Coniferous trees such as pines, spruces, firs, and cedars normally shed needles in the fall. The older, inner needles turn yellow then straw-colored to brown and drop from the tree. Depending on the tree species, the foliage on a given branch may thin progressively over one to three years. For example, spruce and fir needles also turn yellow and drop with age, but retain their needles for several years.
On white pines, a more dramatic yellowing of older needles is observed. By next month, only one year's needles may remain attached to the tree. Austrian and scots pines usually retain their needles for three years. Some needles turn brown rather than yellow as they age such as those of arborvitae and cedars. Adverse conditions in the preceding summer or winter may lead to an earlier or more pronounced needle drop in the fall.
In conifers, nutrient deficiencies can also lead to general yellowing and premature senescence of needles. Analysis of plant tissues is often needed to obtain an accurate diagnosis of the problem.
This article originally appeared in the October 8, 1999 issue, p. 125.