Injected Tracer Reveals Differences in Water Transport Rate, Residence Times and Mixing Dynamics in Tree Species of Contrasting Water Management Strategies

Abstract

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, tracer velocity, 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 monitoring tracer concentration daily in leaves for several weeks postinjection. In a companion study, we injected tracer and collected leaves at subdaily timescales to explore subdaily patterns of tracer arrival in canopy leaves. For the first experiment, we hypothesised 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 subdaily 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 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 in simple ‘in-up-and-out’ (i.e., piston flow) ways and that complex mixing of water reservoirs and water flow paths may occur in some tree species.