Post-fire hillslope erosion response in a sub-alpine environment, south-eastern Australia

doi:10.1016/j.catena.2007.11.003

CATENA

Volume 73, Issue 3, 15 May 2008, Pages 274-285

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Abstract

This paper examines post-fire erosion response in a sub-alpine environment in south-eastern Australia for a period of 2.2 years. Few studies have examined fire impacts on sediment transfer in this environment. Erosion pins were used in grids located at upper, mid and lower slope positions on adjacent burnt and unburnt hillslopes to assess fire effects on the extent of surface level change. The results indicated that there was a significant difference between the surface level change regimes on the burnt and unburnt hillslopes. Estimated erosion rates for the burnt slope over the study period ranged from 2.7 to 94.3 t ha^− 1, which could be considered low given the high slope angles, high precipitation and moderate fire severity. Slope position was critical in modifying post-fire erosion response, as it controlled slope angle and the rate of surface cover regrowth. Analysis of lower slope sites, for which more detailed data was available, indicated a second delayed erosion peak after the initial elevated post-fire response during the following spring snowmelt period. Surface recovery on the lower burnt site was slow, with vegetation cover still comparatively low 2.3 years after the fire. Evidence of post-fire sediment supply limitation was found on this site, with a declining rate of increase in the magnitude of total surface level change, despite limited regrowth and an increasing number of precipitation events > 20 mm for measurement intervals since the fire. Modification of the hillslope surface by fire leads to changing hillslope erosion process dominance in this environment. The post-fire hillslope undergoes erosion by direct rain-drop impact and overland flow, whereas the unburnt slope rarely experiences overland flow due to the thick ground cover. As a result surface level change on the unburnt slope was largely influenced by wetting–drying effects rather than sediment transfer by surface flow. Downslope biotransfer appears to be the dominant sediment movement process in the unburnt sub-alpine forest environment.

Introduction

Wildfire can be an important component in geomorphological change (Shakesby and Doerr, 2006). Modifications to the landscape by fire can lead to significant ongoing changes to erosion rates and the hydrological response of burnt areas (Cerdà and Lasanta, 2005). Fire effects contributing to altered surface processes include reduced ground cover (White and Wells, 1979, Prosser and Williams, 1998), changes to surface characteristics (Mitchell and Humphreys, 1987, Lavee et al., 1995), soil structure effects (Kutiel and Inbar, 1993, Rab, 1996), bioturbation (Dragovich and Morris, 2002b, Shakesby et al., 2006) and alterations to soil hydrophobicity and infiltration (Doerr et al., 1996, Shakesby et al., 2000, Martin and Moody, 2001). Variability in post-fire hydrologic and erosion response is further enhanced by the patchiness of fire severity (Kutiel et al., 1995, Dragovich and Morris, 2002a) and variation in environmental factors unaffected by fire (Kutiel and Inbar, 1993), including rainfall amount and intensity (Atkinson, 1984), slope and aspect (Marques and Mora, 1992) and vegetation type, affecting the rate of recovery (Kutiel and Inbar, 1993).

The complex interaction of the heterogenous fire alterations to the land surface and environmental factors contributes to the variety of post-fire erosion and hydrologic response observed in previous research (Shakesby and Doerr, 2006). The coincidence of large magnitude, high intensity rainfall events immediately after the fire is expected to produce the largest erosion response (Prosser and Williams, 1998). However the peak erosion response may be delayed or subsequent peaks may occur due to higher magnitude rainfall events occurring well after the fire (Inbar et al., 1998, Shakesby and Doerr, 2006). A delayed peak in post-fire erosion response has also been attributed to seasonally dependent soil moisture levels and changing surface characteristics (in the absence of significant regrowth) with time since the fire influencing post-fire runoff and erosion rates during the recovery period (Cerdà, 1998). The re-establishment of ground cover is probably the primary factor influencing the recovery time to pre-fire erosion rates (Benavides-Solorio and MacDonald, 2001). Variability in post-fire erosion response, including delayed peaks in erosion following fire, emphasises the need for longer term continuous monitoring. In particular, Shakesby and Doerr (2006) note that relatively few studies have undertaken continuous hillslope erosion monitoring beyond the first year after fire.

