Hemlock forest (S4)
Hemlock forest is a common community in which a canopy of pure or nearly pure hemlock (Tsuga canadensis) covers a dark, relatively open understory with few herbaceous species. Hemlock is shade-tolerant and appears to maintain itself by out-competing other tree species for light and nutrients. It persists in low abundance in the understory for decades and then takes advantage of periodic canopy gaps. Few other species grow under such dense shade. Older forests tend to have more tip-up mounds that provide bare mineral soil and fallen “nurse” logs important for successful hemlock regeneration. Maximum hemlock ages in the region exceed 500 years. Deer often winter in these areas where movement in light snow cover is easier. Hemlock is an important component in many other types of natural communities where it mixes with hardwoods and/or softwoods; it reaches maximum dominance in this community.
This community typically occurs on sites with rocky, coarse, and/or thin soils poor in nutrients, including ravines, gorges, river and kame terraces, and other microsites below 2,000 ft. elevation. Soils typically have well-developed E horizons (classic Spodosols), are very acidic, high in exchangeable aluminum, and low in available nitrogen and other nutrients. Unlike many other plants, hemlock cycles aluminum. Soils freeze more deeply than sites with hardwoods due to interception of snowfall by the dense hemlock canopy.
Hemlock has long term persistence at some sites (on a scale of centuries or possibly millennia). However, there is some debate as to the relative importance of soil conditions vs. site disturbance on present day composition. Hemlock dominated sites in the Midwest that have been cut significantly often regenerate to hardwoods.
Characteristic vegetation: Floristically, hemlock forests are remarkably similar throughout the region. Hemlock is strongly dominant in the canopy, to the near exclusion of other species. The deep shade and acidic soils of this community result in a typically sparse or absent woody and herbaceous understory. Species characteristic of (but not exclusive to) these forests include northern wood sorrel (Oxalis montana), wood ferns (Dryopteris spp.), Canada mayflower (Maianthemum canadense), Indian pipes (Monotropa uniflora), partridgeberry (Mitchella repens), red maple (Acer rubrum), yellow birch (Betula alleghaniensis), and mosses and liverworts such as Bazzania trilobata. Other species characteristic of northern and transition hardwood - conifer forests may be present in low frequency.
Good examples of this community can be seen at Shingle Pond (
Hemlock forests often occur as part of larger hemlock - hardwood - pine forest systems, and sometimes as part of northern hardwood - conifer forest systems.
Hemlock forest at Sheldrick Forest Preserve in Wilton (photo by Ben Kimball)
hemlock forest on the lower slopes of Mt. Chocorua (photo by Ben Kimball)
Hemlock forest on steep slope at Pawtuckaway State Park (photo by Ben Kimball)