into the atmosphere, including heat dissipated by air 

currents 

LE = heat used in latent heat of vaporization and used 

in evaporation and transpiration 

How G influences temperature variation at the surface 

and in the soil depends on the volumetric heat capacity (C) 

and thermal conduct~ (K) of the surface and lower soil 

horizons. A factor (V KG) called the thermal contact coefficient (Cochran 1969) integrates these two factors. Temperature variations are inversely proportional to {KG. The 

nature of the material, moisture content, and amount of 

air space (thus, compaction, texture, and soil type) influence these thermal properties. Temperatures are more 

extreme on organic surfaces than on mineral soil (Cochran 

1969; Fowler 1974). The thermal properties of the seedbed 

are critically important (Cochran 1969; Fowler 1974). The 

equation predicts that those practices providing shade 

(decreasing Rn), allowing for increasing air circulation (increasing H), or decreasing surface evaporation will lower 

the temperature variation by reducing the amount of heat 

at the surface or by dissipating the heat before it raises 

the temperature to lethal levels. 

Some degree of shading is helpful in reducing seedling 

mortality due to high and low temperatures (Shearer 1967; 

Ryker and Potter 1970; Strothman 1972; Cochran 1975). 
Logging residues and vegetation provide shade, which 

reduces temperature extremes, thus increasing survival 

(Edgren and Stein 1974). 

This paper reports postharvest temperatures at the soil 

surface (surface-air interfaee), in the air (to 4.5 ft), and in 

the soil (to 16 inches) following several levels of overstory 

removal and for four surface conditions resulting from 

several residue treatments. Temperatures are compared 

with adjacent uncut stands and with expected responses 

based on heat fluxes and thermal properties of the surfaces. Consistency of treatment response is examined by 

looking at results from three stands in different habitat 

types. Measured temperatures represent the period immediately following treatment to 8 years later. The potential 

for seedling mortality, based on high and low temperature 

data, is compared with actual seedling survival data. 

STUDY AREAS AND METHODS 

We had two study sites in the Flathead Range of Montana and one in the Gras Ventre Mountains of Wyoming. 

These sites represent a range of climatic conditions that 

occur in the Northern Rocky Mountains. May through 

September precipitation ranges from 25 to 45 percent of 

the annual amount and varies considerably with topography. Average temperatures also vary considerably 

throughout the region. Ordination of these three sites 

along moisture and temperature gradients (fig. 1) is based 

on growing season May through September precipitation 

totals and July temperatures. Climax forest series (Pfister 

and others 1977) are Douglas-fir (Pseudotsuga menziesii 

var. glauca [Beissn.] Franco) and subalpine fir (Abies 

lasiocarpa [Hook.] Nutt.). 

II. 75 0 

IL 

::E 

w 

1- 80 

c 

L 

25 

DRY 

35 

u 

40 

WET DRY 

PRECIPITATION, IN 

Figure 1-0rdination of study areas-Coram (C), 

Lubrecht (L), and Union Pass (U)-along a 

moisture (annual) and temperature gradient 

based on July average maximum temperature (A) 

and July average minimum temperature (B). 

Treatments 

u 

c 

Treatments on the study sites included: (1) a silvicultural 

prescription requiring overstory removal that varied from 

none through thinnings, selection cuts, and shelterwoods 

to clearcutting; and (2) a postharvest residue treatment 

varying from none to treatments of complete removal, 

piling, chipping, and burning. Surface conditions of litter, 

2 

burned, mineral soil, and chips resulting from residue 

treatments were evaluated (fig. 2). This study is superimposed on a larger study. A description of specific 

treatments used for each study area follows. 

Lubrecht Experimental Forest-This site is about 

35 miles northeast of Missoula, MT, at an elevation of 

4,000 ft. The terrain is gently rolling (slopes 0 to 5 percent), with west to northwest aspects. Climatic data, taken 

at the forest headquarters since 1956 (Montana Forest and 

Conservation Experiment Station 1983), indicate an average annual precipitation of 18 inches. Maximum July air 

temperatures average 81.6 °F, and minimum July air 

temperatures average 42.5 °F. 

Tree cover prior to treatment was a mixed size and age 

class Douglas-fir forest. Western larch and ponderosa pine 

(Pinus ponderosa Laws.) were significant components of 

the stand. This area has a long history of selective logging 

where merchantable trees were periodically removed from 

the stands. Habitat type (Pfister and others 1977) is Pseudotsuga menziesii/Vaccinium caespitosum (PSME/V ACA). 

Logging was done in 1977 with ground skidding equipment. The following overstory treatments were compared 

to the uncut stand: 

1. Clearcut. All merchantable trees were removed on a 

15-acre clearcut and the rest of the material was felled, 

leaving 29 tons per acre of residue. 

2. Understory removal. Treatment removed about 

40 percent of the merchantable volume by cutting the 

smaller diameter material and leaving the better sawtimber and large pole stems as an overstory. About 18 

tons per acre of residue was left on the surface. The 

residual overstory transmitted about 50 percent of full 

sunlight. 

3. Shelterwood. About half of the merchantable volume 

was removed, leaving an overstory of small sawtimber and 

pole stems. Dense clumps of saplings and poles were selectively thinned. The residual overstory transmitted about 

60 percent of full sunlight. This was a common treatment 

in this type of stand in 1977. 

4. Uncut. An adjacent uncut area was used as the 

control. 

Residue reduction treatments on the study units included the following: 

1. To simulate close utilization, all residues down to 1 

inch in diameter were removed. The small material was 

bunched by hand prior to skidding. The surface was quite 

uniform looking after treatment (fig. 2B). Except for skid 

trails, the understory vegetation was essentially undisturbed. The surface condition used for temperature measurement was a litter surface, made up of small twigs, leaf 

and needle litter, and duff. About 10 percent of the surface was exposed mineral soil. 

2. A pile-and-burn treatment in the shelterwood overstory treatment left most of the surface (except under the 

piles) in the litter and mineral soil surface conditions with 

an appearance like the close utilization treatment. The 

litter and mineral soil surface conditions were used for 

temperature measurement. 

3. Broadcast burning in the clearcut resulted in a 64 

percent reduction of slash (Steele 1980) creating a completely blackened area with some mineral soil exposed 

., 

r, 
(A) 

(C) 

(fig. 2A). The surface condition on this treatment used for 

temperature measurements consisted of blackened litter, 

ash, and small charcoal pieces. Areas with large, partially 

burned residues were avoided. 

Surface conditions evaluated on each of the overstory 

and residue reduction treatments are shown in the following tabulation: 

Overstory Surface 

treatments Residue reduction conditions 

evaluated treatments evaluated 

1. Clearcut Broadcast burn, Burned, 

close utilization litter 

2. Understory Close utilization Litter 

removal 

3. Shelterwood Pile and burn Litter 

mineral 

4. Uncut Litter 

Small containerized seedlings of ponderosa pine, 

Douglas-fir, and western larch were planted each year 

following treatment (1979 to 1982). In the spring of each 

year, 23 seedlings were planted in each residue subtreatment within each overstory treatment. A 2- by 2-ft area 

was scarified around each seedling at the time of planting. 

3 

(B) 

Figure 2-Postharvest residue treatments for providing different surface conditions used for temperature measurement: (A) broadcast 

burning provided the burned surface; (B) close utilization provided 

the litter surface; (C) residues chipped and spread back on the site 

provided the chip surface of Union Pass. 

Coram Experimental Forest-This site is 5 miles south 

of West Glacier, MT, at an elevation of 5,000 ft. All treatments are on east-facing aspects with slopes from 40 to 60 

percent. The climatic information for the area (Hungerford 

and Schlieter 1984) indicates an average annual precipitation of 33 inches at the treatment units. Maximum July air 

temperatures average 74 °F, and minimum July air temperatures average 47 °F. 

Tree cover prior to treatment was a 320-year-old 

Douglas-fir/western larch forest. The predominant species 

were Douglas-fir and western larch. Subalpine fir and 

Engelmann spruce (Picea engelmannii Parry) were common, with some western hemlock (Tsuga heterophylla 

[Raf.] Sarg.), western white pine (Pinus monticola Dougl.), 

and paper birch (Betula papyrifera Marsh.) present. Habitat type (Pfister and others 1977) is Abies lasiocarpa/Clintonia uniflora (ABLA/CL UN), Clintonia uniflora phase or 

Xerophyllum tenax phase. At the upper elevations the 

Menziesia ferruginea phase was evident. 

