Microclimates in an Urban Oasis
A microclimate refers to the climate of a small, defined area that differs from that of the larger surrounding area. Examples of microclimate sites in rural areas would be the top of a high hill, or the bottom of a valley. In contrast, the shadow zone created by a tall building, or a footbridge, surrounded by concrete and asphalt, are examples of urban microclimate sites.
In rural areas, topography, soil, water, and vegetation are factors that impact and create microclimates. Human activity has been the greatest factor to impact and create microclimates in urban areas. Buildings, roads, traffic, and pollution are all human factors. This buildup creates a “heat island” effect, which is why cities are measurably warmer than the countryside.
Microclimates can provide challenges and opportunities in a setting like Mount Auburn. Here exists both rural and urban microclimate indicators. A plant that does very well in one location on the grounds, may fail in another, even though that plant may be hardy and native to our area. Tender perennials, native to warmer plant zones, may prove successful in some areas as well.
Determining Microclimate Areas
Factors to consider when attempting to determine microclimates include the aspect of a slope, which refers to the direction a slope faces. South facing slopes have the greatest sun exposure, therefore they receive the most solar radiation, creating heat. Plants that require full sun are suited for such a site. The opposite side of the slope will receive more shade and plants should be chosen for this site accordingly. The angle of a slope is another factor. The steeper the slope, the faster the wind will move uphill. Without vegetative, or constructed windbreaks, the wind may also severely impact the leeward slope as it moves back down. The angle of a slope also affects water retention. A steeper slope allows water to runoff faster during rain events. Erosion may be a problem. The soil is not able to capture and store the falling water. Drought tolerant plants are suited to steep, south facing slopes.
Soil rich in organic matter, with diverse and healthy microbial biology will have greater water holding capacity, which in turn, is a factor in a microclimate area. Vegetation is important. By covering the soil, preventing erosion, and providing windbreak and heat trapping services, plants play a vital role in regulating temperature.
Man-made, physical structures are impactful at Mount Auburn in several ways. The stone curbing and monuments absorb heat during the day and release the stored heat at night. Monuments can create windbreaks just as trees and shrubs will. The roads and walking paths, whether asphalt or stonedust, store heat, and also prevent water from percolating into the soil, which are both factors that impact microclimates.
Citizen Science Study
We have decided to investigate microclimates more thoroughly at Mount Auburn with the help of our Citizen Scientists (the program was implemented in 2016 to conduct a phenology study). A trail has been created that includes 18 sites. Volunteers will be asked to follow a trail map and, using an infrared thermometer, record the ground surface, ambient, and dew point temperatures of each site. The information will then be entered into a data sheet. The collected information will help us better understand the challenges that our plant collections face in different areas of the landscape and aid in choosing the right plant for the right place.
How the Data Will Inform Us
Ground surface temperature is important to seed germination and plant root system development. While some vegetable crops, including peas and kale, will germinate well in cooler ground temperatures, most plants prefer warmer conditions for optimal seed germination. Warmer ground surface temperatures also allow roots to grow and extend through the soil to engage in symbiotic relationships with bacteria and fungi, gaining access to the nutrients that diverse and healthy soil biology provides. Ambient temperature will affect flower and fruit development. Low temps can stunt development.
Dew point refers to the temperature that air must cool to for water vapor to condense into water. When the dew point temperature falls below freezing, frost occurs, which may create injurious, but not fatal conditions for some plants and lethal conditions for others. When the dew point is closer to higher temperatures, the air becomes saturated with water, creating high humidity and the oppressive conditions of New England summers. During those sticky summer nights, a plant’s respiration rate increases, resulting in slower drying of leaves during the daylight hours. This may create the ideal conditions for foliar diseases to develop.
Microclimate study is important in this time of increasingly severe weather and the information collected will help Mount Auburn choose plant material and care for the existing collections. The implementation of this study will provide our thoughtful and dedicated citizen scientists to expand their scientific knowledge and have one more reason to explore our urban oasis.