Understanding drivers of River Red Gum condition at Walker Swamp

Across the south-west region of Victoria a decline in condition of River Red Gums has been observed, in part by overabundant psyllids. In 2019 many trees were stripped of their foliage by late December. Psyllids are small insects that feed by sucking sap from the tissues of leaves and young plant shoots. Lerp-building psyllids construct either soft or hard coverings called lerps of various designs and sizes beneath which the immature nymphs shelter and feed.

Fig 1. Looking south west across Walker Swamp towards Mount Murdadjoog (Mt Abrupt) through the ring of River Red Gums.

A small attack on trees will result in some leaf discolouration, wilting and distortion in the tree foliage, but severe infestations by lerp-type psyllids cause extensive foliage discolouration in tree crowns with purple to brown/rusty red spots, leading ultimately to leaf death and defoliation. As psyllids are most active during the summer period, attacked foliage tends to desiccate rapidly due to the warm conditions so that by late summer leaf fall has accelerated to the point where tree crowns carry little foliage. In severe situations tree crowns are totally defoliated. This loss of foliage in turn reduces tree vigour and slows growth.

The available literature (see review by Collett 2001) suggests that a correlation may exist between outbreaks of psyllids on River Red Gums and a succession of dry summers combined with very wet winters within the same year. Despite not being excessively wet, these are the conditions that we have broadly experienced at Walker Swamp and the surrounding region over the last two years. In situations where psyllid attack has been particularly severe (Figure 3), tree death may ensue, often in conjunction with secondary insect pests and excessive levels of mistletoe (Figure 4) infesting the weakened trees.

Fig 4. or, in the worst cases, the tree is dominated by mistletoe.

Fig 2. While some trees are in excellent condition

Fig 3. Most are not! With much of the canopy gone following psyllid attack

In addition to this broad scale attack on tree condition, the trees at Walker Swamp were (until recently) hemmed in by dense Blue Gum plantation. This intensive monoculture competed directly with remnant River Red Gums for both nutrients, light and lowered ground and/or soil water levels. Also after artificial drainage of the floodplain long ago, the Red Gum Swamp Ecological Vegetation Class (EVC # 292) community was left stranded for several decades above the zone of regular inundation achieved in all but extreme flood events (e.g. 2011). Under these conditions the River Red Gum population was stressed and the condition of most was poor, with marked canopy die-off, increased parasitism (mistletoes), predation and low levels of reproductive activity.

The current condition of River Red Gum trees at Walker Swamp is therefore the result of a complex combination of factors, some of which we are hopefully enhancing through restoring natural water levels to the wetland complex. We want to see healthy trees and associated communities restored to the wetland complex. If recovery is to be successful, there must be an understanding and an appreciation of what success means, and importantly to ensure that past activities are opportunities for learning and improving practice.

Fig 5. The sub-set of River Red Gum trees being monitored at Walker Swamp

To begin to understand how the River Red Gums at Walker Swamp respond to the new management regime, a monitoring program was established in December 2018. Removal of the Blue Gum plantation and the much anticipated (but yet to be fully realised, as we await sufficient rain) restoration of more natural historic floodplain water levels (243.5 m AHD) that reach to the base of extant River Red Gums across the project area for two to six months, is predicted to improve the condition of this community and encourage natural regeneration. Using a visual assessment method developed by Nick Souter and others (Souter et al. 2009), 100 trees have been tagged and then surveyed each December starting in 2018 (Figure 5).

Following the completion of the 2019 survey, it is now possible to look at the initial response of trees to the removal of the Blue Gum coppice regrowth around Site 3 in late 2018 (orange dots), and the physical removal of remaining Blue Gums from Site 1 in early 2019. Site 2 (yellow dots) has already received higher levels of water associated with the closure of the main drain through Walker Swamp.
A comparison of River Red Gum Condition index for each tree between 2018 and 2019 shows no significant mean change in overall tree condition between years (Paired t-Test t99=1.733, p=0.086).