Overstory treatments on this study site were: 

1. Clearcut. Clearcuts ranged from 2 to 17 acres. Logging was completed during 1974 using a running skyline 

yarder. Prebunched residues were also removed with the 

skyline yarder. Mechanical scarification was essentially 

limited to the skyline corridors. 
2. Uncut. An adjacent uncut area was used as the 

control. 

Residue reduction treatments on the clearcut included 

the following: 

1. Broadcast burning resulted in a 20 percent duff reduction (Artley and others 1978). High water content in 

the duff layer was responsible for not reaching prescription levels of 50 percent mineral soil exposed. The surface 

condition used for temperature measurement consisted of 

blackened litter, ash, and small charcoal pieces. Areas with 

large, partially burned residues were avoided. Burning was 

done in early September 1975. 

2. To simulate close utilization, all residues down to 

1 inch in diameter were removed. Except for skyline 

corridors, the understory vegetation was essentially undisturbed. The surface was a mosaic of vegetation and 

exposed litter surfaces. The surface condition used for 

temperature measurement was a litter surface made up of 

small twigs, leaf and needle litter, and duff. Little mineral 

soil was exposed. The following tabulation shows the surface conditions evaluated for each overstory and residue 

reduction treatment: 

Overs tory Surface 

treatments Residue reduction conditions 

evaluated treatments evaluated 

1. Clearcut Broadcast burn, Burned, 

close utilization litter 

2. Uncut Litter 

Annually for 4 years following both treatments, 25 bare 

root (2-0) seedlings of Douglas-fir and Engelmann spruce 

were spring planted. Survival and growth measurements 

were made periodically. 

Union Pass-This site is 40 miles southwest of Dubois, 

WY, on the Bridger-Teton National Forest at an elevation 

of 9,200 ft. Slope ranges from 5 to 20 percent on an east 

aspect. Climatic data for this area are not available, and 

the nearest stations are considerably lower in elevation. 

Correcting for elevation and slope, annual precipitation is 

estimated to be 35 to 40 inches. Mean July maximum air 

temperature is about 70 °F, and mean July minimum air 

temperatures are 35 to 40 °F. 

Tree cover prior to treatment was overmature lodgepole 

pine (160 years old) with small amounts of Engelmann 

spruce, subalpine fir, and limber pine (Pinus flexilis 

James). Habitat type (Steele and others 1983) is Abies 

lasiocarpa/Vaccinium scoparium (ABLA/VASC). Understory vegetation was sparse. 

Overstory treatments on this study site were: 

1. Clearcut. Clearcutting was done in 1971 using a 

feller buncher and ground skidding (Benson 1982). 

2. Uncut. An adjacent uncut area was used as the 

control. 

Residue reduction treatments within the clearcut included the following: 

1. Broadcast burning, done 2 years after logging (spring 

1973), resulted in 30 percent mineral soil exposed and 

reduction in litter depth from 1 inch to 0. 71 inch (Benson 

1982). The surface condition used for temperature measurement consisted of blackened litter, ash, and small 

4 

charcoal pieces. Areas with large, partially burned 

residues were avoided. 

2. To simulate close utilization, all residues down to 

1 inch in diameter were removed. This treatment exposed 

mineral soil on 42 percent of the surface and reduced the 

understory vegetation cover considerably. After treatment, 

19 tons per acre of woody material remained. The surface 

condition used for temperature measurement was a litter 

surface made up of small twigs, leaf and needle litter, and 

·duff. 

3. On half of the close-utilization treatment area, an 

amount of residue equivalent to that removed was chipped 

and spread back on the site (fig. 2C). The average depth of 

the chips was about 4 inches. No mineral soil or vegetation 

was exposed. The surface of the chips was used as the surface condition for temperature measurement. 

Surface conditions evaluated on each of the overstory 

and residue reduction treatments are shown in the following tabulation: 

Overs tory Surface 

treatments Residue reduction conditions 

evaluated treatments evaluated 

1. Clearcut Broadcast burn, Burned, 

close utilization, litter, 

chips spread chips 

2. Uncut Litter 

Bare root (2-0) seedlings of lodgepole pine were planted 

in the spring the first year after treatment on all treatments. Spot seeding was also done. At the time of planting and seeding, 18-inch square areas were scarified 

around each spot or seedling. 

Measurements 

Surface, air, and soil temperature were monitored for the 

overstory treatment and surface conditions described for 

each study area. Specific temperature measurements taken 

for each study site and treatment combination are shown 

in the tabulation for each study area and in table 1. The 

time and duration of measurements by site are shown in 

table 2. The term "surface" is used to indicate the surface 

exposed to the atmosphere, whether the exposed material 

is litter, ash (from burning), mineral soil, or chips. Measured surface temperatures are actually 0.04 to 0.08 inch 

below the true surface. Temperature sensors were installed close to the surface by covering with one layer of 

the surface material-for example, one leaf, one thickness 

of needles, one layer of soil (enough to cover the metal), 

or one layer of chips. It was necessary to frequently visit 

the sensors and reset them if they were uncovered. Air 

temperature sensors at 4.5 ft above the surface at 

Lubrecht and Coram were placed in standard Cotton 

Region type shelters. 

At Union Pass, air temperatures were measured at 

8 inches and 4.5 ft above the surface. Sensors were placed 

in specially constructed shielded tubes. Soil temperatures 

in the humus layer at 0.4 to 1.2 inches below the surface 

were measured at Lubrecht and Coram. Temperatures in 

mineral soil at 2, 8, and 15.7 inches deep were measured 

at Union Pass. Sensors at Union Pass were pushed horizontally into the undisturbed soil of the wall of a hole dug 

to the proper depth, which was then backfilled. 
Table 1-Temperature measurements taken by study site, overstory, and surface condition 

Study site 

Lubrecht Coram 

Measurement 

location Surface Air Soil Surface Air 

Overstory treatment: 

Uncut X X X X X 

Clearcut X X X X X 

Shelterwood X X 

Understory removal X 

Surface condition: 

Litter X X 

Mineral soil X 

Burned X X 

Chips 

Table 2-Length of time following treatment that temperatures 

were measured by study site 

Study site 

Lubrecht 

Coram 

Union Pass 1 

X 

X 

2 

X 

X 

Years following treatment 

3 

X 

X 

4 

X 

5 

X 

1Temperatures were measured for only 1 year. 

6 

X 

X 

7 

X 

Temperatures were measured using small, 0.064-inch 

diameter, thermistor beads inserted in 0.094-inch diameter 

by 0.5-inch long stainless steel tubing. Shielded 30-gauge 

wire attached to the thermistor ran 6 to 10 ft to a junction box. Multiconductor cables (18-gauge) up to 2,500 ft 

long were connected from the junction box to a recording 

data acquisition system. Each data acquisition system 

handled from 16 to 40 temperature sensors. Temperatures 

were recorded hourly on a 12-month basis for varying 

lengths of time (table 2). 

Net radiation was measured on all sites for some of the 

treatments. Continuous monitoring of net radiation was 

done for the clearcut and uncut treatments at Coram for 

4 years and at Lubrecht for 3 years. Net radiation measurements were also made over each of the surface treatments at noon on a clear day at Lubrecht and Union Pass. 

These data are used in the discussion to help explain how 

treatments affect temperature. 

Analysis 

Analysis of research results involved: (1) evaluating effects of overstory removal treatment on surface, air, and 

soil temperatures; and (2) evaluating effects of surface 

condition, resulting from residue treatments on surface, 

air, and soil temperatures. The discussion of results is 

presented in that order. Because treatment effects are influenced by the elevation, terrain, and climatic characteristics of the specific locations studied, results are also 

generally presented separately for the three study sites. 

As discussed earlier, the range of overstory treatments 

and surface conditions, as well as temperature observaSoil 

X 

X 

5 

Union Pass 

Surface Air Soil 

X X X 

X X X 

X X X 

X X X 

X X X 

tions, also varies by site. Consequently, between-site 

comparisons are necessarily limited. 

To evaluate treatment effect on surface temperature, we 

used two types of summaries: (1) monthly average maximum and minimum temperatures were calculated, and 

(2) the occurrence of potentially lethal temperaturesreferred to as hot events and cold events-was tabulated. 

