Fire Response Syndromes of Shrubs in Grassy Woodlands in the New England Tableland Bioregion
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348 Cunninghamia 8(3): 2004 Knox & Clarke, Fire response of shrubs in grassy woodlands Fire response syndromes of shrubs in grassy woodlands in the New England Tableland Bioregion Kirsten J.E. Knoxa and Peter J. Clarke Botany, School of Environmental Sciences and Natural Resources Management, University of New England, Armidale, New South Wales 2351, AUSTRALIA. a current: Yorke and Mid-North Region, SA Department for Environment and Heritage, PO Box 822, Clare, South Australia, 5453. Email: [email protected] Abstract: In fire-prone grassy woodlands, fire response and time to reach reproductive maturity are two traits that can be used to provide an indication of the minimum interval between fires needed to maintain biodiversity. This study examined the effects of fire intensity and adult size on shrub mortality together with the primary and secondary juvenile periods of shrub species in the New England Tableland (NET) Bioregion. Most shrub species resprouted via basal lignotubers following fire, irrespective of fire intensity and shrub size. The primary juvenile period of most species was found to be greater than four years and the secondary juvenile period for most resprouting species was less than four years. These results suggest that a minimal interval between fires of eight years may be needed to maintain shrub species in grassy woodlands in the NET Bioregion, and that repeated fires at intervals of less than 8 years should be avoided. The time taken for shrubs in the grassy woodlands of the NET Bioregion to reach reproductive maturity appears to be longer than conspecifics in other Bioregions. Caution is needed when using data collected from outside a Bioregion to determine minimum fire frequency thresholds. Cunninghamia (2004) 8(3): 348–353 Introduction attain fire-tolerance (obligate seeders). Knowledge of patterns of fire-tolerance and percentage mortality in different size- Knowledge of how plant species respond to fire regimes is classes allows greater understanding of how species will fundamental for management of biodiversity. In fire-prone react under different fire regimes, especially when communities, studies of the fate of the standing population, communities are dominated by species that resprout. and of the time taken to reach reproductive maturity post- fire, are of two-fold importance. Firstly, when no The time taken to reach reproductive maturity following fire quantitative data are available about the effects of different is important for both obligate seeders and resprouters, as it is fire regimes on communities, species attributes can be used directly related to generation length and will affect the to predict qualitative changes in species composition (Noble capacity of a population to increase in size (Whelan 1995). & Slatyer 1980). Secondly, the continued study of factors The length of the primary juvenile period (the time taken to affecting the post-fire survival of populations can be used to reach reproductive maturity from seed) is particularly build on current models. important for obligate seeders as population decline or local extinction could potentially occur if the interval between fires The fire response of an individual plant will either be death is shorter than the time taken to reach reproductive maturity or survival and at a population level this dichotomy is often and to accumulate an adequate seedbank (Cary & Morrison applied to describe species that survive fire (resprouters) and 1995, Keith 1996). The time taken for resprouting those killed by fire (obligate seeders). In reality, however, a individuals to flower is referred to as the secondary juvenile continuum from 0–100% mortality of individuals within a period. Population decline may occur if the time between fires population exists among species (Morrison 1995, Bond & is less than the secondary juvenile period of resprouters, Van Wilgen 1996, Morrison & Renwick 2000). particularly those species that do not generally display 100% Characteristics of a particular fire, distribution of size-classes post-fire survival (Keith 1996). Age at first reproduction has and the physiological and anatomical features of a species been found to vary among species within a community will affect the percentage mortality of a population post-fire (Benson 1985, Bradstock & O’Connell 1988), between (Whelan 1995). Bond and Van Wilgen (1996) formalised size- populations (Benson 1985) and between individuals within a specific post-fire survivorship into four survival curves. In population (Carthew 1993). Resprouting species generally summary, the first group (type I) acquires fire-tolerance take longer to flower from seed than obligate seeding species quickly, and