Floodplain ecohydrology: Discerning climatic v. anthropogenic controls from tree-ring δ18O, dendrochronology, and instrumental climate records
Abstract
Vegetation of lowland riparian zones in temperate climates is largely determined by floodplain water availability during the growth season. Floodplain water reservoirs are replenished seasonally by lateral hyporheic water from streamflow, which primarily contributes to phreatic zone water and by infiltration of precipitation, which typically controls seasonal vadose zone soil moisture. Water availability to species rooted to particular depths in the floodplain is subject to interannual variability in climate (e.g., precipitation magnitude, timing, and phase). Co-occurring tree species in the riparian zone may express differential adaptation to water availability and shifting water sources, especially if they are rooted at contrasting depths. We have developed an ecohydrologic approach to assess how climatic variability impacts water availability at different depths in the floodplain and corresponding tree growth in the Rhône River basin, France. We combine dendrochronology, tree ring isotopes (δ18O), and instrumental climate records to discern relationships between tree growth and water sources for two contrasting, co-occurring riparian species—the shallowly rooting Fraxinus excelsior and the obligate phreatophyte, Populus nigra (poplar). We developed growth time series via basal area increment (BAI) and extracted alpha-cellulose from tree rings to assess relative responses to water stress via δ18O contained in each annual ring, and we analyzed these data alongside streamflow, precipitation, and groundwater data. Our initial work on a tributary of the Rhône showed that F. excelsior generally indicates water availability in the vadose zone, while P. nigra provides a window into the phreatic zone. However, the rooting depths and water sources for these species overlap on particularly low topographic surfaces, where phreatic water is abundant for both. In contrast to prior assumptions, we found that P. nigra exhibits more growth sensitivity to drought stress, especially when water tables are regionally suppressed, suggesting the importance of regional climate and climate change to floodplain forest succession and diversity. We build on this work using a new, extensive dataset for a site along the mainstem Rhône, where stream hydrology has been significantly modified by one of a series of engineered diversions. To ameliorate some of the deleterious impacts of such diversion on riparian ecology, flow restoration was implemented. Through our multi-pronged method, we find that this restoration has had a dramatic impact to water tables in Rhône floodplains, providing an influx of hyporheic water that supports more consistent growth of poplar and thus lowering risk of drought stress to this obligate phreatophyte which is ubiquitous in lowland rivers worldwide. This research is improving understanding of the response of lowland riparian forests to climate changes that manifest as increasing air temperatures, decreased precipitation, and shifts in precipitation timing and phase. It is also providing predictive capability to anticipate the consequences of river engineering and/or restoration strategies that produce altered streamflow, regional shifts in the water table, or changes in river base levels.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFMEP41H..04S
- Keywords:
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- 1637 GLOBAL CHANGE / Regional climate change;
- 1803 HYDROLOGY / Anthropogenic effects;
- 1813 HYDROLOGY / Eco-hydrology;
- 1852 HYDROLOGY / Plant uptake