Statistical tests (to be described later) determined treatment differences for maximum and minimum temperature 

by month for May through October. The percentage of 

months, over the measurement period, that differences 

were significant was then compared for the overstory 

treatments and surface conditions. 

We calculated the number of hot events and cold events 

based on surface temperatures, then used these to indicate 

the hazard for seedling mortality. A hot event (HE) is 

defined as 2 or more consecutive hours of temperatures 

that are greater than 122 °F or 1 hour at 133 °F or 

higher. A cold event (CE) is an occurrence of minimum 

temperature of 23 °F or colder. These thresholds-122, 

133, and 23 °F -were picked to represent average lethal 

levels for several tree species (Baker 1929; Hare 1961; 

Levitt 1980). The HE's and CE's are summed over the 

growing season, then used to calculate the percentage of 

days that they occurred within the period. Neither the HE 

nor the CE is intended to suggest that seedling mortality 

will invariably result from these events, but they do represent a significant hazard to survival. We then compared 

HE and CE data with measured seedling survival on the 

study areas. 

To evaluate treatment effect on air temperatures, we 

used daily maximum and minimum values and average day 

and night temperatures. We used the day and night average temperatures because they influenced growth of tree 

seedlings (Cleary and Waring 1969; Hellmers and others 

1970). 

Because soil temperatures are much more stable than 

surface or air temperatures, daily averages were used to 

compare treatments at depths below 2 inches. Temperatures in the humus layer (0.4 to 2.0 inches deep) are included in the results as soil temperatures. Analysis of the 

humus layer includes maximum and minimum temperatures and HE's and CE's. Temperatures in the root zone 
are discussed relative to their effects on seedling growth 

and root development. 

Tests for significant treatment effects on surface temperature were made using one-way analysis of variance to 

determine if treatments were different for maximum and 

minimum temperatures (by month). Also, tests of treatment differences in air and soil temperatures were evaluated using one-way analysis of variance. Analyses were 

run only for data from May through October. Duncan's 

multiple range test was used to test for individual treatment differences. If variances were not homogeneous, we 

used approximate t-tests. All tests were done at the 0.05 

significance level. Differences in the overstory removal 

treatments were compared using temperatures measured 

on the litter surface condition of each treatment. Differences between the surface conditions were compared using 

a common overstory removal treatment (usually the clearcut). Mineral soil and litter surface conditions at Lubrecht, 

however, were compared in the shelterwood treatment. 

OVERSTORY REMOVAL AND 

TEMPERATURE 

Surface, air, and soil temperature data accumulated 

from treated units were analyzed first to evaluate the 

effects of level of overstory removal on temperature. To 

compare overstory treatments on a common basis, data 

from the litter surface condition were used. 

Surface Temperature 

Lubrecht-The amount of overstory removal significantly influenced maximum surface temperatures, mean 

monthly maximum surface temperatures being significantly warmer where all or most of the canopy was removed 

(table 3). From May through October of the first 3 years 

after harvesting, temperatures on the clearcut surface 

were significantly warmer than those on the shelterwood 

surface 40 percent of the months, were equal for 40 percent of the months, and were significantly lower for the 

other months. Over the 3 years, surface temperatures on 

the clearcut and shelterwood treatments were significantly 

warmer for 80 percent of the months than were surface 

temperatures on the understory removal or uncut treatments. Surface temperatures on the understory removal 

for some months were up to 11 oF warmer than on the 

uncut, but for 60 percent of the months differences were 

not significant. 

Maximum surface temperatures were observed as high 

as 165 °F and,often exceeded 133 °F (fig. 3) for the clearcut and shelterwood treatments. Temperatures of 122 °F 

200,-----------------------------------

-.., "' UR,UN_CUT 

5 10 15 20 25 

JULY DAY 

Figure 3-Maximum daily ground surface 

temperatures in July 1980 at Lubrecht for the 

clearcut (cc), shelterwood (sw), understory 

removal (ur), and uncut treatments. Surface condition is litter. The horizontal line through the 

graph is 133 ° F, a seedling survival threshold. 

lO 

Table 3-Average maximum surface temperatures (°F) by month for 1978, 1979, and 1980 for 

the overstory treatments at the Lubrecht study site. Data from the litter surface were 

used for all treatments. Values for a month (months are not compared) having different 

letters are significantly different at the 0.05 level. 

Overstory 

treatment May June July Aug. Sept. Oct. 

1978 

Uncut 61a 90a 102a 95a 75a 75b 

Understory removal 72b 100b 106a 86a 77a 61a 

Shelterwood 72b 106b 126b 117b 93b 79b 

Clearcut 93c 126c 138c 118b 97b 93c 

1979 

Uncut 66a 75a 93a 68a 

Understory removal 77b 86a 91a 70a 

Shelterwood 82b 100b 126c 84b 

Clearcut 104c 126c 113b 81b 

1980 

Uncut 86a 104a 99a 70a 61a 

Understory removal 88a 95a 102a 81b 64a 

Shelterwood 93ab 115b 127c 100c 79b 

Clearcut 97b 115b 118b 82b 70ab 

6 
Table 4-Average minimum surface temperatures (°F) by month for 1978, 1979, and 1980 for 

the overstory removal treatments at the Lubrecht study site. Data from the litter 

surface were used for all treatments. Values for a month (months are not compared) 

having different letters are significantly different at the 0.05 level. 

Overstory 

treatment May June 

1978 

Uncut 34b 39b 

Understory removal 35b 38b 

Shelterwood 36b 40b 

Clearcut 26a 30a 

1979 

Uncut 34b 37b 

Understory removal 34b 36b 

Shelterwood 34b 36b 

Clearcut 25a 29a 

1980 

Uncut 44b 

Understory removal 47b 

Shelterwood 47b 

Clearcut 40a 

or greater were measured from April (12 days after snowmelt) into October. Maximums on the understory and 

uncut treatments rarely exceeded 122 °F. Figure 3 is 

representative of the patterns observed for most summer 

months over the 3 years. 

The degree of overstory removal also significantly affected minimum surface temperature (table 4). Removal of 

up to about 50 percent of the overstory (understory removal and shelterwood) did not cause minimum temperatures to fall lower than in the uncut stand. However, 

removal of all the overstory caused minimum temperatures to be significantly lower than on the shelterwood, 

understory removal, or uncut treatments for all months. 

Minimum temperatures on the clearcut in 1978 were as 

low as 18 °F in June, 25 °F in July, and 21 °F in August. 

By September nearly every night was below 32 °F, 

with a low of 10 °F. Temperatures below 32 °F were not 

observed on the shelterwood or uncut treatments, and 

only occasionally on the understory removal treatment 

in June, July, or August (fig. 4). 

The same pattern seemed to exist in 1979, but instrument failure precluded records in July and August. The 

summer of 1980 was warmer; no temperature below 32 °F 

was recorded in July. 

Hot event days, when the temperature exceeded the 

lethal threshold, occurred on the clearcut and shelterwood 

treatments from May through October on 14 to 37 percent 

of the days (table 5). The probability of an HE was greater 

on the clearcut, but the difference is not significant. No 

HE's occurred on the understory removal treatment, and 

only three were measured on the uncut control over the 3 

years. These results suggest that the potential for seedling 

mortality due to high temperatures is quite high on the 

clearcut and shelterwood treatments, but almost nonexistent in the uncut stand and on the understory removal 

treatments. 

7 

July Aug. Sept. Oct. 

43b 41b 37b 27b 

41b 39b 36b 24b 

44b 41b 38b 26b 

35a 34a 28a 12a 

37c 36c 

33b 26b 

36bc 28bc 

25a 17a 

45c 40c 38b 30b 

45cd 38b 39b 28b 

47d 40bc 39b 31b 

38b 32a 33a 23a 

60 

t:" 

~ 50 1.&.1 

0:: 

~ 

e. 

a.. oi-0 

~ 

1.&.1 

t-

~ 30 

~ ~ 

:::> 

~ 20 

z 

~ 

5 10 15 20 25 

AUGUST DAY 

Figure 4-Minimum daily ground surface 

temperatures in August 1980 at Lubrecht for the 

uncut, understory removal (ur), and clearcut (cc) 

treatments. Temperatures in the shelterwood 

(sw) were the same as the understory removal. 

The surface condition was litter. 

