Population Dynamics of Xanthorrhoea resinosa Pers. Over Two Decades: Implications for Fire Management

Abstract

Fire has an important influence on the biota of Royal National Park and is a factor over which park managers exert some control. Fire management guidelines for biodiversity conservation are expressed as thresholds that define fire-regimes associated with elevated risks of extinction that management must aim to avoid. These thresholds strongly reflect fire-interval effects on fire-sensitive (non-resprouting) species. In contrast, the guidelines are less prescriptive about the characteristics of particular fire events (e.g. intensity, season, post-fire rainfall) or competition because the long-term importance of these factors is less well understood. On-going monitoring is required to determine if conservation goals will be met by management actions under these guidelines, or if they should be adapted to counter previously unidentified negative trends. In particular, populations of resprouting species which appear to be relatively resilient to interval-dependent effects must be monitored to detect subtle, but ultimately dangerous declines. We describe population trends in the iconic grass-tree Xanthorrhoea resinosa based on observations of over 3000 individual plants over a period of 23 years. We identify divergent population trends predicted to result in either local extinction within 200 years or slight declines depending on rates of mortality. Experimental evidence is presented for a strong impact of competition on survival which, at one of the study sites, led to local extinction following a single fire interval of 17 years. The relative importance of mortality associated with heat shock during fire, the effort of post-fire resprouting and flowering and other factors such as disease and competition are discussed. Our results suggest that a focus on minimum fire intervals alone will not guarantee the long-term persistence of key understorey species and that fire regime thresholds should more directly consider functional groups containing species with low potential for population growth.