Riparian field elm (Ulmus minor) is sensitive to drought, and fast-growing trees are susceptible to Dutch elm disease

Dutch elm disease (DED) is decimating field elm (Ulmus minor) populations in Europe. The species is subjected to periodic DED outbreaks leading to a loss of vigor (canopy dieback, radial growth, mortality rate), but drought could also stress these riparian stands and interact with DED. However, we lack a quantitative characterization on the impacts of DED and drought on the vigor of elm populations. To assess how river geomorphology, growth rate and intrinsic water-use efficiency (iWUE) influence elm vulnerability to DED, we measured tree-ring, wood anatomy and δ13C in non-declining (ND) and declining (D) trees in an urban site. We also assessed the elm vigor status in 13 elm stands experiencing different geomorphological (river sinuosity index) and stand structural features along the middle Ebro basin (North Eastern Spain). Tree-ring and wood-anatomy series were correlated with climate variables (temperature, precipitation), a drought index and river flow data. Elm was more abundant at sites with lower river sinuosity and where Populus alba was abundant. On average, 64 ± 12 % (mean ± SE) of the elm basal area corresponded to dead trees. The probability of death was above 50 % for elms with diameter > 21 cm. Dry-warm conditions from the prior to the current summer and low spring river flow decreased elm growth. During the severe drought of 2012, the mean ring width was 36 % lower than the long-term average (2.74 mm). However, elm showed a rapid post-drought growth recovery. The earlywood diameter decreased in response to warm conditions in the previous winter. D trees grew more in the past and showed higher iWUE than ND trees. Our findings contribute to better knowledge of declining elms by showing their vulnerability to drought, and how fast-growing elm trees are prone to DED damage.