has little post-seedling mortality; the second (Abbott 1985, Bell 2001). The secondary juvenile period of group (type II) becomes increasingly fire-tolerant with resprouters, however, tends to be shorter than the primary increasing size; the third group (type III) attains fire- juvenile period for resprouters and obligate seeders (Zammit tolerance during juvenile stages, but loses fire-tolerance with & Westoby 1987). age, and the final group (type IV) are those species that never Cunninghamia 8(3): 2004 Knox & Clarke, Fire response of shrubs in grassy woodlands 349 Knowledge of the fate of the standing population and the time Eucalyptus melliodora and Eucalyptus viminalis. The to reach reproductive maturity after a fire event has increased dominant species in the herbaceous layer were Poa sieberiana steadily over the past decade for many community types in var. sieberiana and Themeda australis. Australia. However, little is known about the fire ecology of Booroolong Nature Reserve (865 ha in size) is 30 km NW of woody plants in grassy woodlands in eastern Australia (see Armidale (National Parks & Wildlife Service 2002). No review by Clarke 2000). In particular there is a dearth of specific data are available on the climate of Booroolong knowledge about the shrub species occurring in the cool Nature Reserve, although the climate is similar to that of temperate regions in the New England Tableland Bioregion nearby Guyra, average maximum temperature in January of northern NSW (NET Bioregion). Some fire response and (summer) is 24.6oC; average minimum temperature in July maturation observations from coastal grassy communities (winter) is 0.6oC, average yearly rainfall is 884 mm). Areas have been made but it is not known if it is appropriate to burnt in experimental fires ranged from 1290–1310 m extrapolate data from conspecifics in different climates for elevation, on yellow podsolic soils, overlying a metasediment use in fire management planning. lithology. The areas were generally flat or had a slight to This study addresses four questions: (i) Which species medium slope. No fires have been recorded at Booroolong resprout following fire and where does resprouting arise? (ii) NR for the past 20 years (National Parks & Wildlife Service Does plant size influence post-fire survival for those species 2002) and the area may not have been burnt for more than 50 that show some degree of resprouting? (iii) Does fire years, based on the lack of fire scars. Vegetation structure at intensity affect mortality for those species that show some Booroolong NR ranges from woodland to open forest with a degree of resprouting? (iv) What is the length of the primary sparse to medium density of shrubs in the understorey and a and secondary juvenile periods of shrub species in grassy dense herbaceous layer. The dominant tree species in the study woodlands? sites were Eucalyptus caliginosa, Eucalyptus dalrympleana subsp. heptantha, Eucalyptus laevopinea and Eucalyptus Methods radiata. Poa sieberiana var. sieberiana and Themeda australis were common in the herbaceous layer. Study area The New England Tableland Bioregion (NET Bioregion) Fire response traits of shrub species covers 3 004 080 ha., about 40% of which, native The fire response of shrub species was determined following vegetation remains (National Land & Water Resources Audit three experimental burns at each Nature Reserve. 2001). About 7.5% of extant native vegetation is within Experimental burning occurred at Imbota NR in Spring 1999, conservation reserves (Benson 1999). Grassy woodlands once and at Booroolong NR in Autumn 2000. Each burn site was covered extensive areas of the NET Bioregion (Benson & approximately 50 × 50 m and had an average fine fuel load of Ashby 2000) but, have been extensively cleared for approximately 8 tonnes/ ha. Before each fire, fine fuel (straw) pastoralism. Remnant patches of grassy woodland occur in was added to half the area of each burn site in order to conservation reserves, travelling stock reserves, roadside increase the fuel loads to at least 16 tonnes/ ha so as to reserves and on private land. Clarke (2003) demonstrated that determine the effects of different fire intensities on plant under pastoralism, shrub abundances decline, although the mortality. Recording the intensity of fires is inherently herbaceous layer remains relatively intact. difficult (Whight & Bradstock 1999) and intensity was not Fire response studies were conducted in two grassy measured in the current study as the rate of spread could not woodland conservation areas: Imbota Nature Reserve and be determined due to the way the fires were lit (along control Booroolong Nature Reserve. Imbota Nature Reserve (218 ha lines and then burnt towards the centre). Kitchin (2001) used in size) is 10 km SE of Armidale (National Parks & Wildlife