and Proceedings of/he Royal Society of Tasmania, Volume 122(2), 19RR 109

SOIL EROSION BETWEEN BIRCHS INLET AND ELLIOTT BAY', SOUTHWESTERN TASMANIA

by M. Pemberton

(with two tabIes, one text-figure and one plate)

PEMBERTON. 1987 (3] :x): Soil erosion between Birchs Inlet and Elliott Bay, sOll!hwestem Tasmania, Pap, Tasln, 122(2): 109-J 14. ISSN 0080-4703, Chemistry and Soils Section, Department of Agricultw-e, l-lnbart, Tasmania,

Organic soils (peats) in areas of southwestern Tasmania have been degraded by fire and subsequent erosive processes, area affected by frequent firing has been examined and mapped to demonstrate the extent and severity of erosion. A high fire frequency is likely to lead to increased peat loss, J decrease in height and cover of vegetarion and ultimately to areas denuded of peat. Implications for land management are discussed, Key Wonls: soil erosion, OJ'ganlc soil, peat, southwestern Tasmania, soil e~lIsion mapping, land degradatiml.

INTRODUCTION Osmund and the Wanderer River, East and south of Mount Osmund acid to intermediate rocks of the Extensive areas of southwestern Tasmania Cambrian Mount Read Volcanics occur while are covered blanket bogs that are particularly Cambrian granites are restricted to the area around vulnerable to degradation by burning. Gore (1983) Elliott The topography, which forms part of describes blanket bogs as organic soils or peats that the lower coastal surface defined by Davies (1959), are not confined to depressions but cover covers an altitudinal range of 0 to 400 m. undulating terrain in response to a very humid conditions prevail around most water courses and climate. This paper examines the extent, severity the rest of the area is dominated by undulating type and topography location of erosion in an area plains or low, terraced slopes and plateaux. south of Birchs Inlet. The literature contains no Organic soil covers most of this terrain. detailed accounts of soil erosion in southwestern Ailuvial deposits are restricted to water courses and Tasmania although nutrient loss following peat sands are associated with dunes around Elliott Bay. combustion has been recorded (Bowman ! 980, Organic soils have fibrous surface horizons which Bowman er al. 1986, Brown & Podger 1982). are usually 5 to 15 cm deep and reddish brown in Bowman & Jackson (1981) have commented on colour, with underlying muck peat horizons that are peat destruction, leaching of ash and overall between 10 and 20 cm and brown to grey in decrease in nutrient levels following a prescribed colour. The muck· peats often grade into sandy burn of sedgeland/heath in this region. Wein (1981) gravels at approximately 30 em, North of Moores has documented peat loss associated with burns in Valley unconsolidated gravels, sands, silts Of northwestern Tasmania and Kirkpatrick & of Tertiary age may underlie the organic soils. In Dickinson (1984) examined Joss of surface soil very drained situations peats may reach organic matter after fire in Tasmanian alpine areas. 150 em in depth.

area with Gymnoschoenus THE STUDY AREA Mclaleuca squamea, M, squarrosa, Leptospermum nitidum, Restio spp, and incarnata The srudy area is situated south of common, while scattered taller individuals of Harbour and extcnds from Elliott Bay in the south Hakea and Banksia marginata often to Birchs Inlet in the north, covering an area of occur. Serub has in some gullies with 550 km' (fig. 1), Interbedded Tertiary gravels, riverine rainforest along rivers (Davies 1983, sands, silts, clays and lignite bands. oecur across the Gibson 1986), Average annual precipitation in the northern extent of the area (i.e. north of Moores study area varies between 1800 and 2000 mm. Valley airstrip) while Ordovician siliceous Two bands of one from Port sediments are restricted to the area between Mount Davey, the other from Low Rocky Point frequented f fO M. Pember/on

the area between Elliott Bay and 13 irchs Inlet and kept it open by firing (Jones 1974). Most of these people were removed from the area in the 1830's. In the following years various explorers, surveyors, whalers and piners visited the region and commented on the open nature of the country (Gowlland & Gowlland 1975). It is likely thaI occasional firing occurred over the next one hundred years. Most of the recent fire,; have been attributed to fishermen, Commonwealth and Tasmanian Government employees and people involved with mineral indication of the location and area affected by recent fires in the study area see table I The study area, parl of the South Wes! Conservation Area, is managed by the Tasmanian Department of Lands, Parks and Wildlife. Minerai exploration is occurring in the region due to interest in the massive sulphide deposits associated with the Mount Read Volcanics (Large et at. 1987).