UNCUT 

. 

cc 

JO 

On the surface of the clearcut, cold event days-when 

the temperature dropped to the lethal threshold of 23 °F 

or below-occurred on 16 to 26 percent of the days in May 

through September the first 2 years after treatment. During the third year none occurred on the clearcut. During 

the 3 years following treatment, only one CE occurred on 

the uncut and understory removal treatments and none on 

the shelterwood treatment. Most of the CE's occurred in 

May and September (75 to 80 percent), but a significant 

number occurred in June, July, and August. Like the 

HE's, the results show that a potential for seedling mortality due to low temperatures exists on the clearcut 

treatment. 
Table S-Hot events (percentage of possible days) for the overstory treatments at several study 

sites. Data represent the May through October period for 1 to 7 years after treatment. 

Data for the litter surfaces on the clearcut were used when they were available. If not, 

data from the burned surface were used. 

Study site 

Overstory 

treatment 2 

Years after treatment 

3 4 5 6 7 

- - - - - - - - - Percentage of possible days - - - - - - - - -

Lubrecht Uncut 1 0 2 

Understory removal 

Shelterwood 

Clearcut 

0 0 0 

Coram Uncut 

Clearcut 

Union Pass Uncut 

Clearcut 

14 

28 

2 

36 

In summary, the clearcut treatment on the Lubrecht site 

creates conditions with a high potential for seedling mortality due to high and low temperatures. The shelterwood 

treatment had a high potential due to high temperatures 

but not to low temperatures. The residual stand canopy in 

the shelterwood is open enough to let incident radiation 

warm the surface, yet it is dense enough to hold heat in at 

night, slowing radiational cooling. Maximum and minimum 

temperatures in the understory and uncut treatments 

typically are not high enough or low enough for potential 

seedling mortality. Figure 5 shows the maximum and 

minimum temperature relationships for each treatment on 

a typical summer day. 

MIN 

0 

23 F 

CLEARCUT 

0 

36 F 

SHELTER WOOD 

16 25 

37 23 

0 0 

27 23 29 29 6 

0 

65 

Other Sites-Only data for the clearcut and uncut treatments are available on the other two sites. Data from the 

Coram and Union Pass study sites also demonstrate that 

clearcutting results in significantly warmer and colder surface temperatures. Data on HE's at Coram and Union 

Pass (table 5) are comparable to the results observed at 

Lubrecht. The potential for seedling mortality (due to high 

temperatures) is quite high on the clearcut and low in the 

uncut stand. The frequency of HE's is high (65 percent) at 

Union Pass; only data for the month of July are available. 

Logging significantly increased the potential for seedling 

mortality at Coram and Union Pass. 

0 

36 F 

UNCUT UNDERSTORY 

REMOVAL 

Figure 5-Dai/y maximum and minimum surface temperatures (°F) by overstory treatment at Lubrecht, August 29, 1978. 

8 

;., 
Minimum surface temperatures at Coram and Union 

Pass were significantly colder on the clearcut treatment. 

Results for these two sites between the clearcut and uncut 

follow the same pattern as at Lubrecht. Average minimum 

temperatures are warmer for Coram than for Lubrecht. 

This appears to be a result of topographic differences. 

At Coram the CE's following clearcutting were not as 

frequent as at Union Pass. The frequency of CE's was low 

at Coram, and they occurred only in May or Septembernone in June, July, or August. The frequency of CE's 

varied £-rom 0 to 45 percent of the nights in July at Union 

Pass. The ei'l'ect of surface condition is discussed later. 

Comparison With Expected Results-The results 

reported here are consistent with and explained well by 

the energy balance equation. The major effect of the overstory removal treatments was to increase energy incident 

upon the surface (Rn) (fig. 6). Increased Rn causes an increase in the heat flux density at the surface (G). Changes 

in convective heat loss (H) and latent heat loss by evaporation and transpiration (LE) are not able to dissipate the 

added heat, which results in increased temperature variation at the surface. Although the temperature differences 

were not significant for all treatments for all months, the 

trend of increasing growing season surface temperatures 

with increasing levels of canopy removal was observed 

(table 3 and fig. 3). The effects of vegetation influence on 

~·"···~ ~ ...... ~ 

IS ,t ' ~~ IS I; 600 

shade will be discussed under the surface condition 

section. 

Because the surface condition of all treatments for the 

overstory removal comparisons is the same (litter), the 

thermal properties did not significantly influence the temperature differences. Higher surface temperatures on the 

clearcut and shelterwood treatments not only increase the 

potential for seedling mortality due to high temperatures, 

but also are effective in drying the surface layers, which 

creates additional moisture stress. The drier surface conditions in turn lower volumetric heat capacity and thermal 

conductivity, which further increases temperature 

variation. 

Minimum surface temperatures for the treatments are 

also explained well by the same factors (table 4 and fig. 4). 

The more exposed surfaces, where more of the canopy has 

been removed, are subject to greater radiational cooling 

and thus lower temperatures. On the shelterwood treatment that admits about 60 percent of full sun, the canopy 

is sufficient enough to limit radiational cooling, which explains the absence of CE's, and to reduce the potential 

mortality due to low temperatures. In addition to the effect of the Rn term, topographic conditions at Lubrecht 

and Union Pass are such that cold air can accumulate, 

creating frost pockets. In these cases the H term is affected. More will be said about this in the discussion of the 

surface conditions. 

I§ I\ 

I i 1 \..- CLEARCUT (16 ACRES) 1: 1: 400 500 

400 

~ 

Vi 300 

~ <( 

$ 20 

200 

t'~ I'; 

lj..-CLEARCUT (0. 9 ACRES) 

I : 363 

I : 

I ~ 

i ~SHELTERWOOD 

i i ~ 219 

i . I : 

i ! I : UNCUT 

i !~ 123 i I I : i i 1 ~ i i 1 ~ 

i ! I~ i 1 1 : i \ I~ ,.,,J \ I: .l • ·~ I~ , \ 1: . ..~ : 

.,•' ·~. I~ 0 0~----------~~--------------~~~-------

-5 

-100 -10 

2 4 6 8 10 12 14 16 18 20 22 24 

HOUR 

Figure 6-Diurnal variation in net radiation by 

cutting method on June 18, 1976, Coram Experimental Forest. Daily totals are given for each 

treatment in cal cm- 2• 

9 
Air Temperature 

Air temperatures at 4.5 ft above the surface in clearcut 

units were not significantly warmer than above the surface in uncut units at any of the sites. Comparison using 

accumulative degree days over the May through October 

season did not reveal significant differences either. Data 

from Coram and Union Pass show that topographic position influences air temperature much more than clearcutting. Monthly average minimum temperatures in openings 

at the toe of a slope were 5 to 12 °F cooler than openings 

a few hundred feet up the slope (fig. 7). Monthly average 

maximum temperatures were at least aE'. warm at the toe 

of the slope as in openings on the slope. 

Soil Temperature 

Lubrecht-Daylight average and maximum temperatures 

in the humus (02) layer 0.4 to 1.2 inches deep at Lubrecht 

over the 3 years were significantly warmer on the clearcut 

than in either the shelterwood or uncut treatments (fig. 8). 

Humus in the shelterwood was significantly warmer than 

in the uncut. Maximum humus temperatures were as high 

as 136 °F on the clearcut, and July average maximum was 

about 115 °F. The HE's occurred, but were not frequent 

(10 during the 3 years). Maximums on the shelterwood 

were nearly as high at 131 °F, with a July average maximum of 111 °F in 1978. Temperatures were much colder 

in 1979 and 1980, with a July average of about 86 °F. 

Only three HE's occurred in the shelterwood, all in 1978. 

The maximum humus temperatures on the uncut site were 

91 °F. 

Minimum and night average humus temperatures were 

significantly colder on the clearcut than in the shelterwood 

70,-----------------------------------

c 

(/) 60 

Ls.J 

Ls.J 

a:: 

8 50 

s 

Q.. 

E 40 

~ 

~ 

~ 

z 30 

~ 

\ 

.. ... ... ... ... SLOPE TOE 

MJ J AS 0 M J J AS 

1975 1976 

Figure 7-Mean monthly minimum air 

temperatures at the toe of a slope and at 

midslope on the Coram Experimental Forest. 

Temperatures are for May through October of 

1975 and 1976. 

10 

or uncut treatments. Minimum humus temperatures observed (May through September) were 18 °F in the clearcut and 21 oF in the shelterwood. Average July minimums 

were 43 °F in the clearcut and 50 °F in the shelterwood 

and uncut treatments. 

