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A comparison of urban and forest microclimates in the midwestern United States
Agricultural Meteorology, 14(1975) 335--345 © Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
A COMPARISON OF URBAN AND FOREST MICROCLIMATES IN THE MIDWESTERN UNITED STATES
F O R R E S T L. JOHNSON,* DAVID T. BELL and STANLEY K. SIPP
Department of Forestry, University of Illinois, Urbana, Ill. (U.S.A.) (Received June 28, 1974; accepted November 25, 1974)
ABSTRACT Johnson, F. L., Bell, D. T. and Sipp, S. K., 1975. A comparison of urban and forest microclimates in the midwestern United States. Agric. Meteorol., 14: 335--345. Weather data for 1973 from seven deciduous forest sites were compared with corresponding data from two nearby urban sites in central Illinois. The forest air and soil temperatures, rainfall, and wind travel were found to be significantly lower throughout the year. Daily temperature ranges were lower in the forest during the period when leaves were present on the trees. Most of the differences between urban and forest microclimates result from structural characteristics of the forest vegetation and their effects on radiation interception, rainfall interception, and reduction of wind velocities and from the physical characteristics of cities which result in greater absorption of solar radiation and retention of heat. INTRODUCTION
Climate is one of the principal environmental factors influencing the occurrence, activity, and interaction of biological organisms. Climatic variables impose many of the primary tolerance limits in the selection of species capable of surviving in a certain area. Edaphic, topographic, and biotic factors influence the climatic variables of light, temperature, humidity, and air movement and act to further delimit the occurrence and distribution of these species within a geographic region. The vegetation types which develop, in turn, strongly influence the microclimate which is found within these vegetation types. Any ecological study must include a description of the climatic variables which affect the biological organisms under consideration. Urban weather stations are often the only convenient source of climatic data. These data, however, are sometimes a misleading representation of the microclimate of a specific ecosystem. Urban microclimates have been measured and discussed since the early 19th century (see Geiger, 1965; and Landsberg, 1956, 1970). Forest micro* Corresponding author.
336 climates, however, have received less attention (Geiger, 1965), and direct comparisons between urban and forest microclimates are seldom made. The current report points to areas of difference occurring in the climatic variables of solar radiation, air temperature, soil temperature, wind, and precipitation measured in urban environments and in the forested ecosystem of the Sangamon River basin in east-central Illinois. AREA DESCRIPTION The climate of the Sangamon River basin in east-central Illinois within 40 km of 40 ° 00'N, 88 ° 30'W has been described as temperate continental (Trewartha, 1968) with cold winters, warm summers, and frequent shortperiod fluctuations in temperature, humidity, cloudiness, and wind direction (Environmental Science Service Administration, 1969). January is normally the coldest month, with a mean temperature at Urbana, Illinois of -3.1°C. July usually has the highest temperatures, with a mean temperature of 23.6 ° C. Precipitation averages 941 mm annually and is distributed fairly evenly throughout the year. Regional elevations range between 180 and 235 m above sea level. Physiographically, east-central Illinois lies in the Till Plain Section of the Central Lowland Province (Hunt, 1974) and consists of broad level uplands between river valleys which have steep sides and wide floodplains. Presettlement vegetation of the area was primarily tall-grass prairie with forested areas along stream courses and occasional large detached prairie groves (Bailey et al., 1964; Geis and Boggess, 1968). Most of the prairie areas are now cultivated. Upland forest in the area is of the oak--hickory type (Braun, 1950) and is dominated by Quercus alba, with Q. rubra, Q. velutina, Carya ovata, C. tomentosa, Ulmus rubra, and U. americana as subdominants. The river floodplains are covered with forests dominated by Acer saccharinum, with Fraxinus pennsylvanica, Celtis occidentalis, Quercus macrocarpa, and Q. imbricaria as sub-dominants (Bell, 1974). URBAN WEATHER STATIONS Regional weather data are recorded at two National Weather Service offices, Urbana and Decatur, Ill., which have provided climatic data since 1889 and 1921, respectively (Page, 1949). Urbana and its contiguous neighbor Champaign, have a total population of 87,000 and Decatur has a population of 89,000. Both cities are on open relatively level upland with no unusual topographic features nearby. Urbana is at an elevation of 235 m, while Decatur is at 210 m. Neither city is significantly affected by air pollution. The urban weather data are collected from stations located in small open areas within the city. The Decatur station is surrounded by a residential neighborhood and the Urbana station is on the University of Illinois campus. Each station records information on daily maximum, minimum, and mean
