Resident pasture growth and the micro-environment beneath young, widespaced poplars in New Zealand

Abstract

Poplar (Populus spp.) trees are planted on pastoral hill country throughout New Zealand, primarily for soil conservation. The effect of wide-spaced trees (16-100/ha) aged 8-15 years on understorey pasture growth, botanical composition, and microenvironmental characteristics, was determined at Pohangina (1997-2000) and Lawrence (1998- 2000) in the lower North and South Islands, respectively. Relative to environments without trees, at Lawrence pasture growth (13 100 kg dry matter (DM)/ha/yr) was reduced by 20% and 34% on north and south sides of trees, respectively. At Pohangina (7915 kg DM/ha/yr), trees had less influence (7% and 14% reduction) on understorey pasture growth. There were also strong seasonal effects at both sites, corresponding to when the trees were foliated. Swards at each site comprised 81-93% grass. At Lawrence, swards had similar botanical composition beneath and beyond trees. Swards on the north side of trees at Pohangina comprised relatively high grass and low legume content compared with those on the south side and in the open. Compared with open pasture, areas beneath young poplars received up to 24% less rainfall and 31% less solar irradiance, and soil water content (0-200 mm depth) was up to 33% lower in summer and autumn. At intermediate soil water contents (0.25-0.35 m3/m3), the profile of soil water content around trees during drying was frequently U-shaped, being lowest beneath the stem and increasing with distance away from the tree. Likely reasons for the pasture responses and variation in tree-pasture interactions between sites are discussed. The results will be used to model changes in tree-pasture interactions over time. Keywords: agroforestry, poplar, Populus, silvopastoralism, soil conservation, soil water, treepasture interactions