Other Sites-Humus or upper level soil temperatures at 

Coram and Union Pass were measured at depths from 1.2 

to 2.0 inches. Absolute maximum temperatures ranged 

from 73 to 90 °F in the clearcut and were significantly 

warmer than in the uncut treatments. Night minimums at 

Coram in the clearcut were not significantly colder than in 

the uncut. The clearcut at Union Pass was significantly 

warmer at night than the uncut for the short measurement period. We do not know the reason for this reversal 

yet the 24-hour cumulative number of degree hours-thos~ greater than 32 °F-is 80 percent higher in the clearcut 

than in the uncut. 

Soil temperatures at depths from 2 to 16 inches were 

only measured on clearcut and uncut treatments at Union 

Pass. Temperatures at these depths were significantly 

warmer in the clearcut than in the uncut. Temperatures at 

2 inches deep averaged nearly 12 °F warmer in the clearcut broadcast burn than in the uncut, 11 °F at the 8-inch 

depth, and almost 10 °F more at the 16-inch depth (fig. 9). 

Summary 

Temperatures increased from the ground surface to 

depths of 16 inches as a result of clearcutting. Site location has little or no effect on the treatment differences 

though the maximums and average temperatures are ' 

higher at some locations. The magnitude of differences 

decreases with increasing depth, yet the differences remain statistically significant. 

100,--------------------------------------

90-

80-

r I 

I \1 

I ' \ ,. I / " ., 

\ 

• cc 

/' I '-..J'-- sw 

70-

60 

501-----~.----~.--~.-----r-,--~.-----~,--~ 

5 tO 15 20 25 30 

JULY DAY 

Figure 8-Daylight average temperatures of the 

humus (02 ) layer (0.4 to 1.2 inches depth) on the 

clearcut (cc), shelterwood (sw), and uncut 

treatments at Lubrecht in July 1980. Surface 

condition was litter. 
70 

A 

L:' 

(/') 

w 60 "', BURNED / w ,_,. ~ 

<.:> 

w 

s 

a.. 50 ~ 

w 

1-

w UNCUT 

(.!) 

<( 

~ 40 

w 

~ 

30 

70 

8 ....--

L... 

(/') 

w 60-

w 

~ BURNED .,. <.:> ' "" ' w / / 

s .... /" - a.. 50- LITTER ~ 

w CHIPS 

1- ,.-~ ;-·-·-·-·- w --·---' , .... ~................................ ~--... .~ ; ~ # "' , 

'-' _________ .,. ..... ~ ... 

<( 

40- UNCUT ~ 

w 

~ 

30 I I I 

70 

c 

60-

BURNED 

-- .,. /-. 

50- ... .. 

LITTER CHIPS 

__ ,__/' ·-·-·-·-·-·-·- , ..... ~ ,... , .... , 

~-----------'' ' ........... ," "",.,., '-,,.' 

- - - u -""' UNCUT 

30~--------~~------~~---------~.------~ 

5 10 15 20 

JULY DAY 

Figure 9-A verage soil temperatures at depths of 

2 inches (A), 8 inches (B), and 16 inches (C) for 

burned, Jitter, and chip surface conditions in 

clearcut and uncut treatment units. Data are 

from Union Pass in July 1979. 

11 

These results for soil temperature seem to be explained 

well by the effects of Rn as influenced by overstory removal. Although '{KG affects the rate of conduction and 

associated temperature changes, this factor was overwhelmed by the differences in Rn at the surface of the 

treatments. These effects will be discussed in more detail 

for the surface conditions. 

SURFACE CONDITION AND 

TEMPERATURE 

Surface conditions resulting from residue treatment included litter, burned, mineral soil, and chips. The effect of 

surface condition on surface, air, and soil temperatures is 

examined by comparing temperatures for the different 

surface conditions under the same overstory treatments. 

The clearcut was used for all but one comparison. The 

shelterwood treatment is used in that one. 

Surface Temperature 

Lubrecht-Maximum surface temperatures on both the 

burned and litter surfaces in the clearcut were greater 

than 133 °F during midsummer (fig. 10). For the first 

2 years after burning, average maximum (monthly) temperature on the burned surface was 9 to 16 oF greater 

than on the litter surface. For 57 percent of the months 

for May through October, these differences were significant. After 2 years, a heavy stand of bull thistle and its 

thistledown on the burned treatment provided enough 

shade to decrease surface temperatures below those on the 

litter surface. 

Hot events occurred on the burned and litter surfaces 

from 17 to 37 percent of the days from May through October (table 6). The HE's were observed for more than 

30 days each year after burning on both litter and burned 

surfaces, even though instrument difficulties resulted in 

45 days when temperatures could not be measured. 

Minimum temperatures were not significantly different 

for the burned and the litter surfaces for most months. 

After the thistle crop developed on the burned treatment, 

minimum temperatures were significantly warmer on the 

burned surface than on the litter surface. Both the litter 

and burned surfaces had significant numbers of CE 's in 

1979-the first year after burning. Cold events were 

observed on 42 percent of the days on the litter surface 

and 26 percent of the days on the burned surface from 

May through September. Only two CE 's were observed on 

the litter surface in 1980 and none on the burned surface. 

Climatic conditions and vegetation regrowth were the likely causes of reduced CE's in 1980. 

In the shelterwood, average maximum surface temperatures of exposed mineral soil were significantly lower-5 

to 34 °F-than temperatures of the litter for 75 percent of 

the months over the 3 years. Average maximum temperatures on the mineral soil surface never exceeded 100 °F. 

Difference between the litter and mineral surfaces was 

least in May and October and greatest in July (fig. 10). 

Hot events on the litter occurred from 14 to 25 percent of 

the days, but no HE's occurred on the mineral soil surface 

during the measurement period. Average minimum temperatures were 1 to 4.5 °F colder on the litter surface 
200,----------------------------------

A 

BURNED,CC ' . 

.,.---... ,,···· • _,•'wiNCRAL,SW 

, 

50 

60 

'1:' 

Vl 

w 

w 

a::: 50 

8 

e. 

a.. 

:E 

w 40 1-

~ 

< 0 

::E 30 ~ 

::E 

z 

~ 

B 

5 10 

' , ' ,, , 

15 20 

JULY DAY 

25 

Figure 10-Maximum (A) and minimum (B) daily 

temperatures of burned, litter, and mineral soil 

surfaces at Lubrecht in July 1980. Burned surface was measured in the clearcut (cc) and 

mineral in the shelterwood (sw). Maximum litter 

temperatures were the same in clearcut and 

shelterwood. Horizontal line for maximum 

temperatures (A) is at 133 °F-a seedling 

survival threshold. 

30 

Table 6-Hot events (percentage of possible days) by surface 

condition at the Lubrecht site. Data are for the months 

of May through October. 

Residue treatment 

Burned surface (clearcut) 

Litter surface (clearcut) 

Litter surface (shelterwood) 

Mineral soil (shelterwood) 

1First year after burning. 2Second year after burning. 

Years after logging 

2 3 

--- Percentage of possible days - - -

1

37 223 

27 19 17 

14 16 25 

0 0 0 

12 

than on the mineral soil surface. These differences were 

significant for only 33 percent of the months. No CE's 

were observed on the mineral soil or litter surfaces in the 

shelterwood treatment, mostly because of the overstory 

canopy. Because the mineral soil surface measured was in 

the shelterwood treatment, it could not be directly compared to the burned surface in the clearcut; minimums 

were much warmer in the shelterwood than on the 

clearcut . 

These results indicate a definite contrast between temperatures on organic (burned and litter) surfaces and 

mineral soil. Because the maximum and minimum temperatures of litter and burned surfaces are similar, the probability of temperature-related seedling mortality may be 

similar. Lower maximum and higher minimum temperatures on the mineral surface suggest that temperaturerelated seedling mortality would be lower than on the 

litter surfaces. 

Coram-Only litter and burned surfaces in the clearcut 

are compared here. From the first through the fourth 

years after burning, average maximum temperatures on 

the litter surface were significantly warmer than on the 

burned surface for 79 percent of the months (May through 

October). During the fifth and sixth years after burning 

there were no differences between treatments. Data from 

another clearcut at Coram, for a shorter period, showed 

the same results. 

Hot events were observed mostly in the first year on the 

burned surface, and then only on 5 percent of the possible 

days. The litter surface had 24 to 42 percent of the days 

with HE's from the first through fourth years. After the 

fourth year HE's were rare-4 percent or less on the litter 

surface and only 1 percent on the burned surface. 