337 air temperature and precipitation. Urbana also records data on soil temperature at 10- and 20-cm depths, solar radiation and wind travel. FOREST WEATHER STATIONS The forest microclimate is monitored at seven stations along the 50-km section of the Sangamon River between Urbana and Decatur. Four stations are located under the open canopy of the floodplain forest and three are located under the denser canopy of the upland forest above the floodplain. Each station consists of a recording hygrothermograph, a recording rain gage, recording pyranometer, totalizing anemometer, and a recording three-point thermograph for soil temperatures at the surface, 10- and 20-cm depths. The hygrothermograph and rain gage recorders are m o u n t e d inside a standard U.S. Weather Bureau shelter. The soil thermograph recorder, rain gage, and pyranometer are m o u n t e d on a platform adjacent to the shelter and the anemometer is m o u n t e d on the roof of the shelter. The floodplain stations are placed on 2.5-m towers to protect the instruments from floodwaters. The 1973 continuous record of temporal variations in air temperature, soil temperature, relative humidity, solar radiation, and precipitation provide forest weather data for comparison to the corresponding urban weather data from Urbana and Decatur. RESULTS Incident solar radiation Monthly totals of incident solar radiation near the ground for the Urbana station, the mean of four floodplain forest stations, and the mean of three upland forest stations are shown in Fig.1. The urban incident solar radiation curv~ is typical for this region, showing a maximum of 16,400 cal. cm-2 in July and a minimum of 3,600 cal. cm-z in December. The incident radiation at ground level is consistently higher in the floodplain forest compared to the upland forest. Maximum ground-level radiation occurred in April with values of 5,700 cal. cm-2 for the upland forest and 6,500 cal. crn-2 in the floodplain forest. Floodplain and upland forest radiation minima occurred in December and were 2,100 and 1,800 cal. crn-2, respectively. If the assumption is made that incident radiation above the tree canopy was the same for that period as that measured at the Urbana station, interception by the canopy was consistently greater in the upland forest. Maximum interception occurred in September, with 84% for the upland forest and 74% for the floodplain forest. Air temperature Mean m o n t h l y air temperatures measured in the forest environment were significantly lower (P > 99.5%) t h r o u g h o u t 1973 when compared to the
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Fig.1. Monthly totals of incident radiation in 1973 for Urbana (solid line), mean of four floodplain forest stations (long dash), and mean of three upland forest stations (short dash).
urban environment (Fig.2). Long-term mean annual difference between the two urban stations is 0.8°C and the yearly mean temperature is 11.9°C. The mean temperature for 1973 was 12.4°C for the two urban stations and 9.7°C for the seven forest stations. Mean July temperature was 21.9°C in the forest and 24.7°C in the cities. Mean January temperature was --3.1°C in the forest environment and --0.9°C in the urban environment. During all months the mean air temperatures in the floodplain forest were slightly higher than those in the upland forest. Mean monthly maximum and minimum air temperatures were both significantly higher (P > 99.5%) in the urban environment than in the upland forest (Fig.3). Minima and maxima in the floodplain forest were slightly but consistently higher (P > 95%) than those in the upland forest. Differences in maximum temperatures between city and forest environments were least in the spring (1.7°C in April) and greatest in late summer (4.7°C in August and September). Differences in minimum temperatures were least in late spring and summer (0.7°C in June) and greatest in early winter (3.3°C in December). Monthly mean daily air temperature ranges for the urban and the two for-
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est environments are shown in Fig.4. Daily temperature ranges were significantly lower (P > 99.5%) in both forest types than in the city during the period June--September. The upland forest also had a smaller range (P > 97.5%) during this period than the floodplain forest. Differences in temperature range between the three environments were not significant during the period of tree dormancy. Daffy temperature ranges in the urban environment were greatest in summer and least in winter, with a mean range of 11.9°C in June imd 8.1°C in December. Both forest types had ranges of about 9.8 ° 10.5°C in spring and fall and smaller ranges of about 7.8°--9.0°C in both summer and winter.
