Patterns and drivers of cottonwood mortality in the middle Rio Grande, New Mexico, USA

Riparian ecosystems are some of the most valuable and vulnerable on the planet. Riparian tree mortality is increasing in the western United States, where altered flows are combining with warming climate. Between 2011 and 2013, one-third of an extensive stand of Populus deltoides var. wislizenii (Rio Grande Cottonwood) died along the Middle Rio Grande on the Pueblo of Santa Ana in New Mexico. Mortality coincided with a severe drought, but it was heterogeneous, with adjacent patches of dead and live trees. Research goals were to determine the drivers of mortality to provide insights into future risks of die-off and potential management interventions. We compared tree diameter, age, competition, tree-ring widths, sediment particle size, and climate influences between live and dead forest patches in a nested plot design. Live and dead trees had similar age, stand density, and particle sizes of shallow sediments. Tree-ring widths had the highest correlations with July – September streamflow (1932 - 2013). All trees had declining ring growth since 1992, coinciding with declining late-summer streamflow. An accelerated decline in growth began in 2002, corresponding to recent warmer droughts. Trees that died had smaller diameters and lower ring growth three years prior to death and in the mid-1900s. Dead trees also had coarser deep sediments < 2.4 m, suggesting that water holding capacity was an important factor for mortality. Water management to increase streamflow during the late summer, especially during times of extended drought, could reduce mortality risk in the face of projected increasingly warm droughts.