Interactions between Widely Spaced Young Poplars (Populus spp.) and the Understorey Environment

Silvopastoral systems involving poplars are common to rural landscapes in many parts of New Zealand. The effect of widely spaced trees of Populus nigra × P. maximowiczii, aged 8–11 years, on the surrounding micro-environment in a tree-pasture system was determined over 3 years at a southern North Island hill country site. Relative to open (unshaded, no trees) pasture, understorey pasture received 33% less radiation while radiation on the north side of trees (North) was similar to that on the south side of trees (South). Around one tree, soil temperature averaged 14.9°C annually on the North and 13.8°C on the South. Soil water content was highest in spring and winter (0.35–11;0.39 m³ m⁻³) and lowest in summer and autumn (0.21–11;0.26 m³ m⁻³), and differences occurred between plots in open pasture and those beneath trees in all seasons except spring. Soil water content of tree aspects differed slightly (<10%) in summer (South > North) and autumn (North > South), but not in spring and winter, when contents were similar. Soil pH was 0.2 units higher beneath trees than in open pasture in one of 2 years. Concentrations of Ca, K, Mg, P, and S were similar in tree and open environments. The study results complement those collected for mature trees, and will be useful in developing tree-pasture models.