METHODS

To assess soil degradation, the study area was initially examined using aerial photographs (1 :40 000 black and white photographs, flown 1984). Field inspection was conducted as part of the South West Land Systems Survey (Pemberton, in press). During this survey, information was collected on soils, soil erosion, vegetation (using TASFORHAB - see Peters 1984) and general site characteristics (e.g. topographic position, slope and drainage). Aerial photographs and 1: 100 000 topo­ graphic map sheets were used for detailed mapping of erosion types, as outlined in table 2. Estimates of percentage bedrock or mineral substrate exposure was made for each square kilometre to produce figure I. Sedimentation of creeks and rivers often occurs in the vicinity of sheet or gully eroded areas. Areas of sedimentation are shown in figure 1.

RESULTS

o ~0r!:~~OCk or minerai substrate The extent of the various forms of erosion Etl SEDIMEr~TATIO~J and sedimentation are shown in figure I, and the -',- SHEET EROSII»~ BOlJNDil,~Uf:'~ extent of sheet erosion is presented in table 2. Sheet STuDY AR[A 80Ut-.JDAI~Y 2 ) --...._ CO_ASTLINE OR RIVEfiS,' CREEKS erosion has affected just over 42% (235 km of the study area. Ground checks have confirmed the -',_ TRACK LX~~£'*..2!.'!:.,-.t existence of sheet erosion over extensive areas FIG. 1 - Extent and severity of various forms of where peat has not been completely removed. It appears that much of the study area, with the soil erosion hetween Birch~ Inlet and Elliott Bay, southwestern Tasmania. Soil erosion in southwestern Tasmania 11 J

TABLE 1 Recent Fires in the Study Area*

Date Locatioll Approx. area burnt (km')

]9t,9-70 North from Low Rocky poim 20 1913-74 South from Birchs Inlet to 90 Moores Valley 1975 Northeast region 55 1976 Westem edge 25 1977-78 Southwestern edge 10 1985 Birchs Inlet to Elliott Bay 335

* From unpub!. map, Department of Lands, Parks and Wildlife.

exception of some swamp and coastal up to 0.3 m of peat have been lost from areas where environments, has been seriously degraded by this bedrock is exposed. form of erosion, Conservative estimates suggest Serious gully erosion appears to be restricted that at least 50 km 2 is almost devoid of soil. to the area between the Conder and Spero Rivers The most severe degradation has occurred and to a small region north of the Spero River. The between Birchs Inlet and the Wanderer River, gullies typically occur at the headwaters of where peat horizons overlie Tertiary deposits. drainage lines and often coalesce to form These peat horizons have been removed by wind, units which can be. over 10m deep and 50 m wide water and fire from well drained slopes, ridges and with gently inclined floors but precipitous walls. crests. They are more susceptible to degradation Large volumes of material from gully and than organic soil of adjoining lowland swamps sheet erosion have probably contributed to an because they are more exposed and tend to dry out increased rate of sedimentation on alluvial flats more rapidly. Sheet erosion has left most of the along sections of the Birchs, Sorell, Spero and steeper country with a denuded appearance Pocacker Rivers where decreased stream gradients (plates I C and I D) and field observations suggest and velocities favour sediment deposition.

