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Journal American Rhododendron Society

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Dr. Glen Jamieson ars.editor@gmail.com

Volume 25, Number 3
July 1971

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Microclimates and Their Effect on Plant Hardiness
Alfred J. Fordham, Arnold Arboretum of Harvard University

        Climate is the principal factor that limits which plants may or may not be grown in a specific area. At the Arnold Arboretum some woody plants cannot tolerate the heat and dryness of summer, but the survival of most is determined by various aspects of temperature which involve cold. Some plants which start growth early can be injured by spring frosts. Others may be killed by freezing in autumn before they have properly hardened in preparation for winter. The principal determining factor; however, is severe winter cold. Therefore, this paper will consider only minimum temperatures. 

Arboretum Weather Station
        Climatological records are significant to botanical institutions. With such data, it becomes possible to compile information which adds to knowledge concerning the climatic tolerances of plants. Frequently, the exact date when winter injury occurred can be pinpointed.
        Since August 15, 1962, the Arnold Arboretum has maintained a simple climatological substation in cooperation with the United States Weather Bureau. Daily at 8:00 A.M., observations of temperature and precipitation covering the previous twenty-four hours are entered on forms which the Weather Bureau provides. Despite the fact that records only apply to a relatively short eight year period, some interesting data have been accumulated.

        The equipment consists of a maximum and minimum self-registering thermometer furnished by the Arboretum and an eight-inch non-recording precipitation gauge provided by the Weather Bureau.

        Those familiar with the Arnold Arboretum are aware of the wide variety of topographical features that are present within the bounds of this relatively small 265 acre area. Elevations range from 50 to 233 feet. The terrain is comprised of summits, ridges, valleys, slopes of varying degrees, plateaus and so on. These features lead to an assortment of exposures which face all points of the compass. With such geographical variation there is also a wide range of climatic differences. These deviations from the overall climatic picture have been termed microclimates. Microclimatic situations are infinite. They can occur at hilltops, slopes, valleys, different sides of a house, either side of a wall, under a tree, over a stone, or in a footprint. Areas concerned can be highly localized and sometimes involve distances as little as portions of an inch.
        In November 1934, Dr. Hugh M. Raup, then a member of the Arboretum staff, chose eight locations in the Arboretum and set up a station at each where temperatures could be recorded. The records remained when Dr. Raup left and from them have been extracted some interesting microclimatic data.
        Station 1 was positioned at the southwest side of the Administration Building where the land slopes gently toward the meadow and where the station was sheltered from the north, east, and west.
        Station 2 was situated on flat land in the shrub collection. The shrub collection is in one of the lowest areas of the Arboretum and the land slopes toward it from all directions, making it an ideal location for a cold pocket.
        Station 3 was located on Bussey Hill about 50 yards south of the summit and on a crest - an ideal position for good air drainage.
        A site for Station 4 was chosen on a gently southeast slope near the Centre Street Path. It was well protected from the north and west by higher elevations and has proven to be one of the most favorable microclimates in the Arboretum.
        Station 5 was located in the Juniper collection on a small plateau well sheltered from wind by surrounding slopes and hills. It is of interest that many Indian artifacts were found here. This would indicate that an Indian camp site existed there despite the fact that it proved to be one of the Arboretum's coldest microclimates according to the Raup records. Indians lived close to nature and though they had no knowledge of microclimates, they did know that some sites were more suitable to comfort than others. This awareness would be of prime importance to those living through a harsh New England winter under primitive circumstances. Several considerations may well account for their choosing this location. Nearby to the south was a free-flowing brook and to the north was a spring. The area is well sheltered from bitter winds of winter and they could avoid the chill factor which can lead to much greater human discomfort than severe cold. Together with these features it seems reasonable to suppose the area at that time was wooded and, therefore, could have a climate which differed somewhat from that shown by the Raup records.
        Station 6, the most sheltered site of all, was placed on Hemlock Hill. The area was then populated by massive hemlocks, many of which were lost later in the 1938 hurricane.
        At Station 7 in the isolated Peters Hill area, observations were recorded for about one month when they were discontinued with the notation "thermometer stolen."
        Station 8 was near the Arboretum greenhouses which where then located off South Street on property of the Bussey Institution. The site was a small plateau with the land falling away in all directions. The discovery of numerous Indian relics reveals that a camp site also existed here. Although shown by the Raup records to be the most favorable microclimate of all, it was exposed to the wind from all directions. Again, the area was perhaps wooded, a condition that would tend to mitigate the chill factor associated with winter winds.
        Interpretation of Some Temperature Readings. The following examples of temperature gradients have been selected from the Raup records. Each morning during the winter of 1934-35 observations pertaining to the previous night were recorded at about 9:00 a.m. For purpose of illustration, easily interpreted extremes have been chosen.


Minima Under Clear Conditions, Winds Light to Very Light.

Station 1 2 3 4 5 6 7 8
  -16.0 -26.0 -16.9 -20.7 -25.1 -17.8   -7.5
  -6.9 -15.4 -8.3 -9.2 -19.4 -6.8   -5.3
  -3.7 -14.3 -2.9 -7.0 -12.0 -3.8   -6.0
  16.7 8.0 17.8 15.0 9.6 18.0   18.0

        Table 1 shows some wide differences in minimum temperatures that come about through radiational cooling. Radiational cooling is typical of calm, clear nights during which the atmosphere loses heat to outer space through radiation. Temperature drop is often greater during winter than at other seasons because the longer nights allow radiation to take place over a greater period of time. In the absence of wind, cold air settles to the ground and drains from higher elevations to lower areas producing frost or cold pockets. These are the nights during which our lowest temperatures occur, and they are the most damaging to plants. At such times, the shrub collection usually had the coldest temperature while the area near the greenhouse had the warmest. It is interesting that the temperature stations which showed the greatest extremes were the closest together - about 200 yards. However, the topography is such that the same differences would prevail at the edge of the greenhouse plateau and in the flat area below which contains the shrub collection - a distance of about 90 or 100 feet with a difference in elevation of 30 feet.
        These examples are typical of calm, clear nights when heat is lost to atmosphere through radiational cooling. In the absence of wind, cold air drains from higher elevations and settles in the low areas.

