Water use efficiency, leaf physiology, and productivity of black willow (Salix nigra Marshall) for short rotation bioenergy production in the southern U.S.

Short rotation woody crops (SRWCs) have potential to provide sustainable aviation fuels and exceptional carbon fiber for numerous products. However, many SRWCs are riparian species and measuring water use will ensure that large-scale plantations do not negatively impact surrounding hydrology. Traditionally, willows (Salix spp.) have been important SRWCs in cooler regions like northern Europe and the northeastern US. However, black willow (Salix nigra Marshall) is native to the southeastern US and could be grown in sites that are marginal for poplar (Populus) including heavy clay soils that tend to remain flooded into the growing season. Our research goals were to measure diurnal water use and its correlation with environmental drivers as well as compare physiological parameters including whole-stem water use efficiency (WUE) and productivity across black willow genotypes planted in the Lower Mississippi Alluvial Valley. Across genotypes, yearly aboveground productivity, and whole-stem WUE were positively correlated with each other and were both positively correlated with stomatal sensitivity to vapor pressure deficit. Likewise, whole-stem WUE and leaf carbon isotope ratios were positively correlated suggesting that the latter can estimate whole-stem water use efficiencies for black willow. Atmospheric parameters and shallow groundwater levels were more important in explaining variation in daily water use than soil moisture implying that trees were tapping groundwater resources. In total, this research suggests that black willow is a water use efficient and promising bioenergy crop for the southeastern US, and more work currently underway will continue to develop genotypes and growing practices that maximize biomass production.