By mid-September, the oldest needles of red pines had turned yellow and had started falling off at their points of attachments on their stems. These points are known as the abscission zones. Before the needles fall, they change into a protective layer several cells thick on the side toward the stem and a separation layer two or more cells thick on the side toward each needle. Just before needle fall occurs, the protective layer cells swell and become waterproofed, thereby shutting down the flow of fluids between needle and stem. At the same time the separation layer cells weaken as their intercellular cement and their own cell walls swell and become soft and gelatinous. Extending through this separation layer are some thick-walled tubes, tracheids ( xylem vessels in flowering trees and shrubs), that conduct solutions of minerals from stem to needles, but they alone cannot support and prevent needle fall. The weight of each needle and the effect of wind, rain or other external factors break the needle from the stem along the separation layer. Other pines, spruces, and balsam also develop abscission zones, but not as noticeable as red pines. Woody, broad-leafed, flowering trees and shrubs also develop abscission zones in the late summer or fall.
Premature needle and leaf fall can be brought about by any of several environmental factors, including water deficit, low temperature, reduced light intensity, increased length of daily darkness, insects and diseases, root damage and girdling.