Minima Under Clear Conditions, Estimated Wind Velocity Medium to Brisk.
     Station  1 2 3 4 5 6 7 8
  8.0 7.1 6.9 7.2 6.4 7.3   6.1
  11.7 11.2 10.5 11.3 10.3 11.5   10.2
  11.2 11.0 10.1 11.0 10.1 11.5   10.8
  17.8 17.0 16.2 17.0 16.1 17.4   16.1

Table 2 shows minimal differences in temperature. Brisk winds led to a mixing and stirring of the atmosphere and therefore only slight variation at all stations. Under conditions of high winds, Station 8, which usually had the warmest microclimate, often showed lower temperatures due to its exposed position.
        Brisk winds led to a mixing and stirring of the atmosphere and therefore temperatures were quite similar at all stations.


Minima Under Cloudy Conditions, Estimated Wind Velocity Light to Very Light.

Station 1 2 3 4 5 6 7 8
  28.0 28.0 27.4 28.0 28.0 28.0   -
  16.3 17.1 17.0 15.1 15.9 15.9   19.5
  17.1 17.0 16.0 16.9 16.2 17.0   16.5
  28.0 28.0 27.5 28.0 27.7 28.0   29.3

        Table 3 illustrates the uniformity that was evident under cloudy conditions. If calm nights such as these had been clear, there would have been wide variations. However, the cloud blanket intercepted and prevented the radiation of heat from below. In the absence of radiational cooling, there is a pattern of uniformity despite light winds. As would be expected, similar gradients frequently came about during periods of rain and snow.
        The cloud cover intercepted and prevented the radiation of heat from below. In the absence of radiational cooling, only slight differences appeared in the minima despite relative calm.
        Those responsible for planting the Arboretum in its early years were aware of the more favorable microclimates in the vicinities of Stations 1, 3, 4, and 6 and set out many plants of questionable hardiness in those areas.
        Boston's official weather station is located at Logan International Airport in East Boston. It is situated seven miles northeast of the Arnold Arboretum. Its climate is modified by proximity to the sea and is quite different from that at the Arnold Arboretum. Temperatures are usually cooler in summer, warmer in winter and have less range in the extremes. To show these differences data concerning the past three years have been brought together in Table 5.

Average Minimum Temperatures at Arboretum Stations January 1935.
Station 1 2 3 4 5 6 7 8
  14.2 11.6 14.0 13.0 10.9 14.2   15.9



Comparative Data Official Boston and Arnold Arboretum Weather Stations.

  1966 1967 1968
  A. A.. Logan A. A. Logan  A. A. Logan
January 24.5 28.8 29.8 35.1 20 25.6
July 73.6 74.9 74.5 73.0 76 75.2
Maximum 100 97 97 94 98 98
Minimum -1 3 -9 -3 -6 -4
Growing season            
     (days) 169 216 154 210 172 249
Last frost Apr. 27 Mar. 29 May 7 Apr. 13 May 8 Mar. 26
First frost Oct. 13 Oct. 31 Oct. 8 Nov. 9 Oct. 27 Nov. 30

Growing Season, The growing season is defined as the number of days between the last day of killing frost in the spring and the first day with killing frost in autumn. This time is determined by the last spring and the first fall temperature of 32 degrees F. or lower.
        Those concerned with gardening realize the considerable year-to-year variation that can occur in length of the growing season. During one year in the Boston area, frost may take place in early September while it might not occur until November in the more favorable microclimates. Note in Table 5 that Boston was favored with 47, 56, and 77 more growing days during the three years concerned than the Arboretum.

Plant Hardiness Zone Maps
Many maps which indicate zones of woody plant hardiness have been prepared. However, two which concern the United States and Canada are more commonly used than others. The Arnold Arboretum map was first published in the Manual of Trees and Shrubs by Alfred Rehder (1927). Since then it has been revised a number of times - most recently in 1967, (Arnoldia 27: No. 6, 1967). The United States Department of Agriculture has a Hardiness Zone Map which was issued in 1960.
        Each of the above maps was compiled from records based on annual minimum temperatures averaged over a period of years and each comprises ten zones. However, differing numbering systems have been used for each and this has led to confusion. The Arnold Arboretum lies in Zone No. 5 when its map is used and in Zone 6b when the United States Department of Agriculture's map is consulted.
        It is obviously impossible for large area hardiness zone maps to go into detail and define individual microclimatic situations. However, they do serve a useful purpose by indicating the probability of a plant's survival in a broadly generalized area and without them there would be no guide whatever.
        In closing it is interesting to return to Table 1 and ponder some temperatures that occurred during the winter of 1934-35. The coldest night of that year was January 27 (line 1). By taking the minimum temperatures for that date and then consulting the figures shown in the Arnold Arboretum Hardiness Map, we find that during one particular night some parts of the Arboretum lay in Zone 5 while others were in Zones 3 and 4. By dropping to line 3, we find a night when various locations in the Arboretum ranged from Zone 4 to Zone 7. This clearly indicates that when one is concerned with plant survival in a specific location, knowledge of its microclimate can be of critical importance.

Volume 25, Number 3
July 1971

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