The effects of competition on tree drought tolerance is poorly understood, limiting our ability to project mixed-species forest responses to future drought. We investigated the extent to which drought tolerance traits differ in quaking aspen (Populus tremuloides) trees grown under competition and drought. Aspen seedlings were placed in a growth chamber in individual pots (control) or in competition pots (treatment) with either another aspen, a narrowleaf cottonwood (Populus angustifolia), or a ponderosa pine (Pinus ponderosa) seedling. All seedlings were subjected to three drought events (soil moisture stress, vapor-pressure deficit (VPD) stress, combination of soil moisture and VPD stress) with a well-watered period between events. We measured leaf turgor loss point (TLP), predawn and midday leaf water potentials, xylem water potential causing 50 percent loss of conductivity (P50), maximum rate of carboxylation (Vcmax), and tree growth (height) throughout the experiment. We hypothesized that aspen trees grown under competition will exhibit reduced growth, lower Vcmax, and more negative leaf water potentials. We further hypothesized there will be more negative shifts in P50, Vcmax and TLP values in aspen trees under competition and drought, and the magnitude of these shifts will be proportional to the water stress experienced.
Results/Conclusions
We found differences in drought tolerance traits between individual and competition aspen trees. At the end of the experiment, aspen trees grew taller under all competition treatments as compared to the individual trees, gaining 3.96, 8.09, and 5.32 cm in height with aspen, pine and cottonwood competitors, respectively. Individual aspen trees only gained 1.35 cm in height. Prior to the drought events, mean Vcmax was higher in individual aspen trees than trees under the competition treatments. Pre-drought mean P50 for individual aspen trees was not significantly different from competition trees. Mean predawn water potentials indicated that aspen trees experienced substantial water stress, particularly during the combination stress event. Midday water potentials were not significantly different for aspens across all competition treatments. The mean TLP for individual aspens was less negative than TLP for trees under the competition treatments. Results from this study will advance our understanding of how tree species adjust their drought tolerance mechanisms and strategies under competition.