Hydrologic Tracer Experiments - Douglas fir and Quaking Aspen

Studies have shown that water can reside inside trees for up to several months, but how the duration of long-term stored water relates to species-specific water management strategies is not known. We studied water residence time, tracervelocity, and internal mixing in two tree species with differing water management strategies by injecting deuterated water into Douglas fir (Pseudotsuga menziesii) and trembling aspen (Populus tremuloides) trees and then monitored tracer concentration in leaves collected daily for several weeks post-injection. In a second study, we injected tracer into trees and collected leaves at sub-daily timescales to explore sub-daily patterns of tracer arrival in canopy leaves. For the first experiment, we hypothesized that the tracer would remain in both species for days to weeks and that the tracer would have a longer residence time in the more isohydric Douglas fir trees. For the sub-daily study, we explored if the tracer would arrive at a sharp peak, or be more spread out over time. The tracer resided in the trees for 7-11 days for both species. Interestingly, the tracer reached the canopy leaves of aspen sooner yet remained in trees for longer compared to Douglas fir, which exhibited sharp pulses of tracer breakthrough in canopies. Surprisingly, the tracer arrival in aspens occurred in two distinct pulses, separated by 1-2 days. Combined, the results from both experiments suggest that water inside trees may not flow through trees in simple “in-up-and-out” ways and that there may be some complex mixing of water reservoirs and water-flow pathways in some tree species.