Fascicle Density and Needle Growth Responses of Red Pine to Water Supply Over Two Seasons

Abstract

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 of 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 along—term, year—to—year stability in photosynthetic potential for red pine.