LAUSR

BACKGROUND AND AIMS The onset of spring growth and vessel formation were examined within three deciduous woody plant species, Acer rubrum, Populus balsamifera spp. trichocarpa, and Quercus rubra. We were broadly interested in the lag between onset of girth expansion to formation of mature and hydraulically conductive vessels within the new xylem. METHODS Dendrometers were installed on 20 trees (6-7 per species), and expansion of both bole and distal stems was monitored throughout the growing season in a common garden. For each species, 4-6 distal stems were harvested every other week for anatomical examination of vessel formation. Additionally, for Populus and Quercus, hydraulic conductivity measures and active xylem staining were completed on all stem samples. KEY RESULTS For all three species, the timing of girth expansion was similar. Expansion of distal branches occurred 12 to 37 days earlier than the bole. Vessel formation initiated several weeks prior to leaf-out, but no new earlywood vessels were mature at the time of bud break for Acer and Populus and only a few were present in Quercus. Initial stem grith expansion occurred 2 to >6 weeks before the maturation of the first current-year vessels, and there was an additional delay of up to 4 weeks before mature vessels became hydraulically functional. Hydraulic conductivity was strongly correlated with the number and diameter of stained vessels. CONCLUSIONS Bud break and leaf expansion relied predominantly on water supplied by vessels formed during prior seasons. Early season activity is likely affected by the function of older xylem vessels, and the environmental factors that influence their structure and function. Understanding the functional lifespan of vessels and the varying contributions of new and older vessels to conductivity are critical to understanding of the phenology and vascular function of long-lived woody plants in response to changing climates.