Minimum temperatures on the burned and litter surfaces 

at Coram varied from being equal to being 10 °F colder 

on the litter during the first 4 years after burning. Mean 

monthly minimum litter surface temperatures were significantly colder for 86 percent of the months by 2. 7 to 

5.4 °F. Cold events were not observed on either surface 

for any year. 

Union Pass-Burned, litter, and chip surface conditions 

are compared on the clearcut. Average maximum temperatures were not significantly different between the burned 

and litter surfaces 6 years after treatment. Hot events 

were observed on 62 percent of the days for both treatments in July. Maximum temperatures measured on the 

chip surface were significantly cooler-13 °F-than on the 

litter or burned surfaces (fig. 11). The HE's were observed 

10 percent of the July days on the chip surface. 

Average minimum temperature for July was significantly colder on the litter surface than on the burned surface 

and colder on the chip surface than on either the litter or 

burned surfaces (fig. 11). Cold events in July were observed on the litter surface 10 percent of the nights, on 

the chip surface 43 percent, and not at all on the burned 

surface. These differences in minimum temperatures and 

CE' s are due more to slope position, which demonstrates 

the effect of cold air drainage (fig. 12). 
200 

c A 

C/) 

&...1 

&...1 

0::: 150- 8 _.,. ___ BURNED 

e -- ...../.-. ....._,., / "' I . ..../.-... ~ 

:::E 'v ,_ .............. y CHIPS &...1 

1- 100-

~ .... .......... ------....... " 

.. .. .. .. -,y C§ .. --- :::E .... ~ ..... #'~--- .... ___ ........ LiN~ur 

~ 50-

:::E 

X 

< :::E 

0 I l 

50 

1:' B 

C/) 

&...1 

L&J 

0::: 

8 

.t-0 

s 

Q.. 

:::E 

&...1 30 1-

~ 

< 0 

::E 20 ~ 

:::E 

z 

2 

5 10 

JULY DAY 

Figure 11-(A) Maximum daily temperatures on 

burned, chips, and untreated surfaces. Litter 

surface is same as burned. (B) Minimum daily 

temperatures on untreated, burned, litter, and 

chip surfaces. Data are from Union Pass in July 

1979. Burned, Jitter, and chip surfaces are in the 

clearcut. Horizontal line tor maximum temperatures (A) is at 133 °F, a seedling survival 

threshold. 

Figure 12-Minimum temperatures at 

Union Pass on July 10, 1979. The lower 

row of numbers represents surface 

temperatures; the upper row represents 

temperatures at 2 inches above the 

surface. 

l 

UNCUT 

.. ,'~ " .... ~ 

' # " ~# 

r 

15 

UNCUT 

13 

In summary, the results for Union Pass are consistent 

with those at Lubrecht for the burned and litter surfaces. 

Vegetation development for the two treatments is also 

similar to that at Lubrecht, except slower. Even after 

6 years, ground coverage of vegetation was low (less than 

25 percent) on both treatments. 

The difference in results for maximum temperatures 

on the burned and litter surfaces between Coram and 

Lubrecht can be explained by observations about the pretreatment vegetation, subsequent vegetation development, 

and severity of the burns. At Coram the burn was not as 

hot as the burn at Lubrecht because of the differences in 

duff moisture (Artley and others 1978; Steele 1980). This 

situation, coupled with habitat type differences, promoted 

rapid postburn vegetation development at Coram. By midsummer of the first year after burning at Coram, a good 

ground cover of grasses and forbs existed that was quite 

effective in shading the surface. At Lubrecht the litter 

surface of the close utilization treatment was disturbed 

more by ground skidding, which stimulated resprouting 

and provided conditions for colonization of weeds. At 

Coram, where a skyline yarder was used, understory 

vegetation was disturbed differently on the close utilization treatment where the litter surface was measured. 

Branches and tops of shrubs and herbs were broken and 

mashed but not severely damaged. As a result, resprouting and regrowth were not stimulated enough to cause 

rapid growth response, yet coverage was enough to provide a significant number of shaded spots for seedlings. 

Comparison With Expected Results-Other published 

results reveal some inconsistencies between burned and 

unburned surfaces. Fowler and Helvey (1981) did not find 

any differences between temperatures on the burned pile, 

broadcast-burned scarified surfaces, and the unburned surfaces on a clearcut in northeastern Oregon. Ahlgren (1981) 

reports that burned surfaces were warmer for the first 

7 years than were unburned surfaces. After 8 years 

burned surfaces became cooler, a trend that was attributed to gradual development of shrubs. Our results seem 

to be consistent with Ahlgren's observation of vegetation 

development. Temperatures of burned and litter surfaces 

were similar (both maximums and minimums), yet the 

amount of vegetation cover and the rate at which it 

developed significantly modified the differences. 

BURNED 0 

21 F 

LITTER CHIPS 
Temperatures on the litter and burned surfaces are 

expected to be similar (under full sunlight), based on the 

energy balance equation, and the thermal contact coefficient ('{i«j) (table 7) that influences G. This situation applies if we assume that (1) both surfaces are dry, thus the 

H and LE terms should be equal; and (2) increased Rn 

contribution to burned surfaces as compared to litter surfaces is offset ~ lower '{KG value for the litter. This 

shows that if V KG were different for either surface, because of moisture, compaction, and so forth, the LE term 

would be different, causing the surfaces to have different 

temperatures. 

The study results reported here, and those published, 

tend to support the expectation that burned and litter surfaces will be similar. At Coram, however, where maximums on the litter surface were warmer and minimums 

colder on the burned surface, Rn on the burned surface 

was apparently reduced by the rapid vegetation growth 

that provided shade and reduced the temperature 

variation. 

The energy balance equation and thermal properties of 

'{KG (table 7) cause us to expect that temperatures on the 

burned or litter surfaces will be considerably more variable (higher and lower) than for the mineral soil surface. 

Lubrecht data are consistent with this expectation. The 

major factors influencing this difference are '{KG and Rn. 

Net radiation is less on the mineral soil (fig. 13) and '{KG 

is considerably greater for mineral soil than for litter, 

which means that less heat to the surface is conducted 

away more quickly than for a litter surface. This keeps the 

surface temperature from rising as much. The same factors allow heat from the soil to warm the surface at night, 

which keeps minimum temperatures of mineral soil warmer than litter or burned surfaces. Although mineral soil 

surfaces in the clearcut were not compared with litter or 

burned surfaces, the differences should be greater than in 

the shelterwood. If mineral soil were more moist, the differences would be accentuated even more. LE and '{KG 

would increase for the mineral soil, which would further 

reduce the temperature variation at the mineral surface in 

comparison to litter or burned surfaces. 

Compared to litter and burned surfaces, the chip surface 

is expected to have less temperature variation (lower maximums and higher minimums). The data at Union Pass are 

consistent with this expectation. Chips have a higher 

Table 7-Average values of albedo or reflectivity and the thermal 

contact coefficient ({KG) for typical surfaces. Values 

are from Fowler {1974). 

Surface Albedo VKc1 

Percent cal cm- 2 sec- 112 oc- 1 

Burned 2 0.0018 

Litter 

(dry needles) 6-10 .0013 

Bark (dry) 20 .0031 

Chips 36 .0026 

Soil 20-35 .0111 

Air .0001 

Water .0361 

1K = thermal conductivity; C = volumetric heat capacity. 

14 

albedo, reducing Rn when compared to a litter or burned 

surface (fig. 13). This lower heat flux density at the surface, coupled with a higher '{KG (table 7) than litter or 

burned surfaces, explains the reduced temperature variation. If the moisture content of the litter surface were 

higher (relative to the chips), the temperature differences 

would be reduced because of increased LE, which would 

lower G. 

Figure 14 shows maximum temperatures for four surface conditions compared to air and humus temperatures. 

The temperatures shown are for a hypothetical site adjusted to a common set of conditions. These are typical 

maximum summer surface temperatures on a clearcut for 

a clear day in full sun. Burned and litter surfaces are 

much warmer than all other surfaces and the humus at a 

l-inch depth. All surface conditions are much warmer than 

air temperature at 4.5 ft. 

