Do gymnosperm needles pull water through the xylem produced in the same year as the needle?

This research investigated the longevity of functional connections between leaf traces and stem xylem in 16 species of conifers to better understand the spatial use of sapwood for water transport. The first question was which ring(s) stained when a vacuum was applied to the distal end of the cut surface of a needle attached to a short stem segment. The vacuum was applied to either 1- or 2-yr-old foliage taken from 4-6-yr-old saplings. The 16 species were then categorized based on the growth ring that most consistently stained when the vacuum was applied to the 2-yr-old needles. There were three distinct stain patterns for the evergreen conifers and one pattern for the deciduous conifers. Three evergreen species could not be categorized. The second question used needle fall data to ask whether the leaf trace appeared to break because of age or stem diameter in 5-yr-old Pseudotsuga menziesii saplings. An apparent threshold stem diameter at which needles tended to be shed was more related to diameter than age. These xylem connection patterns could affect leaf cohort physiology and the spatial pattern of water flux in sapwood.