Changes to Tuart Woodland in Yalgorup National Park Over Four Decades
Total Page：16
File Type：pdf, Size：1020Kb
Proceedings 6th National Conference of the Australian Forest History Society Inc, Michael Calver et al. (eds) © 2005 Millpress, Rotterdam, ISBN 90 5966 026 9 Changes to tuart woodland in Yalgorup National Park over four decades R.D. Archibald, B.J. Bowen & G.E.StJ. Hardy Biological Sciences & Biotechnology, Murdoch University, WA 6150 J.E.D. Fox & D.J. Ward Environmental Biology, Curtin University, WA 6845 Peer reviewed contribution Keywords: fire regime, tree decline, Eucalyptus gomphocephala, Agonis flexuosa, secondary succession, fire management ABSTRACT: The condition of the tuart tree (Eucalyptus gomphocephala), a coastal southwestern Australian woodland species, has declined dramatically within parts of its distribution over the last decade, particularly within Yalgorup National Park. Prior to the park being gazetted in 1968, some of the woodlands were used for cattle grazing. Frequent, light, understorey burns were carried out to encourage grass fodder growth. Earlier, Aboriginal use is believed to have involved a similar re- gime to facilitate hunting and access. Since gazettal, the majority of the park has either been ex- cluded from fire, or burnt infrequently by wildfire and prescribed fire. Consequently, from 1968 to the present, most fires are thought to have been more intense due to increased fuel loads. Altera- tions in disturbance patterns (particularly fire) elsewhere, have been linked with vegetation changes (composition and structure) and in some instances, declining tree health. For tuart woodland, it has been proposed that increased abundance and vigour of the lowerstorey peppermint tree (Agonis flexuosa) and a decline in the health of tuart trees are consequences of reduced fire frequency. Sample plot data from the mid – late 1970s and photographs from 1957 are contrasted with the 2003/2004 situation to describe changes in tuart woodland. Declining tuart health, changes in the health and abundance of some understorey species (for example, fewer Banksia attenuata) and a shift towards peppermint dominance are revealed. The contribution of changing fire regimes to these trends is explored. While a link between fire and changes to the woodland may be estab- lished, factors underlying the loss of tuart dominance remain to be determined. An integrated re- search project is in progress to examine the range of decline factors. 1 INTRODUCTION It is becoming increasingly apparent that the creation of conservation reserves such as National Parks, followed by a passive approach to management, does not guarantee the preservation of an ecosystem in its existing state or condition. This is in contrast to popular public opinion which ig- nores the long (millennial) history of human influence on the biota through modification of fire re- gimes (Hallam 1975, Bowman 1998, Hassell & Dodson 2003) and the recent (decades and centu- ries) changes in the landscape from fragmentation of vegetation, climatic variation, introduction of exotic species and the spread of plant diseases (Soule, 1991). For most Australian ecosystems, dis- turbance, particularly fire, performs a significant role in recycling and renewal. For example, many plant species are markedly dependent on fire for successful reproduction and recruitment (Bell et al. 1993), nutrients are returned from litter and woody debris to the soil following combustion (Grove 363 Millpress koptekst FOREST 1 09-08-2004, 12:37:01 et al. 1986, Adams et al. 2003) and the vigour of some species increases when resprouting after fire dramatically. While the knowledge of fire history for this area is relatively good compared to others, (Wallace 1966, Bowen & Pate 2004). Therefore, maintaining or restoring appropriate fire regimes it is not complete, and there are still only minimal or imperfect data sets available in relation to the is an important tool in the preservation of biodiversity. Where the goal is to maintain biodiversity, vegetation history. Nevertheless, we feel that this preliminary examination of available data is a but where complete knowledge of ecosystem functioning is lacking, maintaining a diversity of fire useful step in testing the assumption that changes to vegetation structure and composition have ac- regimes across a landscape (rather than fire exclusion) is now advocated as a practical, precaution- tually occurred, providing some details on the nature of these changes and proposing further re- ary approach to managing fire prone ecosystems (Bond & van Wilgen 1996, Burrows & Wardell- search direction. A project exploring decline of a tree species (Eucalyptus gomphocephala) from a Johnson 2003). number of research angles and across a wider portion of the landscape is also discussed. The