Submitted to: Forest Ecology and Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/4/2007
Publication Date: 7/15/2007
Citation: Snyder, K.A., Williams, D.G. 2007. Root allocation and water uptake patterns in riparian tree saplings: Responses to irrigation and defoliation. Forest Ecology and Management. 246:222-231. Interpretive Summary: This study investigates how Populus fremontii (cottonwood) responds to above and belowground resource limitations. Specifically, this study documents if allocation to fine and coarse roots, leaf gas exchange, root respiration and water uptake from hydrologically isolated upper and lower soil compartments are altered by drought and defoliation. Implications for the recruitment box model and successful recruitment events are discussed. This study has important implications for understanding how cottonwood recruitment events may respond to declining ground water levels and summer rainfall events.
Technical Abstract: The genus Populus is often the focus of restoration efforts in riparian ecosystems. Its reliance on shallow groundwater tables for successful recruitment is well documented. However, under some circumstances mature trees take up a substantial proportion of their water from the unsaturated zone. From a management perspective, what is unknown is how cottonwood saplings may alter root allocation patterns and water use in response to abiotic and biotic conditions. From a basic biological perspective, changes in patterns of water source use as determined from stable isotope tracers are often inferred to reflect differences in plant rooting patterns, but this notion rarely has been tested. We used a glasshouse experiment with Populus fremontii (Frémont cottonwood) to determine if differences in water source use patterns were correlated with differences in rooting patterns. The objectives were to determine if 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 repeated defoliation treatments and belowground resource limitations were imposed with differential irrigation of lower soil compartments. The lower soil compartments were irrigated to maintain high ("wet") or low ("dry") soil water availability. 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 root proliferation. Results indicate proportional use of water sources was plastic and changed in response to water availability and defoliation. Additionally changes in fine root biomass allocation were associated with changes in water-source use for water-stressed plants. By treatment week 19, defoliated plants in both watering treatments used proportionally less of the surface pulse than undefoliated plants. These data suggest uptake of water by shallow roots is reduced when cottonwood saplings have limited carbon to allocate to roots. In contrast, when faced with water limitations in the deep soil compartment, but with full carbon assimilation capacity (undefoliated saplings) Populus had higher ratio of shallow fine roots to deep fine roots and took up proportionally more water from the surface pulse. Populus exhibited belowground allocation tradeoffs in response to spatial heterogeneity of soil water and carbon limitations. Taken together these data suggest that successful recruitment events may also be influenced by the occurrence of summer rainfall, and by factors affecting canopy carbon gain.