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

Field study

Soil samples from a widely spaced poplar--pasture (PP) area and an adjacent open pasture (OP) area on the same hillslope were collected for analysis from 3 unreplicated farm sites with mature poplars (Populus deltoides clone I78, >29 years old) and from a replicated experimental site with young poplars (Populus spp., 5 years old). All sites had experienced slip erosion and trees had been planted to stop the process. Soil under the trees was sampled on the debris tail, rather than on the headwall. Open pasture soils showed no evidence of recent erosion (later than the tree planting), according to aerial photograph records, and formed an apparently stable headwall (Pohangina 1 and Rongoiti sites). The Pohangina 2 site showed an earthflow form of erosion as opposed to the slip of Pohangina 1 (Eyles 1983). Two of the sites, Pohangina 1 at 37 stems/ha (sph) and Pohangina 2 at 40 sph had 29-year-old trees and were located on a commercial livestock farm in the Pohangina Valley, longitude 175[degrees]53'E, latitude 40[degrees]08'S, 250 m above sea level (asl). One site, Rongoiti, had 40 years old trees at 40 sph. This site was located on a commercial sheep farm in the Rongoiti area (longitude 175[degrees]41'E, latitude 39[degrees]40'S, 500 m asl). The hills of the Pohangina had a north-westerly aspect with a 20-25[degrees] slope, whereas the Rongoiti hill had a northerly aspect and a 18-25[degrees] slope on the debris tail. The fourth site was at Ballantrae, the AgResearch Hill Country Research Station (longitude 175[degrees]50'E, latitude 40[degrees]18'S, 300 m asl), 20 km from Palmerston North. Poplars were planted on the site in 1992 on a north-easterly aspect with an average slope (20-25[degrees]). The hill at Ballantrae had a number of large old slips delineated by full-length headwalls.