Pollarding wide-spaced poplar trees on pastoral hillslopes alters root development

Pollarding is a management tool for controlling height of poplars in pastoral hill country. This tool has potential to enhance understorey pasture growth without severely compromising erosion control functions. However, if pollarding reduces the rate and extent of root development then pollarding could reduce tree effectiveness in soil conservation. We hypothesised that (1) pollarding will reduce the mass, length and extension of the root system and (2) that pollarding at the younger age of 8 years will be more detrimental to the root system than pollarding at 12 years (when the root system is more developed). Three treatments, unpollarded (UP), pollarded at ages 8 years (P08), and 12 years (P12) were imposed on wide-spaced Populusxeuramericana 'Veronese' trees growing on a pastoral hillslope. Tree root systems were excavated at 8 (2 trees), 12 (4 trees) and 16 (4 trees) years and root length (RL), root mass (RM) and root spread of coarse roots determined. A model was developed to predict the effect of pollarding of Populusxeuramericana 'Veronese' on coarse root production. Pollarding at similar to 20 cm diameter at breast height (DBH) when the trees were aged 8 years reduced DBH growth in the following 4 years by 24%, RL by 60% and RM by 66%. Trees in the PO8 treatment had significantly less RL and RM below 0.5 m depth within 2 m of the trunk than trees in the UP12 and P12 treatments. At 16 years, P08 trees had reached a comparable DBH with P12 trees, and while RM was 28% less for P08 trees, RL was 13% greater and specific RL was 58% greater. Trees pollarded either at DBH similar to 20 cm (8 years) or at a DBH of similar to 28 cm (12 years) did not differ significantly in RL or RM at age 16 years. At 4 years, and 8 years, following pollarding, RL and RM for the pollarded trees were 35% and 53% less respectively, and 32% and 68% respectively, than RL and RM modelled for UP trees of the same DBH. The model suggests pollarding trees may promote RL at the expense of RM thereby enhancing soil-root contact.