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

Abstract

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 (δ18O) in α‐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 δ18O, 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 δ18O among individuals and years, but greater δ18O 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 © 2012 John Wiley & Sons, Ltd.

Sub‐annual variability in historical water source use by Mediterranean riparian trees

“…The susceptibility of riparian forests to changes in the availability of water within terrestrial storage reservoirs (i.e. vadose and phreatic zones) is well appreciated (Dufour and Piégay, 2008;Scott et al, 2000;Singer et al, 2013). Yet there is a lack of information regarding the seasonal progression of water partitioning within these floodplain reservoirs and its uptake by trees.…”

“…Isotope tree-ring studies can provide detailed, historical records of the ecohydrological processes occurring during the time of growth, including the source-water uptake, for riparian forests (Alstad et al, 2008;Marshall and Monserud, 2006;Singer et al, 2013;Singer et al, 2014). This is achieved through analysis of oxygen isotope ratios within tree-ring cellulose (δ 18 O cell ), which preserves a signal from source water used by trees during growth.…”

“…By accurately constraining these variables it is possible to use δ 18 O cell to determine the δ 18 O sw used during the growing season (Singer et al, 2014). Yet, such annual (whole-ring) analyses only provide an integrated signal of water-use over the entire growing season (Alstad et al, 2008;Marshall and Monserud, 2006;Singer et al, 2013;Singer et al, 2014), which may belie seasonal dynamics in water availability. There is, however, additional information stored in tree rings at a sub-annual resolution that could contribute to new understanding of the details of how source-water availability may change throughout the critical growing period in response to climate fluctuations and seasonal evolution of water availability (Treydte et al, 2014).…”

“…Furthermore, the use of co-occurring trees with different rooting depths can bolster the interpretation of seasonal changes in source-water utilization by alluvial forests in response to climatically driven changes in hydrology because the trees integrate different water source reservoirs (e.g. Singer et al, 2013).…”

“…Populus spp. ), which are highly vulnerable to xylem cavitation, leading to reduced growth, branch die back and eventual mortality under prolonged periods of water deficit (Amlin and Rood, 2003;Bréda et al, 2006;Rood et al, 2008;Singer et al, 2013).…”