Root allocation and water uptake patterns in riparian tree saplings: Responses to irrigation and defoliation

The genus Populus relies on shallow groundwater for successful recruitment and is often the focus of riparian restoration efforts. Under some circumstances mature trees take up a substantial proportion of their water from unsaturated soil water derived from growing season rainfall, but it is unknown how Populus saplings may alter root allocation patterns and water use in response to water availability and carbon limitations. Although it has been inferred that root allocation differs with changes in water uptake patterns as determined with stable isotope studies, this notion has rarely been tested. We conducted a glasshouse experiment with Populus fremontii (Frémont cottonwood) saplings to determine how allocation to fine and coarse roots, leaf gas exchange, root respiration and water uptake from hydrologically isolated upper and lower soil compartments would be altered by above- and belowground resource limitations. Aboveground carbon limitations were imposed with defoliation. Belowground resource limitations were imposed by maintaining high or low soil water availability in lower soil compartment. Isotopically labeled water was supplied in pulses to upper soil compartments to determine the proportion of transpiration water derived from each compartment. Above- and belowground resource limitations differentially altered use of surface water pulses and affected patterns of fine root allocation. Proportional use of water sources was plastic and changed in response to water availability and defoliation. Changes in fine root biomass allocation were associated with changes in water-source use for water-stressed plants. Defoliated plants in both watering treatments used proportionally less of the surface pulse than undefoliated plants. In contrast, plants that were water limited, but not carbon limited had a higher ratio of shallow fine roots to deep fine roots and took up proportionally more water from the surface pulse. These data suggest that carbon limited saplings take up less water with shallow roots. Thus, P. fremontii exhibited belowground allocation tradeoffs in response to spatial heterogeneity of soil water and carbon limitations. Furthermore, our data suggest that successful recruitment events may be influenced by the occurrence of summer rainfall, and by factors affecting canopy carbon gain.