Spatial variation in above-ground growth in unevenly wastewater-irrigated willow Salix viminalis plantations

Abstract

In several Swedish wastewater treatment plants, an improved level of water treatment has been achieved through irrigation of short-rotation willow ( Salix viminalis) coppice (SRWC) with secondary or tertiary treated wastewater. The spatial variation in above-ground growth in two unevenly wastewater-irrigated SRWC fields in southernmost Sweden was measured during 3 consecutive years. Wastewater was distributed on the soil surface through pipes with emitters spaced at distances varying from 10 to 21 m. During the first year of irrigation, a very marked gradient in growth developed, with stools (i.e. resprouted stubs) standing within 2 m of wastewater emitters growing several times faster than those standing 5 m or more away. This situation prevailed after three growing seasons with wastewater irrigation, indicating that any redistribution of plant roots towards the emitters was insufficient for a spatial levelling of the growth. We conclude that, in order to promote growth and fully utilize the treatment potential of SRWC, wastewater should be applied fairly evenly, i.e. through emitters at spacings of less than 5 m.