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How Big Microclimate?

An Ohio Story

Most of our CED ecosystem controls on climate items have focused on regional and macroclimate impacts. In my view the best big microclimate change due to vegetation paper is by Edgar N. Transeau (1943) in The Transactions of the American Geophysical Union pages 154-166. Transeau was interested in the richness of floras in deep valleys and made the case for atmospheric controls on biodiversity. As we will see vegetation control of the atmosphere is also real.

Transeau instrumented a small valley in the Hocking Hills about 50 miles SE of Columbus, Ohio. The site is a cove called Neotoma. Not many valleys are named after a simple rat. In addition to the native flora of southern Ohio there are species of a more southern climate, of glacial climates and or prairie climates. Neotoma is a refugia of several past climate episodes.

Minimum Temperatures

Within the one half mile long cove the lowest temperature recorded in a frost pocket was -25 F for the winter of 1942-1943. In the same year in a leaf-litter near the covehead, the lowest temperature recorded was above freezing (32.5 F). Within the cove, soils with a good litter didn't freeze that year. The minimum temperature range that year within this little cove was 57.5 F! That is a big microclimate difference. If this were the norm, then winter hardiness zones within this valley range from Zone 3 to Zone 10! In the leaf litter that year, the length of the growing season was 365 days. In the summer minimum temperatures in the 30s were recorded in the frost pocket and minimum temperatures were highest in the covehead forests where minimum temperatures were in the 60s.

Absolute Monthly Minimum Temperatures (F) at Neotoma Habitats
 
JAN
SEPT
Frost pocket
-25.0
29.0
Lower NE slope
-19.5
38.0
Upper NE slope
-15.5
42.5
Ridge top
-19.0
44.5
Covehead
-16.0
40.0
Crevice
+10.0
54.0
Leaf-litter (cove)
+32.5
52.0

In winter the absolute range of minimum temperatures within Neotoma is 57.5 F and in summer 23.0 F. The very large wintertime range accounts for the Neotoma status as a refugia for southern species.

Maximum Temperatures

The highest temperature in 1941 occurred at cliff top (117 F). On the same day temperatures at the head of the cove reached only 76. The maximum temperatures for the year in the three mature forest units in the cove (red maple, chestnut oak and hemlock) occurred in April before leaf-out. Ample water supply and evapotranspiration precludes sensible heating of the air to expected summer highs.

Absolute Monthly Maximum Temperatures (F) at Neotoma Habitats and at the
Lancaster Weather Service Office 15 miles from Neotoma.

 

J

F

M

A

M

J

J

A

S

O

N

D

Lancaster

56

60

62

88

92

95

102

85

92

71

73

66

Cliff Top

68

71

85

102

109

110

113

117

115

107

84

73

Cove

49

43

49

80

68

74

76

76

77

73

57

55

Red Maple

56

54

61

89

86

87

87

88

83

81

69

62

Chestnut Oak

55

61

64

90

89

87

90

93

86

86

76

67

Hemlock

59

54

58

83

87

85

89

90

82

85

71

65

Within Neotoma, at the cove and in the three forest stands, the highest temperature recorded was only 93 F which is 24 F cooler that at ridge top. For the most part mid-summer temperatures are in the 70s and 80s in the cove and adjacent forest stands. To have such low absolute maxima, most of the excess daily radiation must be given off as latent heat rather than sensible heat. Here in Charlottesville, Virginia with a fully charged soil moisture, say after a good soaking rain, temperatures do not get over 90 F.

Why are coves warm places at night and in winter and cool places in the day and in summer and not cold spots or frost pockets due to simple cold air drainage? First coves are usually moist places with ample water supply even in drought years. With ample moisture for evapotranspiration hot days are uncommon. With active vegetation moisture and hydrocarbons added to the air and trapped in the cove airshed, minimum temperatures do not fall as much. High minimum temperatures increase the length of the growing season and a long growing season keeps the cove climate moderate. A synergism between vegetation and climate within a captive airshed is likely. There is as much as 100 days difference in the length of the growing season within Neotoma (See discussion of the montain oasis in an earlier CED.)

Length of the 1941 Frost Free period in Neotoma and at Lancaster
 

Spring Last Frost

Fall First Frost

Frost Free Days

Frost Pocket

25-May

26-Sep

124

Lower NE Slope

14-May

11-Oct

150

Upper NE Slope

22-Apr

29-Oct

190

Cove

3-Apr

11-Nov

209

Crevice

3-Apr

26-Nov

235

Lancaster

5-May

26-Sep

144

Coves Elsewhere

In a more recent paper on cove climates, Gaddy, Suckling and Meentemeyer (1984) [Arch. Met. Geoph. Biocl. Ser. B 34:155-162] found Appalachian Cove in South Carolina in which it was much warmer at cove head than down valley and that the blooming dates at covehead were as much as three weeks earlier than down valley. While coves are low places that might be thought to be filled with denser, cold air catchments, it is clear that processes, which retard night time cooling, must dominate the collection of all those molecules of air with low molecular velocities (the cold ones). The longer growing seasons and advancing of phenological stages weighs in for reduced radiative cooling at night. Transeau found that the cove had the highest vapor pressure of all his study sites. Using an atmometer he found it had the lowest evaporation at 15 to 18 inches above the surface. Wind speeds are low, radiation least, and vapor pressure deficit the smallest. High specific humidities and thus high dew point temperatures keep minimum temperatures warm. In effect there is a cove greenhouse of atmospheric water vapor. The cove keeps itself warm at night and evaporation keeps it cool by day. In the cove, Transeau recorded 1.8 times the canopy evaporation as outside the cove.

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