In 2010, Craig Allen and colleagues published ‘the first global assessment of recent tree mortality attributed to drought and heat stress’ (Allen et al. 2010). In this fantastic paper, the authors collated examples of tree die-off (or mortality) from around the world and – in a very long sentence – they concluded:
… studies compiled here suggest that at least some of the world’s forested ecosystems already may be responding to climate change, and raise concern that forests may become increasingly vulnerable to higher background tree mortality rates and die-off in response to future warming and drought, even in environments that are not normally considered water-limited.
Given that pessimistic prognosis, it’s worth asking; how are trees faring here, in our own backyard?
In July, I invited readers to send in observations of how their local vegetation changed after the Big Wet years of 2010-2011. I expected to receive lots of comments about changes caused by the heavy rains, which I did (see recent blog posts: here, here and here). Unexpectedly, many readers also submitted observations of trees that died after the Big Wet had passed. What caused these deaths? Let’s see, by first reviewing all of the relevant observations.
Most reports of dieback were from dry hill slopes, and occurred last summer. In north-east Victoria, one reader noted, ‘conspicuous bands on north and west aspects where major die off occurred over the summer of 2012/13 … mostly Stringybark and Red Box. There are a range of very obvious bands which showed following the heat, so … this may have been due to the sustained wet, followed by the major dry period.’ The same reader had also seen, ‘many old isolated paddock trees die off rapidly this summer … right across the region.’ Others made similar observations: ‘there has been considerable, albeit patchy, dieback in the Strathbogies, most notably amongst stands of Messmate Stringybark (E. obliqua) on shallower soils and sheet rock.’ Further west, in central Victoria, a reader noted:
‘On western slopes abutting Mount Tarrengower in nearby Maldon there has been considerable dieback of mature eucalypts and wattles, including both natural regeneration and planted stock that had thrived under the wet conditions of 2010-11. This notably includes hardy species such as Grey Box, Drooping Sheoak and less hardy Black and Golden Wattles. It appears that successive dryness over spring-summer-autumn 2012-13 has notably impacted native vegetation in grassy woodland and dry forest vegetation types throughout Victoria, from casual observations. Following minor winter respite some plants have ‘hung on’ and are greening up despite the prevalence of dead plants.’
Trees didn’t only die in the hills: ‘across western Victoria there has been considerable dieback of Banksia marginata stands, many of truly veteran status…. the phenomena appears to be widespread and apparently associated with dry conditions.’
In their global review, Craig Allen and colleagues found that, ‘tree mortality commonly involves multiple, interacting factors, ranging from particular sequences of climate stress and stand life histories to insect pests and diseases.’ Notably, many readers found that dieback was exacerbated by hungry insects.
‘In the Daylesford area… the most noticeable change after the big wet was an explosion of cup moth caterpillars. We are having another explosion of cup moth caterpillars currently and the canopies of Messmates, Long-leaved Box and Candlebarks are looking pretty bad. Last spring the regrowth of the canopies was beautiful. But I fear the long dry summer and autumn recently will stress and kill some of those affected.’
Another reader wrote:
‘I live in Box-Ironbark country in Taradale…. The forest suffered from a massive cup-moth infestation and consequent leaf loss last spring, during the driest 6 months – i.e. lowest moisture levels on record, according to the DSE/DEPI, worse than any time during the 12 year drought that ended in 2010. The stringybarks are now looking very sick, many covered with brown leaves or more advanced stages of dying. It’s looking a lot worse than in the 1982 drought. I wonder what future there is for stringybarks in this forest.’
