Forests dominated by hemlock (Tsuga canadensis): distribution as related to site and postsettlement history

Abstract

Along a transect of mature forests from Wisconsin to Nova Scotia, hemlock attains dominance most frequently where soil is thin or low in stored nutrients. Very probably, soil is also dependably moistened either by climate or seepage. The inability of shade-tolerant competitors to survive suppression on thin or infertile soil may explain hemlock's success, even though hemlock maintains understory populations at very low densities. Hemlock also grows well on moist uplands with richer soil but does not often attain dominance or form large aggregates in a continuously varying mixed forest. This is in contrast to hemlock's codominance on a wide variety of upland sites in the presettlement forest. Recent history, then, may best explain the rarity of hemlock-dominated stands on richer sites: (1) hemlock is sensitive at all stages of its life cycle to disturbances that were intensified after European settlement as well as to the kinds of animal consumption that increase with disturbance; (2) disturbance has been most frequent on sites with richer soils; and (3) hemlock does not compete well with shade-tolerant trees such as sugar maple, beech, and red spruce that are better adapted to disturbance. These species can increase at the expense of hemlock because they have less rigorous requirements for seedling establishment, are not as palatable to consumers, or, in the case of sugar maple and beech, have deeper root systems and can sucker or sprout after extensive shoot damage.