New-England Dieback
Throughout the New England Tablelands bioregion, Eucalyptus dieback, also known as ‘rural dieback’ or ‘New-England dieback’, is a historical and ongoing threat to woodland communities (Nadolny, 2008). While dieback is a natural process often seen in old senescing adult trees, New-England dieback is a syndrome in which the crown and root systems of seemingly healthy trees prematurely decline in condition (Landsberg & Wylie, 1988; Nadolny, 2008). Trees that are affected by New-England dieback present with significantly reduced crown diameters, with foliage on large and small branches displaying damage from insect herbivory, fungal attack, or drought scarring (Figure 1). In certain instances, cankerous lesions may also be present along the bark in the mid to lower trunk (Figure 4). Trees respond by producing epicormic shoots along the trunk and branches, in severe instances presenting new and old epicormic growth from repeated dieback events on the same branch. Concurrently, root systems are reduced in extent within the soil profile from drought and insect attack, limiting the ability for trees to access water and nutrients (Lowman et al., 1987). Dieback is most evident in the crown of susceptible trees and commonly affects New-England Peppermint (Eucalyptus nova-anglica), Red Gum (E. blakelyii), Ribbon gum (E. viminalis), River Red Gum (E. camaldulensis), and various Stringybark species, however, dieback has been documented in most woodland species when conditions are particularly challenging (Figure 2).
Figure 1: A dieback affected stand of Eucalyptus nova-anglica from drought and insect damage.
Extent & ecological impact
Eucalyptus dieback has been an ongoing threat to woodlands across the New England Tablelands since the inception of agricultural landscape modification 160 years ago. Widespread dieback events gained attention throughout the 70’s and 80’s where it was estimated that up to 300,000 hectares of both intact and agriculturally modified woodland was affected (Nadolny, 2002; Nadolny, 2008). Woodlands that experienced the greatest loss of tree cover were those with exposure to increasing agricultural land modification, however, intact woodlands peripheral to agricultural activity also suffered significant losses. More recently, the millennium drought induced dieback events in multiple tree species across woodland and agricultural landscapes throughout the tablelands, the extent of which is yet to be fully determined.
The impact of dieback can have a myriad of cascading effects on woodland condition and landscape ecological processes. As crown cover is reduced and trees switch into survival mode, the transition from outward crown growth to epicormic growth increases light resources within the understory. This typically favours the incursion of weeds and disturbance specialists that rapidly colonise new conditions. As trees affected by dieback are no longer investing energy into the production of seed, any recruitment post dieback is reliant on seed generated prior to the dieback event. As eucalyptus seed generally has a short window of viability (6 to 12 months), the establishment of new recruits is entirely dependent on the success of previous fruiting events. For those seedlings that do emerge, they now have to compete with incursions of weeds, insect herbivory, and a cascade of new and rapidly changing environmental conditions. As such, the ability for woodlands to naturally regenerate is significantly compromised, in severe instances woodlands gradually shift in ‘stable state’ from that of a grassy woodland to a dominant pasture system.
Figure 2: Severe dieback from drought in a Eucalyptus blakelyi woodland near Uralla NSW Autralia.
Causes
New-England dieback syndrome is the result of systemic ecosystem dysfunction whereby the interconnected factors that contribute to maintaining a functioning ecosystem have been severely or completely disrupted (Landsberg & Wylie, 1988; Nadolny, 2008; Rawlings et al., 2010, Yates & Hobbs, 1997). These factors include the removal or reduction of woodland structure (shrubs, tree desnsity), the incursion of exotic weeds and pasture, the onset of extreme seasonal conditions (wet or dry), and the exposure of woodlands to agricultural perturbation (fertilisers and grazing). The insidious nature of dieback is that the reduction of woodland condition happens gradually over time, the effect of repeated disturbance not taking a noticeable toll on tree health until environmental conditions become extreme or symptoms display. As mounting pressures from drought, insect attack, landscape salinisation, fertiliser application, and changes in hydrological cycles (drought or inundation) become more prevalent, the probability of dieback events occurring becomes increasingly probable (Jones et al., 1990; Rawlings et al., 2010).
Insect attack
One of the most notable symptomatic conditions of dieback is the reduction in projected foliage cover within the tree crown by large populations of herbivorous insects (Figure 3). Although the loss of foliage can also be caused by severe and prolonged drought conditions, when conditions within the woodland are conducive, insect herbivory can be a primary driver of dieback (Lowman et al., 1987). Insects often responsible for leaf predation might include scarabs, psyllids, and longicorn beetles, however, insect herbivory does not always occur within the crown of the tree but also throughout the root system (Nadolny, 2002; Nadolny, 2008). The effects of insect activity can be seen by the distinct patterns of leaf damage from biting-and-chewing, rasping, or piercing and sucking insects (Figure 3). While this is most obvious when leaf predation occurs, some insect damage occurs under the bark or within the root zone (Figure 4). A number of insects have been attributed to root herbivory the most common being the Christmas beetle (Anoplognathus spp.). In their larval stage, the Christmas beetle predates on the roots of stressed trees and is capable of significantly reducing root system extent. While insect predation is a naturally occurring disturbance for all eucalypt woodlands, this becomes a serious problem when insects breed to large populations. The build up of large populations of insects is most extreme after wet years following drought when the average rainfall within the previous decade has consistently exceeded the long-term average (unpublished data). High rainfall along with the application of agricultural fertilisers within or adjacent to woodlands generate thick swards of improved pasture that provide abundant resources for larvae. Adequate moisture levels are a critical factor in the emergence of scarab beetles associated with dieback, if conditions are too dry bide their time for wetter years. When combining the rapid increase of herbivorous beetle populations with unfavourable climatic conditions (drought) and agricultural disturbance (grazing and fertiliser application), a ‘perfect storm’ eventuates capable of dramatically altering the structure and function of woodlands. As tree resources are depleted from drought, their susceptibility to insect attack increases, and those that have exhausted available resources are generally killed.