At Union Pass, minimum temperatures and CE departed 

from the expected pattern. Litter surfaces were colder 

than burned surfaces, which should be similar, as at 

Lubrecht, and the chip surface was colder than either, 

where it should be warmer (fig. 11B). The basic explanation for these reversals in minimum temperatures lies in 

the topographic positions of the treatments. The close 

utilization treatment, where litter surface temperatures 

were measured, and the chip spread treatment were in a 

~,<~ro ~~<l- ~~Q o'"' 

v "''~ ~~ ~ C3 ~ tt;,<l-~ ~,~ 

Figure 13-Net radiation over several 

surface conditions in a clearcut at Union 

Pass at noon on July 12, 1979. The differences between burned and litter surfaces and between mineral soil and chip 

surfaces probably are not significant. 
CHIPS BURNED LITTER 

Figure 14-Maximum temperatures expected in a clearcut of a 

hypothetical site for four surface conditions-burned, litter, mineral soil, 

and chips-humus at 1-inch depth and air temperature at 4. 5 ft. 

relatively flat area where cold air accumulates at the toe 

of a slope that is below the burned surface (fig. 12). The 

chips treatment is the lowest. In this case H was reduced 

because of restricted airflow and colder, more dense air 

accumulated in pockets. This aided radiational cooling, 

thus lowering the surface temperatures. Additionally, 

where chips were spread, basins were created in the chips 

to allow seedlings to be planted. These additional ~essions trapped more cold air. Surfaces with lower VKC 

values are more susceptible to lower temperatures and 

frosts, particularly in frost-pocket situations. 

Air Temperature 

Air temperatures above the surface conditions were 

measured only at Union Pass. Surface condition did not 

have any effect on maximum air temperature at 4.5 ft or 

8 inches above the surface. Because surface temperatures 

were higher on burned and litter surfaces than on chip 

surfaces, we expected that air temperatures above these 

surfaces might be different. However, prevailing winds 

and upslope thermals likely caused enough mixing that 

surface differences did not significantly influence maximum temperature at these levels. 

Minimum air temperatures were significantly different 

at both 4.5 ft and 8 inches. Temperatures above the chip 

surface were significantly colder than above the litter or 

burned surfaces at both heights. At 8 inches above the 

litter surface, temperatures were also significantly colder 

than above the burned surfaces. As with the overstory 

treatments, these differences are due, at least partially, to 

cold air drainage caused by slope differences. 

Soil Temperature 

Temperatures at depths of 2 inches and greater were 

measured only at Union Pass. Higher surface temperatures of some surface conditions at Union Pass caused 

15 

greater soil temperatures at depths of 2, 8, and 16 inches. 

Mean temperatures (at all depths) were significantly 

warmer below the burned surface than below the litter 

and chip surfaces (fig. 9). Root zone (2- and 8-inch depths) 

temperatures ranged from 58.1 to 54.5 °F under the 

burned surface, 54.3 to 51.8 °F under the litter surface, 

and 50.1 to 45.5 °F under the chips. Temperatures under 

the burned and litter surfaces are probably not different 

enough to cause differences in root growth, plant development, or microorganism activity. The colder temperatures 

under the chips are likely to decrease root growth, transpiration, and.microorganism activity. Temperatures under 

the chips increase much more slowly in the growing season than under the other surfaces. Near-freezing temperatures and frost were often observed in June under the 

chips. Spreading the chips shortened an already short 

growing season at this high elevation. 

Summary 

Temperatures at the ground surface, to depths of 16 

inches, are warmer for burned and litter surface conditions than for mineral and chip surface conditions. The 

results are explained rather well by the energy balance 

equation and the thermal properties of the surfaces. Differences in temperature between the burned and litter 

surfaces seem to be influenced by the rate of vegetation 

recovery. This is consistent with other published results. 

Minimum temperature differences departed from the expected pattern due to slope position that affects cold air 

drainage. Differences in the soil mimic the differences at 

the surface for the surface conditions studied. 

Surface condition does not appear to influence air temperature. Wind movement likely caused enough mixing to 

mask any differences in maximum temperature. Differences in minimum air temperatures are apparently caused 

by differences in slope position. 
REGENERATION RESPONSE 

Assuming that HE's and CE's are events with a potential for causing seedling mortality, there should be a relationship between these events and actual seedling survival. 

For the sites in this study, the relationship between HE's 

and CE's and regeneration success does seem to be strong 

and consistent. 

Lubrecht 

At Lubrecht, the frequency of HE's and CE's (table 8) 

suggests that first-year survival should be lower on the 

clearcut than on the understory removal for the first 2 

years. Douglas-fir and western larch survival (first year) 

for the 1979 planting was from 0 to 8 percent (table 8) 

on the clearcut treatment, and greater than 64 percent on 

the understory removal treatment where no HE's or CE's 

were measured (Schmidt 1984). 

Survival for ponderosa pine was greater than for 

Douglas-fir or western larch at about 50 percent on the 

clearcut and 96 percent on the understory removal treatment. Even though the survival was higher for ponderosa 

pine, the pattern of survival for all species was consistent 

with the expected pattern, based on HE's and CE's. 

First-year survival for seedlings of all species planted in 

1980 was higher than for the 1979 planting, which might 

be expected because the number of HE's and CE 's was 

lower than for 1979. The clearcut had the highest frequency of HE's and CE' s and much lower survival for 

western larch than for the understory removal. Survival 

differences were not as dramatic as the previous year for 

Douglas-fir. Survival of ponderosa pine was not much different for any of the treatments except the close utilization surface in the clearcut. Overall, for all species in both 

years, survival was not different between the broadcast 

burn and close utilization treatment, which is consistent 

with the pattern of HE's and CE's. 

The frequency of HE's during the growing season seems 

~- better explain the pattern of survival for western larch 

than do the CE's. For ponderosa pine, CE's seem to 

better explain the pattern of survival than the HE's. For 

Douglas-fir, HE's and CE's seem to be about equal in explaining the survival pattern. These results are consistent 

with our ideas about the adaptability of these three 

species. Ponderosa pine is known to be more adapted to 

hot-dry sites and thus may survive the HE's better than 

the CE's. Douglas-fir, the intermediate of these species, is 

better adapted to drier sites than is western larch but not 

as well as ponderosa pine, thus possibly being equally 

susceptible to HE's and CE's at Lubrecht. Western larch 

would be expected to be more sensitive to hot-dry sites. 

Thus, the mortality would likely be the result of the HE's 

and related moisture stress before the CE's occurred. 

Coram 

The only two comparisons at Coram that can be made 

are between the burned surface of the broadcast burn 

treatment and the litter surface of the close utilization 

treatment within the clearcut. Seedling survival was high 

(90 percent) on both treatments for all plantings (Shearer 

16 

Table 8-First-year plantation survival data from Lubrecht for 

ponderosa pine, Douglas-fir, and western larch container stock planted on clearcut and understory removal 

treatments. Broadcast burn and close utilization treatments were used in the clearcut. Hot events and cold 

events for these treatments are shown by year. 

Clearcut 

Species 

(percentage survival) 

Ponderosa pine 

Douglas-fir 

Western larch 

Temperature 

(percentage days) 

Hot events 

Cold events 

Species 

(percentage survival) 

Ponderosa pine 

Douglas-fir 

Western larch 

Temperature 

(percentage days) 

Hot events 

Cold events 

Broadcast 

burn1 

1979 

48 

4 

4 

37 

26 

1980 

100 

52 

8 

23 

0 

1Surface condition was burned. 2Surface condition was mostly litter. 

Residue 

removed2 

28 

8 

0 

19 

42 

60 

56 

16 

17 

3 

Understory 

removal2 

96 

76 

64 

0 

0 

84 

72 

68 

0 

0 

1980) of Douglas-fir and Engelmann spruce even though 

HE's were more frequent on the litter surface. Hot events 

on the litter surface were nearly as frequent as at 

Lubrecht, but they were rare on the burned surface after 

the first year. In both treatments at Coram there was 

more vegetation and debris to provide protection for seedlings than at Lubrecht. Germination and survival on the 

seeded spots were higher on the burned surface than on 

the litter surface. The HE's likely had minimal effect on 

survival of germinating seedlings, but insufficient moisture 

was the major cause of mortality (Shearer 1980). However, moisture was less of a limiting factor at Coram than 

at Lubrecht. The absence of CE's seems to have benefited 

seedling survival at Coram. 

