Towards making willows potential bio-resources in the South: Northern Salix hybrids can cope with warm and dry climate when irrigated

Willow (Salix spp.) is an attractive biomass resource for many regions, but is today grown commercially mainly in cool-temperate areas. It is unclear whether modern willow hybrids bred for cool-temperate climate are capable of regulating strong water losses when exposed to warm and/or dry climate. The objective was to assess leaf scale water relations (evapotranspiration, E; and stomatal conductance, GS) and corresponding leaf traits in six wild and hybrid willows field-grown in Central Sweden (cool and well-watered), Northern Portugal (warm and dry), and Northern Italy (warm and well-watered). Diurnal courses of E, GS and leaf temperature were recorded, plant heights measured, and leaves sampled for assessment of specific leaf area (SLA) and area-based leaf N content (Na). Height growth, GS, SLA and Na varied between the genotypes, but genotype environment interaction was important only for plant height and GS. Thus, genotypic variation in leaf scale E was mostly caused by stomatal (GS) and not by non-stomatal (leaf temperature) genotypic variation. Leaf scale E was positively correlated with Na when assessed across the drought gradient. It is concluded that the willow hybrids bred for cool-temperate climate (in Scandinavia) are capable of regulating strong water losses when exposed to warm and/or dry climate (in Southern Europe), provided that water supply is good. The ability to regulate water losses under warm and dry conditions in the short term is a pre-condition to high water use efficiency and improved growth in warm and dry environments also in the long term.