Are the patterns of regeneration in the endangered Eucalyptus gunnii ssp. divaricata shifting in response to climate? JENNIFER C. SANGER, 1,2 NEIL J. DAVIDSON, 1,2,3 ANTHONY P. O’GRADY 1,2,4 * AND DUGALD C. CLOSE 5 1 School of Plant Science, University of Tasmania, Hobart, 2 CRC Forestry, School of Plant Science, University of Tasmania, Hobart, 3 Greening Australia, Battery Point, 5 Tasmanian Institute of Agricultural Research, University of Tasmania, Hobart, and 4 CSIRO Ecosystem Sciences, Private Bag 12, Hobart,Tas. 7001, Australia (Email: anthony.ogrady@csiro.au) Abstract Increasing drought frequency is a major driver of changes in forest structure and has been implicated in the decline of the endangered tree species, Eucalyptus gunnii ssp. divaricata (McAulay & Brett) in the Central Plateau region of Tasmania, Australia. In this study, we examined patterns of regeneration, aspects of the water relations of E. gunnii ssp. divaricata and its replacement Eucalyptus pauciflora and, whether shifts in stand dominance have occurred where the subspecies co-occurs with E. pauciflora could be related to recent changes in climate. Successful E. gunnii ssp. divaricata seedling regeneration was restricted to micro-sites with relatively deep soils within slight depressions. In contrast, poor E. gunnii ssp. divaricata regeneration and declining adult cohorts of this species all occurred on steeper, concave micro-sites with shallow soils. This apparent shift in suitable regeneration micro-site, from sites with shallow to deeper soils, may be linked to an observed 25% reduction in summer rainfall over the last 50 years. On slopes surrounding waterlogged depressions where E. gunnii ssp. divaricata co-occurs with E. pauciflora, E. pauciflora was in higher abundance than E. gunnii ssp. divaricata in small adult and sapling size-classes, compared with the adult cohorts (>30 cm d.b.h.), a trend consistent with a shift in stand dominance. Despite existing paradigms related to differential drought tolerance between these two species as a driver of this shift in stand dominance, there were no differences in predawn (Y pd) water potentials between species. Further- more, pressure–volume analysis showed that E. gunnii ssp. divaricata had lower values for osmotic potential at turgor loss point (-2.33  0.06 MPa) than E. pauciflora (-2.13  0.03 MPa), suggesting that E. gunnii ssp. divaricata may be more drought tolerant than E. pauciflora, in contrast to the prevailing paradigm that it is more susceptible to drought than E. pauciflora. Key words: Eucalyptus, regeneration, species distributions, water relations. INTRODUCTION During the last 50 years there has been a substantial decrease in rainfall in south-eastern Australia (CSIRO 2007). Some parts of Tasmania have received below average rainfall and frequent droughts continuously since the mid 1970s (Neyland 1999; Calder & Kirk- patrick 2008). Long-term reductions in rainfall are thought to be a leading factor in recent tree decline among some eucalypt species acrossTasmania (Close & Davidson 2004), in other areas of Australia (Fensham 1998; Rice et al. 2004, Jurskis 2005) and in tree species more generally around the globe. As a result, there is increasing focus on the mechanisms underlying of tree mortality and community decline (McDowell et al. 2008; Close et al. 2009; Allen et al. 2010). Differential drought tolerance within communities is thought to be a major factor-driving changes in ecosystem structure, composition and ultimately productivity. Drought can cause differences in decline severity between co-occurring eucalypt species leading to changes in stand dominance, either through direct mortality of adult trees (Davidson & Reid 1989; McDowell et al. 2008) or via effects on recruitment (Lloret et al. 2009). Drought on its own can significantly impact on ecosystem structure and productivity, but so can changes in the temporal distribution and intensity of rainfall. Huxman et al. (2004a) demonstrated that both the timing and magnitude of precipitation events in arid and semi-arid ecosystems have significant impacts on ecosystem productivity. Similarly, Mummery and Batta- glia (2004) used a modelling approach to demonstrate *Corresponding author. Accepted for publication August 2010. Austral Ecology (2011) 36, 612–620 © 2010 The Authors doi:10.1111/j.1442-9993.2010.02194.x Journal compilation © 2010 Ecological Society of Australia