Hydroclimatic drivers of the growth of riparian cottonwoods at the prairie margin: River flows,river regulation and the Pacific Decadal Oscillation

Cottonwoods, riparian poplars, are facultative phreatophytes and can obtain water from shallow soil moisture originating from rainfall, or from the deeper capillary fringe above the alluvial water table that is recharged by river water infiltration. The correspondence between cottonwood growth and river flows should reveal the dependency upon alluvial groundwater and subsequently, the vulnerability to reduced river flows. To explore this association, we analyzed historic growth patterns of plains cottonwoods (Populus deltoides) along the Red Deer River (RDR), which is at the northwestern limit of the North American Great Plains. We developed chronologies of yearly radial increments (RI) and basal area increments (BAI) and explored correspondences with the environmental records from the past century. In this semi-arid region, the RI or BAI were not correlated with local precipitation while negative correlation with growth season temperature (T) (r = −0.37, p < 0.01) could reflect reduced growth with hot summers. There was correlation between growth and annual river discharge (Q, and particularly log Q that approximates river stage) and this increased with two year averaging (r = 0.51, p < 0.01), reflecting carry-over in the watershed hydrology and in the ecophysiological response. There was correspondence with the Pacific Decadal Oscillation index (PDO, r = −0.45, p < 0.01), which provides multi-decade transitions that influence Rocky Mountain headwater precipitation and other weather characteristics, and river flows. The combination of Q, PDO and T provided the strongest multiple regression model, accounting for 44% of the historic growth variation (52% correspondence for 1953–2013). The RDR was dammed in 1983, enabling winter flow augmentation, but summer flows were sustained and cottonwood growth and the streamflow correspondence persisted. This indicates that it is the pattern of dam operation and not damming per se that determines the fate of established riparian cottonwoods downstream. This study revealed that these cottonwoods are phreatophytic and dependent upon alluvial groundwater that is recharged from the river. This provides a research strategy to determine whether riparian woodlands along other regulated rivers are similarly groundwater-dependent and could be vulnerable to river flow reductions from excessive water withdrawal for irrigation or other uses, or with climate change.