Soil recovery from track construction and harvesting changes in surface infiltration, erosion and delivery rates with time

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Abstract

The effects of timber harvesting activities are often characterised by significant changes in surface soil properties, runoff production and erosion rates. Here we quantify trends in surface soil recovery following timber harvesting activities, specifically snig track (skid trail) construction and logging, based on changes in soil surface compaction, infiltration, and erosion with time since initial disturbance. Sites range in age from 0.5 to 5 years. Surface erosion on the snig tracks and general harvesting areas (GHA), and sediment storage and delivery between the two areas, are measured using a large rainfall simulator. Surface soil compaction shows no significant recovery over this time period, illustrating the persistent effects of machinery compaction on surface soil structure. In contrast, surface infiltration rates on the snig tracks increase by a factor of two and snig track erosion rates decline by almost one-order of magnitude over the 5-year period. These findings have important implications for assessing the cumulative impact of timber harvesting activities.

Introduction

Potential impacts of timber harvesting activities are often assessed in terms of changes in surface soil properties, runoff production and erosion rates. A number of studies have documented significant increases in soil compaction following logging operations (Steinbrenner and Gessel, 1955, Greacen and Sands, 1980, Johnson and Beschta, 1980, Jakobsen and Greacen, 1985, Incerti et al., 1987, Rab et al., 1992). Others have reported associated changes in the infiltration capacity and runoff characteristics of areas that have been disturbed during harvesting and road construction (Ziemer, 1981, Riley, 1984, Bren and Leitch, 1985, Huang et al., 1996, Ziegler and Giambelluca, 1997; Croke et al., 1999a, Croke et al., 1999b). The enhanced erosion potential of severely disturbed surfaces such as forest roads and logging tracks has also been well documented in the literature (Reid and Dunne, 1984, Grayson et al., 1993, Reid, 1993, Croke et al., 1999b).

Quantifying the persistence of these effects over time is an integral part of the assessment of forest harvesting impacts but, with few exceptions (Reinhart et al., 1963, Megahan, 1974, Reid, 1993), this aspect has received relatively little attention in previous studies. There also appears to be considerable variation in recovery rates depending upon the index used to assess the impact. Megahan (1974) suggests that erosion rates on forest roads remained one-order of magnitude higher than on undisturbed lands almost 40 years after the period of initial disturbance. Reid (1993) indicates that road erosion rates had reduced to five times the background rate within a period of 5 years after logging. The effects of surface soil compaction have also been reported to persist for between 30 and 40 years after logging has ceased (Dickerson, 1976, Froehlich, 1979, Jakobsen, 1983, Rab, 1998, Rab et al., 1992). It is important to quantify this variability in the recovery of on-site impacts in order to determine potential cumulative impacts over time. Forest harvesting operations are often confined to specific time periods according to inter-harvesting cycles, 30–40 years in this region of southeastern Australia. It is essential that the potential on-site impacts of harvesting practices do not persist beyond these periods of forest regrowth and recovery, both for the sustainability of site productivity and catchment water resources.

The primary objective of this paper is to quantify temporal trends in soil recovery from forest harvesting practices and to examine variations due to the inherent characteristics of the soil and the degree, or severity of, initial surface soil disturbance. Soil recovery is assessed according to changes in surface soil compaction, infiltration, and erosion on highly disturbed snig tracks (skid trails) and less intensely disturbed general harvesting areas (GHA). Sites range in age from 0.5 to 5 years since disturbance. The study draws upon field rainfall simulator studies reported by Croke et al., 1999a, Croke et al., 1999b) which quantified relative differences in runoff and sediment production between snig tracks and the adjacent GHA on sites that had been logged within the past 1.5 years. Here we examine patterns of soil recovery over the longer time period of 5 years.

Section snippets

Study area

This study was conducted in the Eden Forest Management Area (EFMA), a large area of native timber available for harvesting on the southeastern seaboard of New South Wales (NSW) (Fig. 1). The predominant tree species in these forests are Eucalyptus sieberi, Eucalyptus cypellocarpa, and Eucalyptus fastigata which are harvested for both pulpwood and saw logs over a 20–40-year cutting cycle. The experimental plots were located between the Southern Tablelands (680 m asl) and the coastal lowlands (130 

Data collection

Impacts of harvesting practices on these sites were assessed using changes in surface (0–0.1 m)–soil structure, infiltration properties and erosion rates across the nine sites. Changes in the surface soil properties were measured using a mean value of surface bulk density (n=5 on the snig tracks and n=10 on the GHA), using the compliant cavity method (Grossman and Pringle, 1987). Changes in the infiltration properties of the surface soils were assessed using a large (1 m2) drip infiltrometer (

Soil compaction

Snig tracks across all sites have a mean surface bulk density of 1.4 Mg m−3 with no significant differences across the three soil types (Table 2). Surface bulk densities on the snig tracks also show a general trend of increasing with soil depth (Table 2). There is no significant reduction in bulk density on the snig tracks across the three age classes (Table 2). The 5-year old sites retain mean bulk density values of ∼1.3 Mg m−3 indicating the persistence of surface soil compaction over this time

Discussion

This study provides further evidence that forest-harvesting practices, such as track construction and logging can have a significant effect upon surface soil properties and erosion rates. Relative differences in compaction, infiltration rates and surface erosion between the snig tracks and GHA also highlight the notable effect of the degree of initial surface soil disturbance on the magnitude of these impacts. Snig tracks are highly disturbed areas both during the initial construction phase and

Conclusions

This study reviews and extends our understanding of soil recovery on sites subject to timber harvesting activities in the native eucalypt forests of southeastern Australia. Recovery is assessed using a range of indices including surface soil structure, infiltration properties and erosion rates. Soil compaction is the only index that shows no significant recovery over the 5-year time period, highlighting the more persistent nature of machinery compaction on the surface soil structure of

Acknowledgements

This work was funded by the Cooperative Research Centre for Catchment Hydrology, CSIRO Land and Water and the NSW Department of Land and Water Conservation. The authors are particularly grateful to Jim Brophy, Simon Mockler, Matt Nethery and the many people who assisted in the field experiments of 1996–1997.

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