Hydrologic linkages between a climate oscillation, river flows, growth, and wood Δ¹³C of male and female cottonwood trees

Abstract

To investigate climatic influence on floodplain trees, we analysed interannual correspondences between the Pacific Decadal Oscillation (PDO), river and groundwater hydrology, and growth and wood C decreased during low-flow years, especially in trees that were higher or further from the river, suggesting drought stress and stomatal closure, and male trees were more responsive than females (-0.86 versus -0.43‰). With subsequently increased flows, D 13C increased and growth recovered. This demonstrated the linkages between hydroclimatic variation and cottonwood ecophysiology, and we conclude that cottonwoods will be vulnerable to drought from declining river flows due to water withdrawal and climate change. Trees further from the river could be especially affected, leading to narrowing of floodplain forests along some rivers.

Temporal and Spatial Variation in Riparian Vegetation and Floodplain Aquifers on the Regulated Dolores River, Southwest Colorado, USA

“…In August 2011, water levels at LD fluctuated on average 7.83 cm/day in willow-dominated sites (well 1), 7.44 cm/day in Populus-dominated sites (well 2) and 4.99 cm/day in forb and shrub-dominated sites (well 3) -a significantly lower value than on phreatophyte-dominated sites (p = 0.006). Other researchers have observed similar evapotranspiration signals in alluvial groundwater well data (Rood et al, 2013).…”

Water use in a riparian cottonwood ecosystem: Eddy covariance measurements and scaling along a river corridor

“…However, the cottonwood forest soil also had a relatively large water storage capacity (field capacity − wilting point, 1577 − 250 = 1327 mm) compared to the shallow Lethbridge grassland soil (420 − 160 = 260 mm; Hufkens et al, 2016). In addition, the riparian cottonwood trees have roots that extend deep enough in the soil (approximately 2.5 m) to reach the capillary fringe, even when the water table is near its maximum depth below the floodplain soil surface (Rood et al, 2011(Rood et al, , 2013. The deep-rooted cottonwood trees should be able to access groundwater (which extends almost horizontally from the river water) for much of their transpiration requirements in years with river flow rates like that observed in 2015 (Rood et al, 2013).…”

“…In addition, the riparian cottonwood trees have roots that extend deep enough in the soil (approximately 2.5 m) to reach the capillary fringe, even when the water table is near its maximum depth below the floodplain soil surface (Rood et al, 2011(Rood et al, , 2013. The deep-rooted cottonwood trees should be able to access groundwater (which extends almost horizontally from the river water) for much of their transpiration requirements in years with river flow rates like that observed in 2015 (Rood et al, 2013). Despite this, it has been observed that cottonwood growth rates and carbon isotope discrimination declined during years with very low river flow rates, especially in trees that were higher in elevation relative to the river or more distant from the river's bank, implying soil drought effects can affect leaf gas photosynthetic exchange (Rood et al, 2013).…”

“…The deep-rooted cottonwood trees should be able to access groundwater (which extends almost horizontally from the river water) for much of their transpiration requirements in years with river flow rates like that observed in 2015 (Rood et al, 2013). Despite this, it has been observed that cottonwood growth rates and carbon isotope discrimination declined during years with very low river flow rates, especially in trees that were higher in elevation relative to the river or more distant from the river's bank, implying soil drought effects can affect leaf gas photosynthetic exchange (Rood et al, 2013). In addition, the more shallowly rooted herbaceous plants and small shrubs in the cottonwood forest understory are likely even more negatively influenced by the decline in soil moisture that was observed in the cottonwood ecosystem during 2015 (see below).…”

“…Second, the dammed rivers normally only release very low water flows in the later portions of the summer (late June through September) after the natural peak in river flow that occurs in early June. The low stream flow in late summer occurs during the time of warmest air temperatures and high solar radiation input that can lead to drought stress and death of mature cottonwood trees in these riparian forest ecosystems (Rood et al, 2003a(Rood et al, , 2005(Rood et al, , 2008(Rood et al, , 2013.…”