Soil temperature Soil temperatures at Urbana were compared with mean soil temperatures at three upland forest stations (Fig.5). Monthly mean soil temperatures at a depth of 10 cm averaged 4.1°C lower (P > 99.5%) in the upland forest than in the city. In the upland forest, the minimum of--0.8°C was observed in
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Fig.3. Differences in mean monthly maximum temperatures (solid line) and mean monthly minimum temperatures (dashed line) between two city and three upland forest stations. Temperatures were lower in the forest in all cases.
February and the maximum of 21.1°C occurred in July. At the urban station, the minimum o f --0.3°C was observed in January and the maximum of 25.2°C in July and August. Differences between soil temperatures in the upland and floodplain forest were not significant.
Wind Omnidirectional wind travel distance for the seven forest stations is shown in Fig.6. Maximum wind travel was observed in the forest in March. Almost no wind travel was recorded in the forest during the period of June--October.
Rainfall Long term mean annual precipitation for the urban weather stations averages 941 mm with a mean annual difference between the two city stations of 3 mm. Yearly precipitation for 1973 averaged 1,324 mm for the t w o urban
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Fig.4. Monthly mean daily temperature ranges for two city stations (solid line), four floodplain forest stations (long dash), and three upland forest stations (short dash).
stations and 1,003 mm for the seven forest stations. Monthly precipitation was consistently greater (P > 97.5%) for the city stations, with the greatest differences occurring in the period June--October. DISCUSSION
Differences in microclimate between the city and the forest relate most strongly to the modification of the environmental parameters by forest structure. The interception of solar energy, precipitation, and the recorded wind distance is strongly regulated by the phenological status of the forest canopy. The tree species of both forest associations are entirely deciduous. Leaves begin to appear on the trees in early May and both forest types are in full leaf by early June. A full leaf canopy is present until October when leaf senescence begins. Some dead leaves remain in the canopy until late December. Incident radiation near ground level is at its m a x i m u m in the forest in April (Fig.l) just prior to canopy development. Higher values for solar radiation interception in the upland forest are due to the fact that the forest surrounding the upland stations is more dense than that surrounding
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Fig.5. Mean monthly soil temperatures at a depth of 10 cm for Urbana (solid line) and three upland forest stations (dashed line).
the floodplain stations. The upland forest stations are situated in areas with an average of 140 trees ha-1 with a basal area of 27.4 m 2 ha-1 while the floodplain forest areas average 83 trees ha-1 with a basal area of 20.4 m 2 ha -~ . Use of solar radiation data from Urbana as incident radiation above the nearby forest canopy is justified because there is very little, if any, reduction of incident radiation in the city due to air pollution. The larger 1973 total solar radiation at Urbana for July than for June is a result of more cloud cover in June. In the following discussion, emphasis will be placed on differences between the city and upland forest environmental data. The microclimate of the floodplain forest is somewhat intermediate between that of the city and the upland forest. The a m o u n t of radiation intercepted by the upland forest canopy accounts for 72% of the variance (P > 99%) in the difference between m a x i m u m temperatures (Fig.3) in the forest environment and the city environment. The large differences in m a x i m u m temperature observed in the summer and fall are, therefore, probably due to the density of the leaves in the tree crowns. Interception of radiation by the canopy, however, accounts for only 5% of the variance in the difference between minimum temperatures (Fig.3). The
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Fig.6. Monthly wind travel, average of seven forest stations.