This study examines hillslope erosion response following fire in a sub-alpine environment in south-eastern Australia. Few studies have examined fire impacts on sediment transfer in this environment. Alpine and sub-alpine environments in Australia are limited in extent, occurring mainly in Tasmania and the Australian Alps in south-eastern Australia. Previous research in these environments has reported higher catchment runoff and sediment yields following fire, with a return to pre-fire hydrologic patterns within 4 years at the small catchment scale (Brown, 1972). Costin et al. (1960) noted the high hillslope erosion hazard following fire in sub-alpine woodlands when the ground cover of herbs and herbaceous litter had been destroyed. Good (1973), in a preliminary, largely qualitative survey of erosion after fire which burnt 12 000 ha above 1360 m noted some areas of severe erosion existed, but mostly only minor erosion was evident.

The fire which burnt through the study area occurred in January 2003 during a period of drought in eastern Australia. Severe electrical storms started 140 lightning strike fires throughout Victoria, New South Wales and the Australia Capital Territory (ACT); these eventually combined to burn 1.73 million hectares. This study investigates the erosion response following this fire in a sub-alpine environment by monitoring surface level change on adjacent burnt and unburnt hillslopes. It examines the erosion response for a period of 2.2 years (795 days).

Section snippets

Study area

The study area was located in the Snowy Mountains, an elevated plateau region ranging between 1500 and 2228 m. Deeply incised by rivers, there are extensive areas with steep slopes. Glaciation during the late Pleistocene affected only a limited area (Costin, 1954). The geology of the region is dominated by granites, with gneisses, porphyries and some Tertiary basalts and metamorphosed sediments (Costin, 1954). Shallow gravelly soils are common on steeper slopes and organic-rich humus and peaty

Methods

The erosion response of the post-fire landscape was interpreted from the magnitude of surface level changes recorded from erosion pin grids and compared with changes measured on the unburnt hillslope. Surface gain indicates a reduction in pin exposure, often as a result of deposition. Surface loss reflects an increase in pin exposure, often due to erosion (Haigh, 1977, Toy, 1983, Loughran, 1989). Site characteristics determined the use of pins rather than plots due to the problem of installing

Precipitation

Precipitation during the study period was mostly below average (Fig. 3). A total of 3399 mm fell during the period, below the cumulative monthly mean for the period of 3863 mm. Exceptions to this include August 2003 when 311 mm fell (compared to mean 192 mm), although much of this would have fallen as snow on higher slopes. Notably 89 mm of rain fell on 14 August 2003 associated with relatively mild conditions which resulted in a widespread reduction in snow cover. November 2004 was also well

Discussion

To assess the nature of post-fire erosion response burnt surface level change was examined for the whole slope, individual slope positions and compared to previous research. Monitoring of comparable sites on the unburnt hillslope provided the ability to assess the post-fire erosion response relative to rates of surface level change recorded on the unburnt slope. Analysis of different burnt slope positions enabled distinction of the influence of different environmental factors unaffected by

Conclusion

In this study of post-fire erosion in a sub-alpine environment it was found that the difference in net surface level change between burnt and unburnt hillslopes was significant although estimated net soil loss from the burnt slope was low compared to rates reported in other environments. This is noteworthy in the context of high slope angles, relatively high precipitation, slow vegetation regrowth and moderate fire severity. Erosion response at the total slope scale displayed an initial

Acknowledgements

The authors gratefully acknowledge Alan Smith, Tim Middleton, Jacob Waugh and Viola Bloomfield for their assistance with fieldwork. The NSW National Parks and Wildlife Service are thanked for permission to undertake research in Kosciuszko National Park, with particular acknowledgement of Dr Ken Green for assistance with site selection. An anonymous reviewer is thanked for their comments which helped improve the manuscript.

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Post-fire hillslope erosion response in a sub-alpine environment, south-eastern Australia

Abstract

Introduction

Section snippets

Study area

Methods

Precipitation

Discussion

Conclusion

Acknowledgements

Journal of Hydrology

Catena

Catena

Geomorphology

Catena

Catena

Geomorphology

Catena

Review of Palaeobotany and Palynology

Geoderma

Forest Ecology and Management

Earth-Science Reviews

Journal of Hydrology

Forest Ecology and Management

Erosion damage following bushfires

Journal of Soil Conservation, New South Wales

Post-fire runoff and erosion from simulated rainfall on small plots, Colorado Front Range

Hydrological Processes

Sediment yield from runoff plots following bushfire near Narrabeen Lagoon, NSW

Search