Stress response, annual and diurnal patterns in root sap flow of Populus tremuloides

Background/Question/Methods

Quaking aspen (Populus tremuloides Michx.) stands provide a variety of crucial ecosystem services but have been declining in the western United States due to a decrease in wildfire and an increase in browsing. Additionally, a recent phenomenon dubbed “Sudden Aspen Decline” has hastened this trend. Aspen produce mainly vegetatively by suckering from the parent root system, and clonal integration (the sharing of resources via roots) determines the number and survivorship of new individuals. Knowledge about the effects of clonal integration on ramet development and responses to environmental factors will aid aspen management, especially in respect to aspen’s response to the current mountain pine beetle epidemic. To better understand the mechanisms of clonal integration, we installed Dynamax sap-flow sensors on the roots between connected aspen ramets and monitored water flux throughout the 2011 growing season (May 31^st to September 20^th). Flow was measured unidirectionally towards the older, donor ramet (inflow), between both ramets, and away from the younger, receiving ramet (outflow). We hypothesized that flow rates show annual fluctuations and correlate with leaf expansion and local weather conditions. At end of growing season, we severed all tap roots of the receiving ramet, and hypothesized that clonal integration would increase.

Results/Conclusions

Flow rates increased considerably with leaf-out (June 6^th) and peaked after maximum leaf expansion (July 8^th) until early to mid-August. By mid-August, flow rates decreased steadily. Water distribution from donor to receiving ramet (i.e. clonal integration) was lower than expected (averaging around 60 g/h during peak season), compared to outflow at the distal root of the receiving ramet (avrg. 560 g/h) and inflow at the proximal root of the donor ramet (avrg. 400 g/h). The difference between inflow and outflow is likely a result of water uptake by the both ramet’s taproots. Clonal integration doubled after severing all roots of the receiving ramet (avrg. 120 g/hr), indicating a nurturing effect of the donor ramet. These results suggest that aspen become less dependent on neighbor ramets as they age but may still utilize neighbor ramet resources, especially when under stress. This data provides valuable insight in the physiological processes involved in clonal integration within aspen stands and will aid in development of optimum management and restoration efforts.