differences in minimum temperatures, which are greatest in the winter, are probably due to factors such as increased heat absorption b y streets and buildings and local heat production in the city (Landsberg, 1970). Daffy temperature ranges (Fig.4) are strongly influenced b y incident radiation at ground level. Incident radiation accounts for 76% o f the variance (P > 99%) in daily temperature ranges at the Urbana station. Interception of radiant energy by the forest canopy accounts for 69% of the variance (P > 99%) in the difference between daily temperature ranges in the upland forest and the urban locality. Soil temperatures are also strongly influenced by incident solar radiation reaching the ground. The amount of incident radiation intercepted by the upland forest canopy accounts for 71% of the variance (P > 99%) in differences in soil temperature (Fig.5) between city and upland forest stations. Soil temperature at the urban station was always higher than in the forest, b u t differences were greater during the summer and fall when interception by the forest canopy was greatest. The correlation between wind travel (Fig.6) and interception of solar radiation at the seven forest stations is --0.918, accounting for 84% of the
344 variance. This correlation does not imply that the two variables are interdependent but that both are partially dependent on the presence or absence of leaves in the tree canopy. Interception of radiation is high and wind travel is low during the period June--October when leaves are present on the trees. Studies reported by Geiger (1965) indicate that wind velocities outside a forest are usually from 4 to 10 times greater than those measured under the tree canopy. The approximately 100-fold difference measured between urban and forest locations in this study may be slightly high. The influence of the forest on the environmental parameters of relative humidity and interception of precipitation were not analyzed in the present study. Urban weather station data on relative humidity were not available for the full 1973 year and a quantitative analysis of rainfall interception requires a much larger number of rain gages (Czarnowski and Olszewski, 1968). Comparisons made here indicate that most differences between urban and forest microclimates are a result of the structural characteristics of the forest vegetation and their effects on radiation interception, rainfall interception, reduction of wind velocities, and the altered heat-absorption characteristics of the city land surface. In using urban weather station data to describe the physical environment of forested ecosystems, where direct measurements are not available, the substitution should only be made with the realization that compensations should be made for the differences in the two environments. ACKNOWLEDGEMENTS The work reported in the paper is supported by funds under the auspices of the Illinois Agricultural Experiment Station from the Illinois State Division of Waterways and the U.S. Army Corps of Engineers. This study is a contribution of the Springer-Sangamon Environmental Research Program, which is designed as a multi-disciplinary research program to investigate the structure and function of streamside and aquatic ecosystems of the Sangamon River basin and to determine impacts of water resource management projects on the ecosystems of the area.
REFERENCES Bailey, L. W., Odell R. T. and Boggess, W. R., 1964. Properties o f selected soils developed near the forest--prairie border in east-central Illinois. Soil Sci. Soc. Am. Proc., 28: 257-263 Bell, D. T., 1974. Tree stratum composition and distribution in the streamside forest. Am. Midl. Nat., 9 2 : 3 5 - - 4 6 Braun, E. L., 1950 Deciduous Forests of Eastern North America. Hafner, New York, N.Y., 596 pp.
345 Czarnowski, M. S. and Olszewski, J. L., 1968. Rainfall interception by a forest canopy. Oikos, 1 9 : 3 4 5 - - 3 5 0 Environmental Science Service Administration, 1969. Climates of the States -- Illinois. U. S. Department of Commerce, Washington, D.C., 19 pp. Geiger, R., 1965. The Climate Near the Ground. Harvard University, Cambridge, Mass., 611 pp. Gels, J. W. and Boggess, W. R., 1968. The prairie peninsula: its origin and significance in the vegetational history of central Illinois. The Quaternary of Illinois. Univ. Ill. Coll. Agric. Spec. Publ., 1 4 : 8 9 - - 9 5 Hunt, C. B., 1974. Natural Regions of the United States and Canada. Freeman, San Francisco, Calif., 725 pp. Landsberg, H. E., 1956. The climate of towns. In: W. L. Thomas (Editor), Man's Role in Changing the Face of the Earth. University of Chicago, Chicago, Ill., pp. 584--606 Landsberg, H. E., 1970. Man-made climatic changes. Science, 1 7 0 : 1 2 6 5 - - 1 2 7 4 Page, J. L., 1949. Climate of Illinois. Univ. Ill. Agric. Exp. Stn. Bull., 532, 342 pp. Trewartha, G. T., 1968. An Introduction to Climate. McGraw-Hill, New York, N.Y., 408 pp.
