Seasonal Fluctuation in the Foliage-Moisture Content of Well-Watered Conifers

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Abstract
The foliage-moisture content (FMC) of Douglas-fir, grand fir, ponderosa pine, sugar pine, and incense-cedar of several ages was studied throughout the year under well-watered and well-fertilized conditions. The results of this study have allowed some speculation into the processes that affect water content of leaves under low soil-moisture stress. The very low FMC''s known to occur for evergreen conifers in early spring, may not be caused by water stress as was previously believed but may be due to other causes, such as a dry-matter buildup while water content remains constant. The late-spring succulence of mature conifer foliage, heretofore associated with bud break and flushing, can actually occur as much as a month later. Bud break and flushing probably have a lower temperature threshold than rise in foliage succulence. A drop in FMC, similar to that occurring in the fall with the onset of cooler weather, was found in hot weather under a long-day photoperiod for plants that by July had completed in the greenhouse what was equivalent to a full summer''s growth cycle. Some seasonal changes in FMC may be caused directly by environmental changes, whereas others are only indirectly tied to changes in the environment and are probably more closely allied with metabolic or growth changes within the plant. The latter, of course, may accompany changes in the environment.