Earlywood formation was observed in 60-year-old forest-grown red oak trees in southern Michigan. Extreme care in removing samples from the cambial region of the main stem at 1.4 m and 18 m, and from small branches at about 24 m, permitted the following conclusions. First vessel elements were initiated in the second or third xylem derivative radially removed from the previous year's latewood, possibly in overwintering derivatives, simultaneously throughout the bole and branches of the tree, some 2 weeks before bud enlargement. Vessel elements enlarged first in the tangential dimension (to about 200 μ.) within a few days after initiation of differentiation. Enlargement in the radial direction required up to 2 weeks to grow 300 μ, occurring as the entire xylem mother cell zone was displaced outward by cambial growth to either side—tangentially—of the vessel element. The duration of earlywood formation was about 10 weeks, while the duration of shoot elongation was less than 2 weeks. First earlywood vessels were fully mature about 5 weeks after initiation, coinciding with the unfolding of first leaves. All foliage was mature several weeks before complete maturation of later formed earlywood vessels. Detailed stem analysis and bark peeling studies revealed that stem sections clear of branching contained few lateral junctions between axial vessels. There were many such junctions where twigs joined larger limbs and where limbs joined the main stem; all such junctions were between adjacent vessels from the same limb.

The ontogeny of terminal growth in red pine (Pinus resinosa Ait.) suggests that the growing season in Michigan be divided into three 2-month periods: April-May, June-July, and August-September. It is during the middle period that mitotic activity at the shoot apex initiates dwarf shoot primordia in the bud for the subsequent season's foliage; thus, preconditioning one year has a strong effect on next year's growth.Water was applied to or withheld from the soil under 23-year-old trees in various combinations of the 2-month periods over two growing seasons. All effects of drought were greatest on the terminal shoot of the main stem, diminishing on terminals of branches with distance from the apex, and disappearing at the base of the crown.Shoot extension was governed equally by conditions of the middle period of the previous year and of the early period of the current year. Each year fascicle density was regulated only by water conditions of the early period of the current year, regardless of preconditioning. Spacing of needles on shoots depended on elongation of the stem internodes between dwarf shoots, and was independent of the number of fascicles.Number of fascicles per shoot was governed only by conditions of June-July of the previous year, with drought during this period reducing the number by 30%. Length of needles was related to two factors: the number of fascicles produced per shoot and the water treatment of the period during which needles elongated. Thus when the water supply of the middle of the previous year was deficient, fewer but longer needles were produced currently; when water during the middle of the current year was deficient, needles were reduced in length by about 30%.It is concluded that the interaction of two seasons' water supply are usually compensative, rarely additive. The average net effect on foliage production is a long-term, year-to-year stability in photosynthetic potential for red pine.

Annual ring widths and ring areas from 131 even-aged, natural, well-stocked stands of loblolly pine (Pinustaeda L.) in the Piedmont region were analyzed to reveal possible causes of a previously reported decline in radial growth. A linear aggregate model was used to separate independent factors that are known to contribute to radial growth variation in this species. Stand, site, and climatic conditions were reconstructed for each stand for the 36-year period 1949–1984 from previous inventories and from weather records at appropriately located stations. Within each of six 5-year age-classes, the model identified declines in both ring width and ring area associated with stand density, climate changes, and the passage of time. Regional climate first ameliorated this decline as pine stands passed from droughty conditions early in the 36-year period to a favorable climate during the middle of the period, and the decline accelerated later with the return of dry conditions toward the end of the period. The tree-ring model simulates a decline in radial increment in trees in natural pine stands between the ages of 20 and 45 years in the Piedmont which has averaged 1% per year since 1950. Part of the downward trend was attributed to increased competition, part to regional drought, and a considerable part to unidentified factors, possibly regional atmospheric deposition.

A method is described for computing daily values of moisture stress on forest vegetation, or water deficits, based on the differences between Thornthwaite's potential evapo—transpiration and computed soil—moisture depletion. More realistic functions are used for soil—moisture depletion on specific soil types than have been customary. These functions related daily rates of depletion to characteristics of soil—moisture tension. Separate functions account for surface soil wetting and drying processes following rain and during dry periods. Two methods of summarizing the day—to—day distribution of moisture stress are illustrated. One utilizes directly accumulated stress values during such logical growth intervals as periods of leaf flushing or bud setting. In an example of a regression of red pine shoot growth on water deficits, 72% of the variation in annual growth was accounted for by moisture stress during certain periods of both previous and current growing seasons. The second method computes moisture stress and weather variables as functions of time over two growing seasons and utilizes the coefficients of orthogonal polynomials as independent variables in regressions of growth. In an example of western white pine basal area growth utilizing this latter method, moisture stress accounted for a 28% reduction in the variance of growth remaining after the effects of temperature and precipitation per se had been removed. The complete model accounted for 78% of the total variation.

Various portions of the root systems of bigtooth aspen (Populus grandidentata) suckers were severed, and the subsequent height and radial growth of stems were measured. Aspen vegetative regeneration is heavily dependent on the parent roots for at least 25 years following initial suckering. The distal portion of the parent root contributes more to sucker growth than does the proximal. New roots at the base of suckers contribute little during the first 6 years, then become progressively more important with age, and by 25 years account for about half of annual stem growth.