Figure 3: Different types of leaf herbivory on Eucalypts. Top-left: Biting and chewing. Top-right: Piercing and sucking. Bottom: Skeletonising/rasping.
Figure 4: Insect grazing throughout the sap-wood of Eucalyptus nova-anglica. The external damage is likely caused by a combination of rot from fungus and the activity of birds predating on the bark-eating insects. In this stand of trees near Guyra, many had significant damage that had reduced projected foliage cover through crown dieback.
Loss of woodland structure
Woodlands affected by drought and agricultural pressures such as grazing and fertiliser application often present an observable loss of woodland structure. Woodland structure refers to the presence of structural layers such as herbs, grasses, and shrubs. The removal of shrubs from woodlands across the New-England tablelands has occurred since agricultural settlement, primarily from grazing animals such as sheep and cattle, but also via mechanical removal from landscape clearing. An infamous example of this is the removal of the shrub Bursaria spinosa, a common woody shrub associated with multiple woodland communities across the tablelands. The active and passive removal of Bursaria was encouraged in the early days of agricultural settlement as it tarnished the quality of wool when graziers allowed stock access to woodlands. Systematically, Bursaria was removed from properties across the tablelands and replaced with exotic pasture species as stock encroached into woodland habitat. Shrubs like Bursaria, Indigofera, Callistemon, Leptospermum, Olearia, Acacia and other native species are integral to a functioning woodland ecosystem as they provide a myriad of ecosystem services for the woodland and surrounding environment. Shrubs are important habitat for woodland birds, pollinating insects, and mammal communities that rely on their structural habit for nesting or procuring of food resources. Insectivorous birds and bats are capable of feeding on large numbers of insects and are a valuable resource for pest control (Kolkert et al., 2020). Furthermore, predatory insects attracted by annual blooms of nectar rich flowers aid in the control of abundant pest species when in large populations. For many of the remaining woodlands susceptible to dieback, where important structural elements have been removed their susceptibility to perturbation is in turn increased.
The woodland communities most affected by dieback are those severely fragmented, small in size, and lacking connectivity to other woodland patches. Dieback may be more evident in some years than others and trees will often appear to ‘bounce back’ following a bout of dieback. How individual trees fare in the long run will depend on many external factors including the effect of landscape management at the farm level. The use of grazing or fertiliser application within or adjacent to woodlands significantly increases the probability of dieback and decreases the survival rate of dieback-affected sites. Under extreme circumstances such as prolonged drought, combating the probability of dieback by increasing the size of the woodland through natural regeneration, removing or reducing agricultural pressures (grazing & fertiliser), improving structural diversity through plantings of shrub species, and increasing connectivity between woodlands may all help mitigate future dieback events.
References
Jones, A. D., Davies, H. I., & Sinden, J. A. (1990). Relationships between eucalypt dieback, land, and land use in southern New England, New South Wales. Australian Forestry, 53(1), 13-23.
Kolkert, H., Andrew, R., Smith, R., Rader, R., & Reid, N. (2020). Insectivorous bats selectively source moths and eat mostly pest insects on dryland and irrigated cotton farms. Ecology and Evolution, 10(1), 371-388.
Landsberg, J., & Wylie, F. R. (1988). Dieback of rural trees in Australia. GeoJournal, 17(2), 231-237.
Lowman, M. D., Burgess, A. D., & Higgins, W. D. (1987). The biomass of New England peppermint (Eucalyptus nova‐anglica) in relation to insect damage associated with rural dieback. Australian Journal of Ecology, 12(4), 361-371.
Nadolny, C. (2002). Dieback and what to do about ir. Department of Land & Water Conser;ation, Sydney, 7 pages.
Nadolny, C. (2008). Eucalypt dieback: an increasing threat in rural landscapes?. Australasian Plant Conservation: Journal of the Australian Network for Plant Conservation, 16(4), 26-27.
Rawlings, K., Freudenberger, D., & Carr, D. (2010). A guide to managing box gum grassy woodlands. Commonwealth of Australia.
Yates, C. J., & Hobbs, R. J. (1997). Temperate eucalypt woodlands: a review of their status, processes threatening their persistence and techniques for restoration. Australian Journal of Botany, 45(6), 949-973.
