DYNAMICS IN LATE-SUCCESSIONAL HEMLOCK–HARDWOOD FORESTS OVER THREE DECADES

Abstract

Permanent plots in old-growth hemlock-northern hardwood forests of Mich- igan's upper peninsula have been remeasured over periods of 16-32 yr. A gradient from hemlock (Tsuga canadensis) to sugar maple (Acer saccharum) dominance is associated with increasing soil pH and calcium. Secondary species include yellow birch (Betula al- leghaniensis) and basswood (Tilia americana). From 1962 to 1994 hemlock increased in basal area and dominance in most plots. Sugar maple showed little overall change, while basswood and especially yellow birch showed sporadic but often large declines in basal area. Birch populations declined due to lack of recruitment, and sugar maple and basswood may be subject to similar decline; only hemlock showed a fairly stable size structure. Mortality rates were lowest for hemlock (0.3%/yr) and highest for yellow birch (1.6%/yr), corresponding to canopy residence times of 357 and 61 yr, respectively. Stem maps allowed assessment of neighborhood influences on growth and mortality. Growth and mortality rates were negatively correlated for all species. Growth rate was influenced by tree size and site conditions for all species, but hemlock and sugar maple growth rates were also affected by size- and distance-weighted indices of neighbor influence. Old-growth stands several centuries old continue to undergo compositional change related to both stand history and current population interactions. Yellow birch and basswood are probably maintained by significant disturbances and will decline under a disturbance regime of small gaps. Hemlock may be the ultimate competitive dominant in most sites but may require well over a mil- lennium without major disturbance to displace sugar maple.