modification of natural fire regimes, usually by way of increasing frequency, decreasing in- tensity and bringing forward the time of burning within the season, has been a practice common to 2 SITE: PHYSICAL FEATURES, VEGETATION AND FIRE HISTORY the world’s aboriginal peoples living in fire prone environments (Pyne 2001). Australian Aborigi- nes used fire as a tool to increase the abundance of resources (plants and animals), in hunting and Yalgorup National Park is located on the southwestern coastal plain of Western Australia between to facilitate movement through the landscape (Hallam 1975, Bowman 1998). Historical accounts Mandurah and Bunbury. A Mediterranean climate prevails with approximately 80 % of the mean of early European settlers indicate frequent burning in the summer months (Hallam 1975, Abbott annual rainfall (870mm) occurring between May and September (Australian Bureau of Meteorol- 2003) with grasstree stem analysis suggesting burning every 3 to 5 years on average in central ogy 2004). Soils are predominantly calcareous sands underlain by limestone which occasionally southwestern forests and woodlands (Ward 2000, Ward et al. 2001). outcrops near ridges (Portlock et al. 1993). Chains of lakes are a conspicuous feature of the Park as From grasstree data (Ward 2000, Ward et al. 2001), dating of fire scars on jarrah tree cross sec- is the diversity of vegetation structural types from herbfields and shrublands to forests and tall tions (Burrows et al. 1995) and the compilation of historical accounts (Hallam 1975, Abbott 2003, woodlands (Portlock et al. 1993). Tuart is a dominant tree species in many of the Park’s woodland Ward 2000), there seems little doubt that a general trend towards declining fire frequency has taken formations. Tuart is restricted to the western margin of the coastal plain of southwestern Western place in parts of southwestern Western Australia since European settlement in 1829. Further, it fol- Australia, however clearing is estimated to have reduced the area of tuart woodlands and forests by lows, and evidence from Burrows et al. (1995) supports this, that greater fuel loads accumulating 73 % since European settlement, consequently there is now a community-wide desire to conserve after longer fire free intervals have contributed to a reduction in the return interval for intense fire remnant tuart woodlands (Government of Western Australia 2003). events. Such events have the potential to bring about drastic and long-term changes to vegetation Ward (2000) has documented the history of burning in the area, which extends back to well be- (Hopkins & Robinson 1981). fore European settlement. Insight into Aboriginal and European burning practices within the wood- A succession of fires with sufficient intensity to cause substantial defoliation or bark damage, lands was gained from the analysis of 58 grasstrees scattered across the National Park and nearby could conceivably impact on the condition of resprouting canopy dominants by eroding their re- private property, plus interviews with local identities and the examination of historical literature. genenerative capacity (depletion of storage carbohydrates or destruction of epicormic buds). Beard An average fire return interval of 2-4 years up until the 1850s (Aboriginal burning) was then main- (1967) proposed such a process, in combination with historical timber cutting and present day in- tained at the shorter end of this range in areas where cattle grazing occurred. A substantial and sud- sect borer attack, to underlie the decline of tree canopies in woodland of Kings Park, Western Aus- den increase in fire return time (between 1 and 0 per decade) has occurred within the National Park tralia. Insect and fungal attack have been observed in other areas where wood has been exposed by area and private property since the 1960 – 1970 period. Today, grasstrees unburnt for 80 years or fire damage (McCaw 1983, Ashton 2000). Beard (1967) inferred that the increased vigour of the more indicate the presence of some very long unburnt areas within the Park. understorey was both a response and reinforcing agent (through competition) in the opening of the Regular burning, two or more times per decade by Aborigines, and then by European graziers, canopy. Given the strong limitations imposed by moisture availability on Leaf Area Index (LAI) in was likely to have involved two features perpetuating one another: low intensity fire and an open Mediterranean climates and the decline in southwestern winter rainfall totals since the 1950s (IOCI understorey dominated by a non-woody, possibly grassy fuel-bed (Bunbury quoted in Ward 2000, 2002), redistribution of leaf area (from the overstorey to the understorey) following disturbance, Bradshaw 2000). This has been reported for other areas in the southwest (Scott 1994)