Changes in Moisture Contents and Dry Weights of Buds and Leaves of Forest Trees

Abstract

Changes in bud moisture content were studied in northern Wisconsin during 1963, from mid-April until bud opening, in Betula papyrifera Marsh., Acer saccharum Marsh, Tilia americana L., Quercus rubra L., Pinus strobus L., Pinus resinosa Ait., and Tsuga canadensis (L.) Carr. Changes in leaf moisture contents and leaf dry weights were studied throughout the 1963 growing season for the above species and for Abies balsamea (L.) Mill. In gymnosperms zero-age (current) needles and 1-year-old needles were studied separately. A similar study of changes in moisture content and dry weight of leaves of Quercus ellipsoidalis Hill was performed in the greenhouse. Moisture contents (% dry weight) of buds of angiosperms and gymnosperms increased progressively from early May until buds opened. The increase in moisture content was traceable primarily to rapid trans-location of moisture into the buds. Moisture contents of angiosperm leaves decreased rapidly in the early part of the growing season and slowly after midsummer. The seasonal decrease was traceable primarily to greater increase in leaf dry weight than to decrease in actual water content. In fact, early in the season moisture content, as % dry weight, decreased rapidly while the actual amount of water in the expanding leaves increased. The rate of increase in leaf dry weight exceeded the rate of water uptake for the same period. In most angiosperms the actual amount of water in the foliage changed little from mid-June to the end of the season. The seasonal trend in moisture content of gymnosperm leaves varied with age. In current-year needles the moisture content declined progressively, while in 1-year-old needles it increased at first and then tended to level off. These differences were caused largely by dry weight changes resulting from rapid carbohydrate translocation into current-year needles and some translocation out of the older needles. These experiments emphasized that changes in moisture content (% oven-dry weight) often occur without appreciable changes in actual water content of tissues.