Overall the majority of trees rated a good or very good condition (Figure 6), but there was a non-significant, and unexpected trend for trees to be in slightly worse condition in 2019 than in 2018. Unpacking why this might be gives an insight into the ecology of these trees and the pressures they are under, both at the regional level and between each of the sites. The average condition of trees at site 3 has declined significantly (Figure 7), while those at both sites 1 and 2 have shown a trend to improved condition. Trees at site 1 may have benefitted from the changes associated with Blue Gum harvesting, but they show the same trend as the control site 2.

Fig 6. Comparison of River Red Gum Condition between 2018 and 2019.

Fig 7. Mean River Red Gum condition between sites in 2018 and 2019.

Despite a far higher level of leaf damage by insects in 2019 across all sites, the resultant loss in leaves and drop in the density of canopy was most marked at site 3. This decline in condition at site 3 was mainly due to a significant drop in the density of leaves in the canopy (Figure 8 and 9).

Fig 8. Change in extent of leaf damage by insects between 2018 and 2019.

Fig 9. Change in mean crown density of leaves at each site between years.

It is not immediately apparent why the trees at site 3 would experience a greater level of leaf loss. Perhaps their ability to tolerate the high levels of leaf damage seen this year is lower than for the trees at sites 1 and 2. At these sites, changes arising from our onsite efforts may have increased their resilience to the effects of the psyllids.

One very interesting and positive aspect from the monitoring is the almost ubiquitous and high level of reproductive activity (either budding, flowers or fruiting) evident in the River Red Gums across all three sites this year (Figure 10). In 2018 only 19%  of trees had evidence of moderate to high levels of reproductive activity. But in 2019, 91% of trees had moderate to high levels of reproduction, with over three quarters having their canopies absolutely dominated by flowers and/or fruit – particularly evident in the near absence of a leafy canopy in many trees.

Fig 10. Level of reproductive activity in River Red Gums in 2018 and 2019.

The extremely high level of reproduction observed in 2019 is intriguing… as for River Red Gums typically as tree condition declines so does its ability to reproduce. River Red Gums in poorer condition (measured primarily as crown condition) may produce fewer seeds, have reduced flowering, have stunted bud size, slower rates of bud development, fewer trees in bud, and the timing of seed fall may be altered (George 2004). It may be that the River Red Gums are showing a postulated biennial cycle of flowering (Souter et al 2009). If this is the case, 2018’s relatively low level of flowering and fruiting may be due to the trees being in a low reproductive year, in a landscape-scale biennial flowering cycle. Or alternatively, it could reflect a high level of stress in the trees at that time. Resolution of this confounding set of possibilities will only come as data are collected into the medium term. Either way, the correlation between high levels of insect damage to the canopy, very low canopy densities and the high level of reproductive activity observed in 2019 raises a number of questions about the previously observed relationship between high reproductive output and good tree condition.

So what has been the effect on River Red Gum condition of our efforts to remove Blue Gums, remove drains, and raise sill levels to retain water in the landscape? Well, we can’t really say for sure yet… it is just too early. But the prospect of collecting a long-term data set which may provide some insights into the drivers of tree condition at Walker Swamp is exciting.

REFERENCES:

Collett, N. 2001 Biology and control of psyllids, and the possible causes for defoliation of Eucalyptus camaldulensis Dehnh. (river red gum) in south-eastern Australia – a review. Australian Forestry 64(2) pp. 88-95.

George A. K. 2004 Eucalypt regeneration on the Lower Murray floodplain, South Australia. PhD thesis. The University of Adelaide.

Souter, N. J., R. A. Watts, M. G. White, A. K. George, and K. J. McNicol. 2009. Method manual for the visual assessment of lower River Murray floodplain trees. River Red Gum (Eucalyptus camaldulensis). February 2009. Government of South Australia, through Department of Water, Land and Biodiversity Conservation., Adelaide.

Greg Kerr
Greg Kerr