A COMPARISON OF URBAN AND FOREST MICROCLIMATES IN THE MIDWESTERN UNITED STATES
F O R R E S T L. JOHNSON,* DAVID T. BELL and STANLEY K. SIPP
Department of Forestry, University of Illinois, Urbana, Ill. (U.S.A.) (Received June 28, 1974; accepted November 25, 1974)
ABSTRACT Johnson, F. L., Bell, D. T. and Sipp, S. K., 1975. A comparison of urban and forest microclimates in the midwestern United States. Agric. Meteorol., 14: 335--345. Weather data for 1973 from seven deciduous forest sites were compared with corresponding data from two nearby urban sites in central Illinois. The forest air and soil temperatures, rainfall, and wind travel were found to be significantly lower throughout the year. Daily temperature ranges were lower in the forest during the period when leaves were present on the trees. Most of the differences between urban and forest microclimates result from structural characteristics of the forest vegetation and their effects on radiation interception, rainfall interception, and reduction of wind velocities and from the physical characteristics of cities which result in greater absorption of solar radiation and retention of heat. INTRODUCTION
Climate is one of the principal environmental factors influencing the occurrence, activity, and interaction of biological organisms. Climatic variables impose many of the primary tolerance limits in the selection of species capable of surviving in a certain area. Edaphic, topographic, and biotic factors influence the climatic variables of light, temperature, humidity, and air movement and act to further delimit the occurrence and distribution of these species within a geographic region. The vegetation types which develop, in turn, strongly influence the microclimate which is found within these vegetation types. Any ecological study must include a description of the climatic variables which affect the biological organisms under consideration. Urban weather stations are often the only convenient source of climatic data. These data, however, are sometimes a misleading representation of the microclimate of a specific ecosystem. Urban microclimates have been measured and discussed since the early 19th century (see Geiger, 1965; and Landsberg, 1956, 1970). Forest micro* Corresponding author.
336 climates, however, have received less attention (Geiger, 1965), and direct comparisons between urban and forest microclimates are seldom made. The current report points to areas of difference occurring in the climatic variables of solar radiation, air temperature, soil temperature, wind, and precipitation measured in urban environments and in the forested ecosystem of the Sangamon River basin in east-central Illinois. AREA DESCRIPTION The climate of the Sangamon River basin in east-central Illinois within 40 km of 40 ° 00'N, 88 ° 30'W has been described as temperate continental (Trewartha, 1968) with cold winters, warm summers, and frequent shortperiod fluctuations in temperature, humidity, cloudiness, and wind direction (Environmental Science Service Administration, 1969). January is normally the coldest month, with a mean temperature at Urbana, Illinois of -3.1°C. July usually has the highest temperatures, with a mean temperature of 23.6 ° C. Precipitation averages 941 mm annually and is distributed fairly evenly throughout the year. Regional elevations range between 180 and 235 m above sea level. Physiographically, east-central Illinois lies in the Till Plain Section of the Central Lowland Province (Hunt, 1974) and consists of broad level uplands between river valleys which have steep sides and wide floodplains. Presettlement vegetation of the area was primarily tall-grass prairie with forested areas along stream courses and occasional large detached prairie groves (Bailey et al., 1964; Geis and Boggess, 1968). Most of the prairie areas are now cultivated. Upland forest in the area is of the oak--hickory type (Braun, 1950) and is dominated by Quercus alba, with Q. rubra, Q. velutina, Carya ovata, C. tomentosa, Ulmus rubra, and U. americana as subdominants. The river floodplains are covered with forests dominated by Acer saccharinum, with Fraxinus pennsylvanica, Celtis occidentalis, Quercus macrocarpa, and Q. imbricaria as sub-dominants (Bell, 1974). URBAN WEATHER STATIONS Regional weather data are recorded at two National Weather Service offices, Urbana and Decatur, Ill., which have provided climatic data since 1889 and 1921, respectively (Page, 1949). Urbana and its contiguous neighbor Champaign, have a total population of 87,000 and Decatur has a population of 89,000. Both cities are on open relatively level upland with no unusual topographic features nearby. Urbana is at an elevation of 235 m, while Decatur is at 210 m. Neither city is significantly affected by air pollution. The urban weather data are collected from stations located in small open areas within the city. The Decatur station is surrounded by a residential neighborhood and the Urbana station is on the University of Illinois campus. Each station records information on daily maximum, minimum, and mean
