Seasonal Basal Growth Area as an Expression of Competition in Northern Hardwoods

Abstract

A 2—year study of basal area change of six tree species was conducted with vernier growth bands in the northern conifer—hardwood sub—formation in north—central Wisconsin. Curves of basal area change per unit of stem circumference satisfactorily expressed differences in initiation of stem expansion, rate and duration of growth, and winter shrinkage for various species and crown classes. Seasonal growth for all species and crown classes was represented by a sigmoid curve beginning with a short period of slow basal area increase in early spring and ending with a longer period of declining growth rates leading to relative stability through late summer and fall. Winter shrinkage, most marked in basswood, began in early winter and often exceeded total summer growth by midwinter. Recovery was complete by early spring. Hemlock began rapid growth earlier and ceased growth later than any other species studied. Basswood began and ceased growth later than sugar maple, white and yellow birch, and trembling aspen. Timing of rapid growth initiation was somewhat affected by weather and tree condition. The decreasing order of vigor in competition of the major species was (1) sugar maple, (2) hemlock, and (3) yellow birch and basswood. Differences in amounts of cambial growth of species and crown classes were traceable to differences in both rate and duration of growth. Delay of rapid stem expansion in lower crown classes was much less marked than in lower crown classes of pure coniferous stands studied elsewhere, probably because of less canopy shading in spring. Differences in times of cessation of growth of crown classes were more marked, with the most suppressed trees ceasing growth earliest. Effects of particular stand conditions were most evident in growth of suppressed crown classes.