TABLE 2 Erosion Types, and Areas Affected by Sheet Erosion

Type Description Areas affeded PCI'I:entage (km') of study area

Gully Erosion that forms large gullies at headwaters of drainage lines (plates J A and IB)I Sheel Removal of soil layers by fire, wind and water (p.latcs 1C and I D). Sheet-eroded areas were identified where bedrock or mineral substrate exposure was evident. Sheet erosion has been classified into: Category I _.- >60% bedrock or mineral substrate exposed; 20 3.6 Category 2 ~ 60-30% bedrock or mineral substrate exposed; and 45 8.2 Category 3 -- <30% bedrock or mineral substrate exposed. 170 3(,0

Total 235 42.8 ...... N

~ f

A B

"

PLATE 1 (A) Gullies forming at the headwaters of drainage lines after peat removal. (B) Small gullies coalesce to form larger units. Geology: unconsolidated Tertiary gravels, sands and clays. (C) Severe sheet erosion on well drained positions in the Moores Valley area. (D) Sheet erosion on crests, slopes and ridges near Moores Valley. Soil erosioll in southwestern Tasmania / J3

DISCUSSiON soils are very important llutrient sources in the blanket (Bowman & Jackson Tht' stud, area includes some of the masl 1981) particularly as they often overlie infertile eroded and degraded iand in southwestern siliceous substrates. They can be degraded Tasmania. firing during firing, and some authors have thaI, have initiated soil erosion it is although fires have occurred in the region for firing in recent limes has thousands of years, have increased over Large iracts of land in the recent limes (Kirkpatrick 1983). Although soils and arise from strikes 1983) affectcd sheet erosion result from acts of arson or are started acc]dent sunounding Point Vivian, the Snch fires are most to start hot, Ped Point Hills, the New summer conditions when peat burns are Rowitta Plains, Mount Rugby, Lost Hazard reduction burns have been carried World Plateau. North Broken Hili and the Lawson out for the last couple of decades in the Giblin River Plains, Piners Peak and the some have peat horizons. Organic soils Denison, De Wilt, Erskine and Propsting near the Scotts Peak Road were destroyed a The vegetation associated with hazard reduction burn (Bowman & Jackson 198 j) peat soils in Tasmania is prone to firing due to the while another in t.he Vale of the Rasselas extended flammability of many of the species (such as up the slopes of the Denison Range with similar sphaerocephalus and various results. Tyson (1983) reported that a hazard Melaleuca, Lepfospermum, Baeckia and Boronia reduction burn in the Port Davey area did little spp.), and the hot, dry conditions, which are damage to root systems as the soil was saturated, relatively common during summer. which suggests there was little or no loss of peat. promotes sheet erosion be removing The most recent fire in the study area p"e'.,nF·d vegetation whIch hinds the organic soH but more from a bum intended to create a favourable habitat peat and producing an ash that for the rare orange .. bellied parrot. Re .. assessment of removed by wind and water. Frequent, this kind of land management is needed in a winds, and a annual average where it is impossible for the peat-forming process the problem. Sheet erosion can to pace with soil loss. It may take decades to occur relatively rapidly on peats compared to restore organic soil cover in the absence of further mineral soils, with destruction of up to 0.2 m of disturbance. If the area is 10 be stabilised, attempts organic soil in one prescribed burn (Bowman & must be made to limit firing. If frequent Jackson 1981). continues, peat loss on well drained sites is Peat absorbs water and reduces the rate of inevitable, with an increllse in gully erosion, runoff. Therefore any substantial loss of this soil sedimentation, and flooding. Some sheet-eroded results in increased surface water flow. Reduced areas have minor peat accumulations where peat vegetation cover on areas of denuded peat could occur. Frequent firing ill intensifies this problem. Both of these factors southwestern Tasmania has consumed organic soil, contribute to the spread and of sheet initiated sheet erosion and reduced the nutrient and gully erosion. level of 1''''''''''''''' Gullies appear to have in some The rate of peat formation has not been places after the removal of peat horizons sheet measured in the but it is likely to be erosion. The main occurrences are between the slow. Horizons which took Spero and Wanderer Rivers I). They have centuries to can hum in a few hours. formed on sands. silts and Considerable work on aspects of peat which are the extent of peat erosion, rate of peat to erosion. Ero~ion appears to be and the effects of from both the headwaters and sides of the if Tasmanian peats and of sediment, derived from animals are to be appro· gully and sheet eroded areas, is creeks and rivers. This increas\:s the as there is more matenal in lines and iess peat in the catchments. An increase in lines could in SOIT1C cases lead to J 14 }d. Pemberton

