Summer storms tend to follow three tracks through Acton: one to the south of the Arboretum, passing through West Acton toward South Acton, one to the north of the Arboretum. These two seem to follow along the two major brooks, Fort Pond Brook and Nashoba Brook. The third track storms follow is Route 2, which brings them right through the Arboretum area.
Prevailing winds at the Arboretum are westerly. The winds move from Taylor Road across the parking area and upper field. The esker provides a break in the winds traveling across the lower area.
For the Acton area the normal temperature in January is 27.80 degrees F and 72.00 degrees F in July. The normal annual precipitation is 43.02 inches.
The previous discussion covers the broad generalization of climate for the Acton area. For any given location other factors, either natural or man-made, work to modify the climate thus altering the plant species that can grow in any given area. These areas are called microclimates. Factors that create a microclimate include slope or exposure, of the site, the moisture content and the depth of the soil, and the existing vegetation especially the density of the canopy. Man-made factors that can create microclimates include wind-breaks, buildings, and pavement.
Microclimates can exist both horizontally on the landscape as well as vertically. For example, the south face on the down wind slope is generally warmer and more protected than a north-facing slope, a horizontal effect of microclimate. The upper portion of a tree canopy is warmer and subject to a higher rate of evaporation than the shrub understory, which is cooler and more protected. The process of plant succession creates microclimates allowing successive stages of communities to move in following the pioneer species. In this case, it is the vegetation that creates the microclimate by changing the soil conditions.
Examples of microclimates can readily be seen in the landscape of the Arboretum, such as alpine lichens and plants growing on the ledge outcrops, mosses growing in the shade of evergreen trees or buildings. Some species, especially evergreens, because of the dense shade they offer, create a cooler environment for other species. At the Arboretum, one of the notable examples of a microclimate is the open field area and rock outcrop with the Tupelo trees. Tupelos are generally found in wet areas, and this spot seems to have a supply of moisture at the base of the ledge outcrop, which allows these trees to grow out of their usual environment.
South-facing slopes, which dominate the Arboretum, afford the opportunity to add species preferring a slightly warmer range, especially if these are also slopes protected from the prevailing wind. Wind breaks can be utilized to cut the drying and cooler effect of the winds. Species more adaptable to the cooler range of temperatures could be placed on a north- facing slope.
Careful consideration needs to be given to the siting of plant materials in relation to microclimates that they will create as they mature. The effect of maturing plantings on adjacent areas, such as shade and or pulling of moisture from the soil, should be factored into any planting plan. Close attention to site-specific differences also allows for the expansion of the range of material that can be grown successfully.