Forest Soils

Forest Soils

Glaciers have advanced and retreated across Massachusetts many times over the past several million years. The last glacier retreated over 10,000 years ago. Our soils are the product of the interaction of vegetation and climate on what the glaciers left behind. 

Most upland, forested soils have developed out of what is called `glacial till,` that is, soils that were created and mixed by glaciers moving across the landscape. They are typically stoney, fine-textured soils on top of bedrock. They may or may not have hardpan layers. 

We also have pockets of `glacial deposit` soils which were left by streams and rivers created by melting glaciers. These deposit soils are made of mixtures of sand and gravel and are found where there were edges of tongues of the retreating glaciers--mostly in river and stream valleys. Many hillsides have scattered pockets and bands of these soils among the predominantly glacial till soils. 

There are also lacustrine and alluvial soils. These are both silty soils, the former from deposits on old lake bottoms and the latter from flood deposits along rivers and streams. Most of these soils are in agricultural use. 

Different tree species are adapted to different types of soils. Some species, such as red maple, have very broad adaptations and will grow just about anywhere. Other species, such as butternut, will only grow well on deep, loamy soils. Most species have more intermediate ranges of adaptation.

Northern hardwoods such as white ash, sugar maple, beech and yellow birch are found mostly on glacial till soils west of the Connecticut River where the parent material of schist and gneiss created tills of fine texture and good nutrient status. Oak-hardwoods are generally found on glacial till soils east of the Connecticut River where the parent material of granite created tills of medium texture and nutrient status.

White pine stands are common on glacial deposit soils which have coarse-textured, sand and gravel soils. Pine is also frequently found on abandoned agricultural land for reasons related to preferential browsing of livestock.

The productivity of forest soils is a function of their ability to hold and make water available to trees (available water capacity--AWC). Their nutrient status is also important. Available water capacity is a function of the rooting depth and texture of soils. Rooting depth may be limited by bedrock, dense hardpan, or seasonal high water table. Texture may be from coarse gravels to fine loams and silts, with sands and sandy loams in between.

Coarse soils have low AWC, but great depth; water tables below the rooting zone may provide water by capillary action. Fine textured soils over hardpans or bedrock have high AWC, but little depth; permeable hardpans and rock fissures may provide water by capillary action. 

Copies of US Soil Conservation Service soil surveys are available from your county SCS office. These surveys show soil series mapping units overlaid on copies of aerial photographs. There are descriptions of the soils and their abilities and limitations for different uses, including forestry.

CLIMATE

Our forests are the product of the interaction of climate, soils and human history. Our soils and human history are actually to a large extent the product of climate too. This is because climate controls the advance and retreat of glaciers, which controls the numbers of people. Our current climate is interglacial temperate, with cold winters and hot summers.

We have an annual frost-free growing season of 120-160 days, with greater numbers to the south at lower elevations, and lower numbers to the north at higher elevations. The range of species is limited more by cold than by heat. All northern species will grow here, but southern species will not because they can't survive the winters. We do get good rainfall here--up to 20% more than other parts of the Northeast. 

Climate also restricts the times of the year when loggers can work in the woods. When soils are saturated, heavy logging equipment causes rutting and soil compaction. On fine textured soils this is the case in the spring and fall when tree leaves are not drawing water out of the ground. Forest soils are likely to be dry during the summer and frozen during the winter. 

However, with climate change, we're getting warmer and wetter conditions throughout the year. We have less time with frozen ground in the winter and more time with wet ground in the summer. On the other hand, we're getting up to 10% more photosynthesis due to more CO2, more precipitation and longer growing seasons.