Review of rural tree decline in a changing Australian climate D.C. Close1,2,3* and N.J. Davidson1,3 1Cooperative Research Centre for Sustainable Production Forestry, Private Bag 12, Hobart 7001 2Schools of Agricultural Science and Zoology, Private Bag 54, Hobart 7001 3School of Plant Science, University of Tasmania, Private Bag 55, Hobart 7001 Abstract native animals. Superimposed on these factors are below-average rainfall and above-average Rural tree decline (RTD) in Australia is a temperatures over the past three decades, complex biological issue involving dieback of phenomena that are associated with climate native trees following the removal of native change. Climate change has been linked to vegetation, the introduction of improved eucalypt decline in landscapes with little or no pasture species, soil fertilisation and the rural land use, and to tree decline across Europe. grazing of livestock. The extent and severity The contributions of climate change and rural of RTD has reached historically high levels in land use to RTD are considered in this review and the past two to three decades. As this period recommendations for management are discussed. has coincided with an unprecedented increase in atmospheric carbon dioxide concentration, it may be speculated that climate change, a Introduction result of the Greenhouse Effect, has played a signifi cant role in RTD. Ecological similarities Rural tree decline (RTD) and dieback in between two geographically and climatically Australia have long been observed (Norton distinct Australian case studies (the Midlands 1886) but in the past three decades have of Tasmania and the New England Tablelands of reached historically high levels (Williams New South Wales), both characterised by severe and Nadolny 1981; Rees 2000). RTD has RTD, suggest that the underlying causes of this been particularly well documented for phenomenon have a common basis. two agricultural regions, the New England Tablelands of New South Wales (Williams The biological and ecological effects of rural land and Nadolny 1981; Nadolny 1984; Curtis use are manifold. These include exposure of 1989, 1991a, b; Jones et al. 1990) and remnant trees to increased wind and soil drying; the Midlands of Tasmania (Grice 1995; decreased water availability due to competition Neyland 1996; Kirkpatrick et al. 2000; Doyle with improved pasture and decreased infi ltration 2001), where RTD is particularly severe due to soil compaction by cloven-hooved livestock; (Photo 1). Rural tree decline has also been increased salinity; loss of ectomycorrhizal fungal documented in Queensland (see Wylie et diversity; changed nutrient balance due to direct al. 1993 for a review; Fensham and Holman addition of fertiliser and/or nitrogen fi xation by 1999) and Western Australia (Wills et al. clover-based pastures; changed pest/predator 2001), although the severity of RTD in the and biodiversity balance; and lack of seedling Tablelands and the Midlands may indicate recruitment due to browsing by livestock and/or a future, more widespread phenomenon * Corresponding author as both are at high altitude relative to the e-mail: [email protected] regions studied in Queensland and Western Tasforests Vol. 15 1 June 2004 Photo 1. Dead and dying paddock trees with no evident eucalypt regeneration in the Midlands of Tasmania. Australia, and the Greenhouse Effect has Eucalyptus gunnii alpine woodlands in the largest consequences for higher altitude central Tasmania has been attributed to plant communities (Fanta 1992). below-average rainfall over the last two decades (Potts et al. 2001a). The decline Rural tree decline is associated with serious of eucalypts in regenerating, even-aged environmental and economic costs. Trees forests of New South Wales has occurred in agricultural regions provide shelter to in response to severe competition for water livestock, crops and pastures, prevent soil (‘self-thinning’) (Glasby et al. 1987) and RTD erosion and salinisation, conserve endemic in the Western Australian wheatbelt has species and provide habitat for native fl ora been attributed to salinity stress (Greenwood and fauna. It has been estimated that it et al. 1995). The overall objective of this would cost in excess of $20 billion to replace review is to consider climate change as a trees (assuming successful revegetation; factor in RTD; research conducted in two see Close and Davidson 2003a, b) currently relatively well-studied cases, the Midlands of at risk of RTD in agricultural regions of Tasmania and the New England Tablelands Australia alone (Reid and Landsberg 2000). of New South Wales, will be reviewed. Furthermore, considering the biological Rural tree decline in Eucalyptus species is effects of rural land use, recommendations characterised by a thinning of the crown for addressing RTD are suggested. that begins at branch ends and progresses towards the trunk. Dead branches typically protrude beyond the remaining foliage Climate change and