337 air temperature and precipitation. Urbana also records data on soil temperature at 10- and 20-cm depths, solar radiation and wind travel. FOREST WEATHER STATIONS The forest microclimate is monitored at seven stations along the 50-km section of the Sangamon River between Urbana and Decatur. Four stations are located under the open canopy of the floodplain forest and three are located under the denser canopy of the upland forest above the floodplain. Each station consists of a recording hygrothermograph, a recording rain gage, recording pyranometer, totalizing anemometer, and a recording three-point thermograph for soil temperatures at the surface, 10- and 20-cm depths. The hygrothermograph and rain gage recorders are m o u n t e d inside a standard U.S. Weather Bureau shelter. The soil thermograph recorder, rain gage, and pyranometer are m o u n t e d on a platform adjacent to the shelter and the anemometer is m o u n t e d on the roof of the shelter. The floodplain stations are placed on 2.5-m towers to protect the instruments from floodwaters. The 1973 continuous record of temporal variations in air temperature, soil temperature, relative humidity, solar radiation, and precipitation provide forest weather data for comparison to the corresponding urban weather data from Urbana and Decatur. RESULTS Incident solar radiation Monthly totals of incident solar radiation near the ground for the Urbana station, the mean of four floodplain forest stations, and the mean of three upland forest stations are shown in Fig.1. The urban incident solar radiation curv~ is typical for this region, showing a maximum of 16,400 cal. cm-2 in July and a minimum of 3,600 cal. cm-z in December. The incident radiation at ground level is consistently higher in the floodplain forest compared to the upland forest. Maximum ground-level radiation occurred in April with values of 5,700 cal. cm-2 for the upland forest and 6,500 cal. crn-2 in the floodplain forest. Floodplain and upland forest radiation minima occurred in December and were 2,100 and 1,800 cal. crn-2, respectively. If the assumption is made that incident radiation above the tree canopy was the same for that period as that measured at the Urbana station, interception by the canopy was consistently greater in the upland forest. Maximum interception occurred in September, with 84% for the upland forest and 74% for the floodplain forest. Air temperature Mean m o n t h l y air temperatures measured in the forest environment were significantly lower (P > 99.5%) t h r o u g h o u t 1973 when compared to the
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Fig.1. Monthly totals of incident radiation in 1973 for Urbana (solid line), mean of four floodplain forest stations (long dash), and mean of three upland forest stations (short dash).
urban environment (Fig.2). Long-term mean annual difference between the two urban stations is 0.8°C and the yearly mean temperature is 11.9°C. The mean temperature for 1973 was 12.4°C for the two urban stations and 9.7°C for the seven forest stations. Mean July temperature was 21.9°C in the forest and 24.7°C in the cities. Mean January temperature was --3.1°C in the forest environment and --0.9°C in the urban environment. During all months the mean air temperatures in the floodplain forest were slightly higher than those in the upland forest. Mean monthly maximum and minimum air temperatures were both significantly higher (P > 99.5%) in the urban environment than in the upland forest (Fig.3). Minima and maxima in the floodplain forest were slightly but consistently higher (P > 95%) than those in the upland forest. Differences in maximum temperatures between city and forest environments were least in the spring (1.7°C in April) and greatest in late summer (4.7°C in August and September). Differences in minimum temperatures were least in late spring and summer (0.7°C in June) and greatest in early winter (3.3°C in December). Monthly mean daily air temperature ranges for the urban and the two for-
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est environments are shown in Fig.4. Daily temperature ranges were significantly lower (P > 99.5%) in both forest types than in the city during the period June--September. The upland forest also had a smaller range (P > 97.5%) during this period than the floodplain forest. Differences in temperature range between the three environments were not significant during the period of tree dormancy. Daffy temperature ranges in the urban environment were greatest in summer and least in winter, with a mean range of 11.9°C in June imd 8.1°C in December. Both forest types had ranges of about 9.8 ° 10.5°C in spring and fall and smaller ranges of about 7.8°--9.0°C in both summer and winter.