ACKNOWLEDGEMENTS GORE, AJ.P., 19i\3: ECOSYSTEMS OF fHE WORLD. 4.4 - MIRE.)'.' SWAMP, BOG. FFN AND Juhn Grant is thanked for his heip in the MOOR. Elsevier, Amsterdam: 350 pr. field. lhe following are acknowledged for their GOWLLAND, R. & GOWLLAND, ·K., 1975. TRAMPLED WiLDERNESS - THE lIfSTORY criticism and comments: Ray Carpenter, John OF SOUIJI-WEST TASMANIA. Richmond and Davies,John Grant, Piene Horwitz, Kevin Kiernan, Sons, Devonport: 160 pp. Fiona Preston, Lindsay Richley, Mike HODGE, G., J983: The practical aspects of fire control. Smith and Steven Smith. John Davies and John in Gee, H. & Fenlon, J. (Eds): THE SOUTH Grant are especially thanked for many hours of WEST BOOK. Au:itralian Conservatioa dIscussion about vegetation and' soils in Foundation, Collins: 30g PI'. southwestern Tasmania. The South West Land JONES, R, 1974: Tasmanian uitles. Tindaic, N.B. Systems Survey was conducted by the Chemistry (Ed.): ABORIGINAL TRIBES OF A.USTRALlA. and Soils Section of the Department of Agricultur~, ANU Press: appendix KIRKPATRICK, J.B., 1983: Fire and plant communities Tasmania. Funding for this survey has been in the South West. In Gee, H. & Fenlon, J. (Eds): provided under the Commonwealth Government THE SOUTH WEST BOOK. Australian National Soil Conservation Programme (NSCP) Conservation Foundation, Collins: 30/\ pp. under the Soil Conservation (Financial Assistance) KIRKPATRICK, J.B. & DICKINSON, K.J.M" 1984: Act 1985. The impact of fire on Tasmanian alpine vegetation and soils. Aust. 1. Bot. 32: 613-629, LARGE, R.R., HERRMAN, W. & CORBETT, KD., REFERENCES t 987: Base metal exploration of the Mount Read Volcanics, western Tasmania: part 1. Geology and exploratioH, Elliott Bay. Econ. Geol. 82(2): BOWMAN, D.M.J.S., 1980: Fire, nutrient and vegetation 267-289. interactions in South West Tasmania. Unpub!. PEMBERTON, M., in press: LAND SYSTEMS OF ROllS thesis, Un;v. Tasm. Bot. TASMANIA. REGION 7 - THE SOUTH WEST. BOWMAN, D.M.J.S. & JACKSON, W.D., 1981: Department of Agriculture, Tasmania. Vegetation succession in South West Tasmania. PETERS, D.G., 1984: TASFORHAB. In Meyers, K & Search 12(10): 358-·362. Musto, L (Eds): PROCEEDINGS OF A BOWMAN, D.M.J.S., MACLEAN, A.R. & CROWDEN, WORKSHOP ON SURVEY METHODS FOR R.K., 1986: Vegetation·soil relations in the NATURE CONSERVAII0N. CSIRO, Canberra: lowlands of South West Tasmania. Aust. 1. Ecol. 179-192. ll: 141-153. TYSON, R., 19X3: Fire management in National Parks. BROWN, M.J. & PODGER, F.D., 1982: Floristics and In Gee, H. & Fenton, J. (Eds): THE SOUTH fire regimes of a vegetation sequence from WEST BOOK. Australian Conservation sedgcland-heath to rainforest at Bathurst Foundation, Collins: 30R pp. Harbour, Tasmania. Aust. J. BOI. 30: 659-676. WElN, R.W., 1981: Characteristics and suppression of DAVIES, I.B., 1983: Huon pine survey 1983. Tech. Rep. fires in organic terrain in Australia, Aust. For. 83/2. Tasm. Natl Pks Wildl. Serv.: 46 pp. 44(3): 162--169. DAVIES, J.L., 1959: High level erosion surfaces and landscape development in Tasmania. Aust. Geogr. 7: 193-203. GIBSON, N., 1986: Conservation and management of Huon pine in Tasmania. Tech. Rev. 86/3. Tasm. NaIl Pks WildL Serv.: 9l pp. " (accepted 12 February 1988