Union Pass 

At Union Pass, seedling survival after 5 years was 

92 percent on the broadcast burn treatment, 46 percent on 

the close utilization treatment, and 26 percent on the chip 

spread treatment in the clearcut (Schmidt 1982). Survival 

for each of these treatments is more closely related to 

CE's (table 9). The HE's apparently did not influence survival. The CE 's and survival seem to be related to the 

topographic position of the treatments in a frost pocket 

rather than to the surface conditions themselves. Stocking 

on the seed spots and natural regeneration spots also 

decreased as the frequency of CE's increased. 
Summary 

In general, probability of seedling survival increased 

whenever the frequency .of HE's and CE's decreased. It 

appears likely that CE's caused freezing of plant tissue, 

which contributed to seedling mortality. For HE's, the 

damage may have been a direct heat injury that caused 

death, but it is just as likely that the high temperatures 

caused dessication of the soil, creating moisture stress 

Table 9-Piantation survival data after 5 years (1982) for lodgepole pine at Union Pass, WY, on a clearcut with three 

residue treatments. Hot and cold events are given for 

July 1979. 

Species 

(percentage survival) 

Lodgepole pine 

Temperature 

(percentage days) 

Hot events 

Cold events 

Broadcast 

burn1 

96 

62 

0 

Clearcut 

Close 

utilization 

46 

62 

10 

Chips 

26 

10 

43 

!Temperatures were measured on the burned surface on the broadcast 

burn, litter surface on the residue removed, and the chip surfaGe on the 

chips. 

conditions that led to seedling mortality. Higher radiation 

loads on the seedlings in the clearcut and above some surface treatments, such as the chips, caused the elevated 

surface and soil temperatures and also greater vapor 

pressure deficits (increased LE) and thus greater moisture 

stress in the seedlings. 

Imposing treatment practices to reduce temperature 

variation must be used with caution. Treatments that have 

been shown to reduce temperature variation (exposing 

mineral soil) also increase evaporation and can create a 

moisture deficiency problem. On sites where high temperature is not a problem but moisture is, leaving a mulch 

(litter) or providing one may reduce moisture losses 

(Cochran 1969). Another major factor is the condition of 

planting stock. At Lubrecht, condition of the containerized 

seedlings was quite variable (Shearer 1985). Seedlings that 

are already stressed may be more likely to succumb when 

planted in more stressful situations. Thus, these results do 

not provide proof of the relationship between temperature 

and mortality. But they do provide strong evidence that 

the effects of treatment on site energy balance and thermal properties should be evaluated in relationship to the 

potential for seedling survival. 

CONCLUSIONS AND IMPLICATIONS 

FOR MANAGERS 

Several points from this and other studies have a bearing on surface temperature modification and successful 

regeneration of stands. 

1. It is crucial to be able to identify potential mortality 

problems due to temperature and moisture at the time 

17 

silvicultural prescriptions are prepared. If problems are expected, steps can be taken to avoid problems by altering 

silvicultural and subsequent reforestation practices. 

2. Current reforestation guides indicate that high temperatures are a problem on south and west aspects on 

slopes over 30 percent at lower elevations. In this study, 

potentially lethal high temperatures were also observed on 

level ground, on east-facing slopes, and at high elevations 

on clearcuts in Douglas-fir and subalpine fir habitat types. 

Potentially lethal cold temperatures were measured on 

level sites at high and low elevations in both habitat 

series. The pattern of seedling survival on these sites indicates that mortality was in fact related to the hot and 

cold temperature events. 

3. Results of this study show that shelterwood or partial 

cuttings with 50 percent or less sunlight transmission to 

the forest floor significantly reduce the occurrence of high 

temperatures and low temperatures. Seedling survival was 

higher in these stands. 

4. Seedbed preparation and residue treatments for slash 

reduction significantly influence the occurrence of potentially lethal seedbed surface temperatures by altering the 

thermal properties of the surface. In general, practices 

that increase the thermal conductivity and volumetric heat 

capacity (thus, VKC-thermal contact coefficient) of the 

surface materials will decrease maximum temperatures 

and increase minimum temperatures. Treatments that expose mineral soil reduce surface temperature variation 

(decrease maximum and increase minimum) compared to 

natural litter-covered surfaces. Burned and natural littercovered surfaces on clearcuts may be equally susceptible 

to high temperatures, based on results reported here. 

However, this may vary depending on the thermal properties of the surface materials. 

5. Where it is not practical to alter the surface 

materials (thermal properties), excessively high and low 

temperatures can be reduced by providing shade. Leaving 

enough residues onsite to provide adequate shade protection will reduce temperatures and increase seedling survival. Residues can also conserve moisture at the surface 

by reducing evaporation. Allowing some vegetation growth 

(provided moisture is not limiting) will also provide adequate shade and thus temperature modification. Seedlings 

planted on these sites should be positioned to take advantage of the modified microsites-on the north or east sides 

of logs, stumps, branches, rocks, etc. 

6. On sites where soil moisture is more of a problem 

than is high temperature, mulches (plant litter, artificial 

materials; dry, loosened soil; etc.) left on the surface can 

help retard evaporation and thus increase survival. However, these practices must be planned carefully because 

they will increase surface temperatures. 

7. Frost-pocket problems can be aggravated by overstory and residue treatment. The ability to identify potential frost pockets while developing the silvicultural prescription is important. Subalpine fir in drainages or flat 

areas or Vaccinium caespitosum in the understory are 

often good indicators of potential frost pockets. 

8. Shade from partial overstories, residue, or understory 

vegetation can provide varying degrees of protection for 

seedlings in frost pockets. Only the most frost-resistant 

species should be used in frost-pocket areas. 
9. Clearcuts and other cutting units should be laid out 

to provide for adequate air drainage through a cutting 

unit, if it is not the lowest depression. This will minimize 

cutting-related frost pockets. Smaller cutting units (group 

selections) will reduce radiational cooling and frost problems on sites that may have potential problems. To be effective, widths of strips or patches should be less than two 

tree (border tree) heights wide. 

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19p. 

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N1-N18. 

18 

Fowler, W. B.; Helvey, J. D. Soil and air temperature 

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2d ed. New York: Academic Press; 1980. 497 p. 

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Department of Agriculture, Forest Service, Intermountain Research Station, Forestry Sciences Laboratory, 

Missoula, MT. 

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In: Benson, Robert E. Management consequences of 

alternative harvesting and residue treatment practices-
lodgepole pine. General Technical Report INT-132. 

Ogden, UT: U.S. Department of Agriculture, Forest 

Service, Intermountain Forest and Range Experiment 

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Shearer, Raymond C. Insolation limits initial establishment 

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Service, Intermountain Forest and Range Experiment 

Station; 1967. 8 p. 

Shearer, Raymond C. Regeneration establishment in 

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consequences of timber harvesting in Rocky Mountain 

coniferous forests: Proceedings of the symposium; 1979 

September 11-13; Missoula, MT. General Technical 

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Shearer, Raymond C. [Personal communication]. 1985. 

U.S. Department of Agriculture, Forest Service, Intermountain Research Station, Forestry Sciences 

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Silen, Roy R. Lethal surface· temperatures and their interpretation for Douglas-fir. Corvallis, OR: Oregon State 

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INT-293. Ogden, UT: U.S. Department of Agriculture, 

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Steele, Robert; Cooper, Stephen V.; Ondov,.David M.; [and 

others]. Forest habitat types of eastern Idaho-western 

Wyoming. General Technical Report INT-144. Ogden, 

UT: U.S. Department of Agriculture, Forest Service, 

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1983. 122 p. 

Strothman, Rudolph 0. Douglas-fir in northern California: 

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-f:r U.S. GOVERNMENT PRINTING OFFICE:1987- 773-039/41039 19 
.~ .. · 
"l I 

Hungerford, Roger D.; Babbitt, Ronald E. Overstory removal and residue treatments 

affect soil surface, air, and soil temperature: implications for seedling survival. 

Research Paper INT-377. Ogden, UT: U.S. Department of Agriculture, Forest Service, Intermountain Research Station; 1987.19 p. 

Potentially lethal ground surface temperatures were measured at three locations in 

the Northern Rocky Mountains but occurred more frequently under treatments with 

greater overstory removal. Observed maximum and minimum temperatures of exposed surfaces are directly related to the thermal properties of the surface materials. 

Survival of planted seedlings was consistent with the pattern of potentially lethal 

temperatures. Care in manipulating overstory canopies and residues can enhance the 

potential for tree seedling survival. 

KEYWORDS: temperature, seedling survival, timber harvesting, thermal properties 
l 

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