Soil temperature Soil temperatures at Urbana were compared with mean soil temperatures at three upland forest stations (Fig.5). Monthly mean soil temperatures at a depth of 10 cm averaged 4.1°C lower (P > 99.5%) in the upland forest than in the city. In the upland forest, the minimum of--0.8°C was observed in
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Fig.3. Differences in mean monthly maximum temperatures (solid line) and mean monthly minimum temperatures (dashed line) between two city and three upland forest stations. Temperatures were lower in the forest in all cases.
February and the maximum of 21.1°C occurred in July. At the urban station, the minimum o f --0.3°C was observed in January and the maximum of 25.2°C in July and August. Differences between soil temperatures in the upland and floodplain forest were not significant.
Wind Omnidirectional wind travel distance for the seven forest stations is shown in Fig.6. Maximum wind travel was observed in the forest in March. Almost no wind travel was recorded in the forest during the period of June--October.
Rainfall Long term mean annual precipitation for the urban weather stations averages 941 mm with a mean annual difference between the two city stations of 3 mm. Yearly precipitation for 1973 averaged 1,324 mm for the t w o urban
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stations and 1,003 mm for the seven forest stations. Monthly precipitation was consistently greater (P > 97.5%) for the city stations, with the greatest differences occurring in the period June--October. DISCUSSION
Differences in microclimate between the city and the forest relate most strongly to the modification of the environmental parameters by forest structure. The interception of solar energy, precipitation, and the recorded wind distance is strongly regulated by the phenological status of the forest canopy. The tree species of both forest associations are entirely deciduous. Leaves begin to appear on the trees in early May and both forest types are in full leaf by early June. A full leaf canopy is present until October when leaf senescence begins. Some dead leaves remain in the canopy until late December. Incident radiation near ground level is at its m a x i m u m in the forest in April (Fig.l) just prior to canopy development. Higher values for solar radiation interception in the upland forest are due to the fact that the forest surrounding the upland stations is more dense than that surrounding
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Fig.5. Mean monthly soil temperatures at a depth of 10 cm for Urbana (solid line) and three upland forest stations (dashed line).
the floodplain stations. The upland forest stations are situated in areas with an average of 140 trees ha-1 with a basal area of 27.4 m 2 ha-1 while the floodplain forest areas average 83 trees ha-1 with a basal area of 20.4 m 2 ha -~ . Use of solar radiation data from Urbana as incident radiation above the nearby forest canopy is justified because there is very little, if any, reduction of incident radiation in the city due to air pollution. The larger 1973 total solar radiation at Urbana for July than for June is a result of more cloud cover in June. In the following discussion, emphasis will be placed on differences between the city and upland forest environmental data. The microclimate of the floodplain forest is somewhat intermediate between that of the city and the upland forest. The a m o u n t of radiation intercepted by the upland forest canopy accounts for 72% of the variance (P > 99%) in the difference between m a x i m u m temperatures (Fig.3) in the forest environment and the city environment. The large differences in m a x i m u m temperature observed in the summer and fall are, therefore, probably due to the density of the leaves in the tree crowns. Interception of radiation by the canopy, however, accounts for only 5% of the variance in the difference between minimum temperatures (Fig.3). The
343
1
~
3
4
5
] 7
5
i ~
i
9
I0
111
118
~m~h
Fig.6. Monthly wind travel, average of seven forest stations.
