The effect of water availability on stand-level productivity, transpiration, water use efficiency and radiation use efficiency of field-grown willow clones

The effect of water availability on stand-level productivity, transpiration, water use efficiency (WUE) and radiation use efficiency (RUE) is evaluated for different willow clones at stand level. The measurements were made during the growing season 2000 in a 3-year-old plantation in Scania, southernmost Sweden. Six willow clones were included in the study: L78183, SW Rapp, SW Jorunn, SW Jorr, SW Tora and SW Loden. All clones were exposed to two water treatments: rain-fed, non-irrigated treatment and reduced water availability by reduced soil water recharge. Field measurements of stem sap-flow and biometry are up-scaled to stand transpiration and stand dry substance production and used to assess WUE. RUE is estimated from the ratio between the stand dry substance production and the accumulated absorbed photosynthetic active radiation over the growing season. The total stand transpiration rate for the 5 months lies between 100 and 325 mm, which is fairly low compared to the Penman–Monteith transpiration for willow, reaching 400–450 mm for the same period. Mean WUE of all clones and treatments is 5.3 g/kg, which is high compared to earlier studies, while average RUE is 0.31 g/mol, which is slightly low compared to other results. Generally, all clones, except for Jorunn, seem to be better off concerning biomass production, WUE and RUE than the reference clone. Jorr, Jorunn and Loden also seem to be able to cope with the reduced water availability with increase in the water use efficiency. Tora performs significantly better than the other clones concerning both growth and efficiency in light and water use, but the effect of the dry treatment on stem growth shows sensitivity to water availability. The reduced stem growth could be due to a change in allocation patterns.