tree decline in natural (Photo 2). New leaf development ecosystems subsequently occurs from epicormic buds and may re-occur for years before eventual The global climate has warmed by more death. Factors affecting RTD are generally than 0.5°C during the past 125 years, and thought to affect the water relations of trees. is estimated to rise by up to 4°C by the For example, the recent decline in the mid twenty-fi rst century (Gates 1990a). Tasforests Vol. 15 2 June 2004 Photo 2. Early stages of rural tree decline are indicated by dead branches protruding beyond the remaining foliage. By then, rainfall in many regions will are predicted, which is nearly ten times decrease, rainfall patterns are predicted the known natural rate (Gates 1990b) and to alter and the incidence of drought to will exceed the dispersal abilities of many increase (Gates 1990a; Fanta 1992; Körner plant species (Dyer 1995), particularly 1992). High latitudes and altitudes will given artifi cial barriers such as cultivated warm more than lower ones and winters agricultural lands and monospecifi c more than summers (Gates 1990a). An coniferous forests (Fanta 1992). increasing rate of climate change in the past three decades has been indicated through Pollen records indicate that a temperature tree-ring chronology of the climatically increase of as little as 1ºC over 100 to 200 sensitive Huon pine (Lagarostrobos franklinii) years, coupled with intense agricultural land (Cook et al. 1991) growing on the west use, led to large-scale forest decline on the coast of Tasmania. Movements of forest dry and poor sandy soils of the north- zone boundaries at a rate of 2.5 km/yr western European lowlands (Lamb 1984). Tasforests Vol. 15 3 June 2004 Hughes et al. (1996) calculated the mean northern Queensland, following three severe annual temperature range and mean annual droughts this century. This was particularly rainfall ratio (the ratio of the highest to severe following recent drought between lowest rainfall per annum) of 819 eucalypt 1990 and 1997 (Fensham 1998; Fensham and species Australia-wide using the GUMNUT Holman 1999). Further, tree decline in many database that contains over 44 000 point- Eucalyptus wandoo woodlands in south- location records for the species. Twenty-fi ve western Western Australia has been per cent of the Eucalytpus species investigated attributed to below average rainfall from have a natural temperature range of less 1997 to 2000 and the lowest rainfall on than 1°C, 53% less than 3°C and 70% less record (since 1887) for 2001 (Wills et al. 2001). than 5°C in mean annual temperature Decline of E. gunnii on the high altitude (Hughes et al. 1996). The natural mean central plateau of Tasmania has been annual rainfall ratio is less than 1.1 for 17%, attributed to below-average rainfall over less than 2.0 for 60% and less than 3.0 for the last two decades (Potts et al. 2001a). 80% of the eucalypt species investigated (Hughes et al. 1996). Note that these data do not indicate that species do not survive Climate change and rural tree decline outside their exhibited natural ranges, as in Australia demonstrated in agricultural or ornamental plantings. But even considering factors Meteorological data indicate that rainfall additional to rainfall and temperature in the Midlands of Tasmania (42°18’S, that confer an ecological niche, such as 147°22’E) has been below the long-term competition, disturbance, or soil type, the average for 20 of the last 25 years (Figure 1) predicted change in future rainfall and and that the average annual rainfall over the temperature patterns has serious implications past 25 years has been 492 mm compared for Eucalyptus. For example, conservative to the long-term average of 552 mm. estimates indicate that by 2070, populations The seasonal pattern of rainfall has also at the southern boundary of 53% of eucalypt changed, from a more even distribution to species are predicted to be exposed to a pronounced dry period in late summer- warmer temperatures than any presently autumn (Kirkpatrick et al. 2000). In addition, tolerated by their most northerly populations an increase of 1.5°C in the mean daily (Hughes et al. 1996). Furthermore, in forest maximum temperature occurred over the communities containing eucalypt species period 1945–1995 (Kirkpatrick and Gibson that can persist under warmer conditions 1999). The worst affected areas of RTD in and changed total and seasonal rainfall the Midlands are those in areas of lowest patterns, the new conditions may confer rainfall (Grice 1995) (see Figure 2). competitive advantage to other species (Lowman and Heatwole 1992). Meteorological data indicate that whilst rainfall in the New England Tablelands of Widespread decline of forests across Europe New South Wales (30°31’S, 151°40’E) has in the 1980s was considered to be due to only been below the long-term average for drought resulting from the Greenhouse 15 of the last 25 years (Figure 3), greater Effect (Pearce 1990).