Observations of root growth of young poplar and willow planting types

Background: Poplars and willows are used widely in soil conservation in New Zealand to prevent shallow landslides on hill country. Determining when plants become effective at soil stabilisation is important for developing guidelines and policy for land management. The aim of this study was to determine if the size of plant material (0.5-m stakes, 1-m wands, 3-m poles) influences the early below-ground plant growth of poplars and willows used in conservation plantings. Methods: In a small field trial, the above- and below-ground growth of three forms of vegetative planting material (poles, wands, stakes) of two clones of poplar (Populus spp.) and willow (Salix spp.) were determined 9 months after planting. Whole trees were excavated and destructively sampled. Results: Plant growth was rapid and exceeded previously reported New Zealand rates. Willows performed better than poplars in above-ground growth metrics (net tree height, above-ground biomass) and 3-m poles performed the best of the planting types tested (net tree height, above-ground biomass, below-ground biomass, total root length). Lateral roots grew more than 2.5 m and up to 5.5 m from the stem in 9 months. Conclusion: While poles significantly out-performed smaller plant materials in most growth metrics and will continue to be the material of choice in most soil conservation applications, the growth of lesser-grade materials was still rapid and suggests that lesser grade materials could be used for erosion control especially where stock are excluded from planted areas. Root observations such as those from this trial, contribute to improving the understanding of how and when, and at what planting density, plants used in soil conservation become effective for controlling shallow landslide erosion in New Zealand.