Soil properties of a widely spaced, planted poplar (Populus deltoides)--pasture system in a hill environment.

Introduction

Soil erosion is common in New Zealand on sedimentary mudstones, siltstones, and pumice soils (Kelliher et al. 1995). Most of the slip, slump, and gully erosion has been a consequence of forest clearing for pastoral agriculture dating back to European settlement (Hicks 1995). Studies of slip scar recovery on hill pastures have shown that 20 years after the erosion event pasture production has not recovered to pre-erosion levels (Lambert et al. 1984). If erosion scars are not protected, secondary events may occur eroding the bedrock, particularly on unconsolidated sandstone. In the past 40 years, extensive areas of pastureland susceptible to erosion have been reforested with Pinus radiata. On land valuable for grazing, widely spaced poplars or willows have been planted to increase soil stability, while retaining the pastoral land use (Hicks 1995; Wilkinson 1999).

New Zealand research on the effect of tree planting on soil properties under widely spaced trees has been restricted largely to P. radiata agroforestry systems on flatland (Hawke and O'Connor 1993; Perrott et al. 1995), 7b date, the effect on soil properties of exotic broadleaved, deciduous trees such as poplar (Populus deltoides) has not been studied. Elsewhere, Singh et al. (1989), Park et al. (1994), and Thevathasan and Gordon (1997) have reported that poplar trees increase soil organic matter, nutrient status, and earthworm populations on cultivated land. Increases in soil pH, mineral nutrients, and organic matter have been reported under poplar forest and plantation stands (Bowersox and Ward 1977). Most soil changes have been attributed to mineral leaching from the canopy (Mahendrappa 1983) and leaf litter accumulation and decomposition (van Cleve and Noonan 1975). Care must be taken in extrapolating these findings to widely spaced poplars as most of the published work is based on densely planted systems.

A young (5 years old) and a mature (>29 years old) poplar--pasture system were compared with a stable, open hill pasture, all grazed with sheep or cattle, with the objective of examining whether poplar impacted on key chemical, physical, and biological properties of the soil. The productive potential of the soil was also evaluated for its effect on the germination and growth of commonly used pasture species by the addition of poplar leaf litter to soil collected from the mature poplar--pasture and open pasture areas.

Material and methods