Abstract: Human-caused climate change is predicted to bring more frequent droughts and higher temperatures in the western U.S., which threaten ecologically important trembling aspen (Populus tremuloides) forests. In the past 15 years, aspens have been undergoing a sudden aspen decline (SAD) that are due to these climate change type droughts and have lasting effects. During drought stress, the pressures within their water-transporting xylem tissue exceed the tolerances of vessels and the membranes within their pits. When this occurs, gasses are pulled into vessels and create emboli which block the ability of vessels to transport water. Emboli can spread from one vessel to another through vessel pit membranes. SAD appears to be associated with multi-year accumulated damages in the xylem due to “cavitation fatigue”, where even if the vessels are refilled, they are no longer cavitation resistant due to “leaky” pits. With more droughts to be expected, the goal of this project was to determine if xylem damages accumulate as the xylem ages. Our approach to addressing this question was to conduct vulnerability curves on specific tree rings of aspen branches along a climatic gradient in Utah and Colorado. We paired this method with dye perfusions to better understand how accumulated damage and xylem age affects xylem vulnerability. Additionally, because vessel diameter is an important factor for hydraulic conductivity, vessel diameters were measured for each ring. Here we show that rings older than three years old have a significant decline in water transport, especially at -1 MPa. Additionally, there was significantly more damage in the driest site compared to the wettest site. These differences seen between rings were not due to differences in the diameter of the vessels, but a difference of how much xylem was active between older and newer rings, suggesting the presence of damage. In conclusion, this study suggests that older tree rings are more vulnerable at average summer water potentials and that damages due to drought may accumulate to lethal levels if the xylem does not acclimate in newer growth.
