Contrasting water-uptake and growth responses to drought in co-occurring riparian tree species

Growth responses of riparian trees to changes in water availability are poorly understood, thereby limiting understanding of regional drought implications in river corridors. We used dendrochronological methods to develop growth series and analysed oxygen isotopic ratios (18O) in -cellulose from individual tree rings for two co-occurring riparian species, Fraxinus excelsior and Populus nigra. Trees were sampled along the Ain River (France) to assess growth and annual source water availability, inferred from tree-ring 18O, compared with variations in streamflow and precipitation. Both species exhibited decreased growth during drought years, but F. excelsior demonstrated less variation in annual growth across sites compared with P. nigra. The latter species expressed a smaller range of 18O among individuals and years, but greater 18O sensitivity to river discharge, indicating a more consistent hyporheic water source. P. nigra also suffered more growth inhibition than F. excelsior in dry years when water tables were suppressed. These differences suggest contrasting patterns of water use, wherein F. excelsior uses primarily water from the vadose zone to maintain moderate growth rates, and P. nigra demonstrates more consistent phreatic water usage and suffers in drought years when water tables are suppressed. These observations are consistent with data on floodplain rooting depths, which show that F. excelsior maintains its roots above the gravel layer, whereas P. nigra generally roots deeply into phreatic zone gravels. These results indicate that surprisingly, poplars may be more sensitive to drought than ash trees and may be vulnerable to climate changes affecting water availability in riparian corridors. Copyright (c) 2012 John Wiley & Sons, Ltd.