differences in minimum temperatures, which are greatest in the winter, are probably due to factors such as increased heat absorption b y streets and buildings and local heat production in the city (Landsberg, 1970). Daffy temperature ranges (Fig.4) are strongly influenced b y incident radiation at ground level. Incident radiation accounts for 76% o f the variance (P > 99%) in daily temperature ranges at the Urbana station. Interception of radiant energy by the forest canopy accounts for 69% of the variance (P > 99%) in the difference between daily temperature ranges in the upland forest and the urban locality. Soil temperatures are also strongly influenced by incident solar radiation reaching the ground. The amount of incident radiation intercepted by the upland forest canopy accounts for 71% of the variance (P > 99%) in differences in soil temperature (Fig.5) between city and upland forest stations. Soil temperature at the urban station was always higher than in the forest, b u t differences were greater during the summer and fall when interception by the forest canopy was greatest. The correlation between wind travel (Fig.6) and interception of solar radiation at the seven forest stations is --0.918, accounting for 84% of the
344 variance. This correlation does not imply that the two variables are interdependent but that both are partially dependent on the presence or absence of leaves in the tree canopy. Interception of radiation is high and wind travel is low during the period June--October when leaves are present on the trees. Studies reported by Geiger (1965) indicate that wind velocities outside a forest are usually from 4 to 10 times greater than those measured under the tree canopy. The approximately 100-fold difference measured between urban and forest locations in this study may be slightly high. The influence of the forest on the environmental parameters of relative humidity and interception of precipitation were not analyzed in the present study. Urban weather station data on relative humidity were not available for the full 1973 year and a quantitative analysis of rainfall interception requires a much larger number of rain gages (Czarnowski and Olszewski, 1968). Comparisons made here indicate that most differences between urban and forest microclimates are a result of the structural characteristics of the forest vegetation and their effects on radiation interception, rainfall interception, reduction of wind velocities, and the altered heat-absorption characteristics of the city land surface. In using urban weather station data to describe the physical environment of forested ecosystems, where direct measurements are not available, the substitution should only be made with the realization that compensations should be made for the differences in the two environments. ACKNOWLEDGEMENTS The work reported in the paper is supported by funds under the auspices of the Illinois Agricultural Experiment Station from the Illinois State Division of Waterways and the U.S. Army Corps of Engineers. This study is a contribution of the Springer-Sangamon Environmental Research Program, which is designed as a multi-disciplinary research program to investigate the structure and function of streamside and aquatic ecosystems of the Sangamon River basin and to determine impacts of water resource management projects on the ecosystems of the area.
REFERENCES Bailey, L. W., Odell R. T. and Boggess, W. R., 1964. Properties o f selected soils developed near the forest--prairie border in east-central Illinois. Soil Sci. Soc. Am. Proc., 28: 257-263 Bell, D. T., 1974. Tree stratum composition and distribution in the streamside forest. Am. Midl. Nat., 9 2 : 3 5 - - 4 6 Braun, E. L., 1950 Deciduous Forests of Eastern North America. Hafner, New York, N.Y., 596 pp.
345 Czarnowski, M. S. and Olszewski, J. L., 1968. Rainfall interception by a forest canopy. Oikos, 1 9 : 3 4 5 - - 3 5 0 Environmental Science Service Administration, 1969. Climates of the States -- Illinois. U. S. Department of Commerce, Washington, D.C., 19 pp. Geiger, R., 1965. The Climate Near the Ground. Harvard University, Cambridge, Mass., 611 pp. Gels, J. W. and Boggess, W. R., 1968. The prairie peninsula: its origin and significance in the vegetational history of central Illinois. The Quaternary of Illinois. Univ. Ill. Coll. Agric. Spec. Publ., 1 4 : 8 9 - - 9 5 Hunt, C. B., 1974. Natural Regions of the United States and Canada. Freeman, San Francisco, Calif., 725 pp. Landsberg, H. E., 1956. The climate of towns. In: W. L. Thomas (Editor), Man's Role in Changing the Face of the Earth. University of Chicago, Chicago, Ill., pp. 584--606 Landsberg, H. E., 1970. Man-made climatic changes. Science, 1 7 0 : 1 2 6 5 - - 1 2 7 4 Page, J. L., 1949. Climate of Illinois. Univ. Ill. Agric. Exp. Stn. Bull., 532, 342 pp. Trewartha, G. T., 1968. An Introduction to Climate. McGraw-Hill, New York, N.Y., 408 pp.