Other insects were abundant in other regions. Catherine wrote, ‘I have just finished my honours thesis looking at Eucalyptus viminalis dieback in the Monaro region, which appears to be the result of drought along with an infestation of the eucalyptus weevil (Gonipterus sp.). However there doesn’t seem to have been any recovery in the last few years despite the comparatively wet conditions.’ And closer to the coast…
‘… we have had a massive dieback event of E. moluccana (Grey Box) in Western Sydney during the last few years. The dieback is due to psyllid attack and has been specific to the one species of eucalypt. This is a dominant tree in threatened Cumberland Plain Woodland and has been the subject of several dieback psyllid attacks since the 1990′s. Opinion seems to be that it is related to the extreme dry/wet climate events in recent years and that the recent event is the worst on record. … This is the most significant event in our region in the recent wet years I think.’
Many of the comments describe unusually dry weather last summer. To see how dry it was, I downloaded the Bureau of Meteorology data for Malmsbury Reservoir in central Victoria, not far from Taradale and Daylesford.
To make the patterns easy to see, I added up the rainfall for each quarter of each year (summer = Dec to Feb, autumn = March to May, etc). I then subtracted the actual rainfall that fell in each quarter from the expected quarterly rainfall, based on the long-term average. This allowed me to create a chart that shows the deviation from the expected (or average) amount of rainfall in each quarter. In the chart below, the bars point down if the quarterly rainfall was less than average, and up if the quarterly rainfall was above average. For example, in summer 2011, Malmsbury received 289 mm more rain than would fall in an average summer. The black bars show each summer (Dec-Feb) so you can clearly see the start of each year.
I’ve divided the chart into four zones. (1) On the left, the wet years of the 1990s are notable, although 1994 was exceptionally dry. (2) The cruel Drought of the Century, the Big Dry, extended from 1997 to 2010. (3) The long drought ended abruptly in the flooding rains of the Big Wet, in 2010-2011. A brief period of ‘near average’ rainfall in 2011-2012 was followed by (4) nine months of below average rainfall from spring 2012 to autumn 2013. This is when most of the dieback occurred. Fortunately, winter rainfall this year was again above average, as the final bar on the chart shows.
When compared to the average rainfall in this way, the 2012-2013 dry spell was one of the driest 9-month periods in the last 30 years. At Malmsbury, similar dry spells occurred in 1982, 1994, 2006 and 2009, as can be seen on the chart above.
How will the forests change in the future? Unfortunately, we have very few long-term studies of how alternating dry and wet periods affect Australian ecosystems. In one long-term study, the late, great David Ashton found that dry rocky slopes on Mt Towrong in the Macedon Ranges were hit by six droughts in 25 years. The vegetation waxed and waned during the dry and wet periods. On shallow soils, the most drought-sensitive species, Messmate Stringybark (Eucalyptus obliqua), died back in each drought and then re-colonised in the next wet period, then died off and re-colonized again.
Many of the observations above mention dieback of stringybark species. The fate of the stringybarks will depend on how well they recover in future wet periods. As climate change intensifies, we can expect drought-sensitive species like these to decline and be replaced by more drought-tolerant species. Perhaps that’s already occurring in some places.
Unfortunately, we have no long-term monitoring system to document how forests across south-east Australia change over time. That’s why it’s important to collate observations like these. To make these observations really useful, we need to record how the forests continue to change. We often notice short, sharp changes, especially deaths, but tend to overlook slow, gradual changes between the periods of dieback. Will dying trees recover or will they die off completely? Will new plants re-colonize the dry slopes in wet years? If so, which species, the same ones or new species?
I wonder how the dieback forests have fared after this winter’s good rains. Have they recovered? Once again, the only way we’ll know is if lots of people record and share their observations. Thanks once again to everyone who wrote in earlier, and to Anna Foley for her great Banksia photo. If you have more observations of dieback or recovery in your area, please leave a comment below.
Allen CD, Macalady AK, et al. (2010). A global overview of drought and heat-induced tree mortality reveals emerging climate change risks for forests. Forest Ecology and Management 259(4), 660-684. [You can download a full text copy of the article for free from this link].
Ashton DH & Spalding DK (2001) The ecology of a stressful site: Mount Towrong, Central Victoria 1967-1997. Australian Forestry 64, 143-150. [You can download a full text copy of the article for free from this link].
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