The Big Wet regeneration pulse

Red Gum Seedlings
Red Gum seedlings regenerate after the Big Wet flood waters recede

In an earlier blog I asked, ‘What impact did the big wet of 2010-2011 have on native vegetation? Did heavy rains promote lots of regeneration in some areas, but not others? Or did nothing much happen at all?’ In response, many readers submitted lots of great observations from across south-east Australia.

I collated the observations of wetland and rivers and grasses, herbs and weeds in earlier posts. But what happened to trees and shrubs? Did the heavy rains trigger lots of regeneration or nothing much at all?  Initially I thought there’d be lots of regeneration in most places but, as it turned out, readers noted abundant regeneration in some areas but none in others. What caused these patterns? Read on to see what everybody found….

In the summary below, the phrases in normal font are mine while readers’ observations are in italics. Again, you can read the original, unedited comments on the initial blog post.

* * * * *

Many readers across central Victoria observed abundant tree regeneration. After flood waters receded near Numurkah, about 200 red gum saplings appeared along our seasonal creek, all growing very strongly. Further south, the Big Wet caused massive regeneration of grey box and river red gum around Castlemaine in paddocks and roadsides. In the Boho hills, a reader observed fairly prolific regrowth of River Red Gum, Grey Box and Cassinia in a paddock where grazing ceased immediately prior to the rains / end of the drought. Pleasingly…

The road from Elphinstone to Bendigo through Sutton Grange passes through magnificent granite country to the east of Mt Alexander. I used to stress about the lack of ‘succession plan’ for the mature river red gums that are a major feature of the grazing land in that area. After 2010-2011 I noticed lots of saplings growing in paddocks, especially in those being rested. The big wet must’ve provided favourable conditions for germination and growth in spite of grazing.

Sutton Grange Red Gum 2
Old Red Gums near Sutton Grange in Victoria (click on the image to enlarge it).

Shrubs also regenerated in some areas. Near Moss Vale in NSW, Bursaria spinosa seems to germinate very well post ‘big wets’. Similarly, in the more marginal / less fertile box -ironbark country around Rushworth / Heathcote, the re-invigoration of the shrub layer has been quite noticeable. Further west, in the Mallee woodlands, the rain worked wonders and created abundant growth of understory and lots of shrub regeneration. The coincidence of good rains and burning triggered dense regeneration in some places…

In a DSE controlled burn area in the forest to the NW of Taradale, there are areas where the local wattle – rough wattle – is so thick as to be almost impenetrable. Very uncharacteristic of this forest where understorey is typically sparse to non-existent. This growth has occurred post 2011. The area has been burned several times over the past 10-15 years. This is the first time I’ve seen this response.

Trees didn’t regenerate everywhere; around Mossvale in NSW the regeneration during the wet period was not amazingly different to other years. Out in the Mallee,

there was no regeneration of the Mallee eucalypts themselves, except in areas where they have been broad-scale cleared. Within their stable woodlands, surviving on their lignotuber, they seem to live “forever” so there is no place for regeneration. The climate, combined with competition, never seem to allow for it, regardless of how wet it gets.

Importantly, a number of readers suggested that amount of tree regeneration was influenced by how much grass there was. On the northern plains of Victoria, 2010-2011 was extremely, continuously wet, causing rank grass to grow, which prevented the regeneration of eucalypt trees in most places (despite climatic conditions being suitable). The exception was where land was ploughed, grazed bare or flooded. In these locations regeneration was prolific. A reader from Mossvale noted, I would imagine that a general picture of wet years would show a flush of grasses and therefore increase the cover (decrease inter-tussock spaces) and reduce the suitability for Eucalyptus seed germination. I have seen Eucalyptus germinate better when there is poor grass cover. And in the Boho hills,

…in the back paddock…there has been little change, even though the paddock is specifically set aside for regeneration under an agreement with our regional NRM body and the summer rains of 2010 and 2011 would appear to have been the bees knees for seed germination. This may be so, but I suspect the absence of recruitment may be due to excessive grass growth during that time smothering any seedlings that may have germinated…. competition faced by seedlings, rather than germination triggers per se, may be most influential for regenerating trees.

grassy woodland
Thick grass after the Big Wet creates a hard site for trees to regenerate.

Seedling ecology

We know a lot about the ecology of eucalypt regeneration. Many studies have found that seedling regeneration is usually limited by soil moisture levels, and many seedlings die in dry soils, especially in summer. Small eucalypt seedlings find it very difficult to regenerate among grasses, as fine grass roots rapidly extract soil water and thick grass reduces light levels, which makes survival even more difficult. On the other hand, eucalypt seedlings grow well in low nutrient soils, so nutrient competition from grasses isn’t as big a problem.

Because of this, eucalypts usually regenerate better on poor soils where there are few grasses, and regeneration can be very rare on fertile, grassy sites. However, these patterns can be altered by disturbances such as burning and grazing. High intensity fires often promote dense seedling regeneration, by increasing seed fall, removing competition and increasing soil nutrient levels. Heavy continuous grazing usually prevents regeneration – which is why most grazed paddocks have few young saplings – but intermittent (or rotational) grazing can improve regeneration, by reducing grass levels and allowing seedlings to grow between grazing periods.

Observations from the Big Wet are consistent with this documented ecology. Many observers found few recruits among dense grasses and found more regeneration in disturbed soils and on grazed sites. On one hand, the Big Wet provided the best conditions possible for regenerating trees, by increasing soil moisture levels. On the other hand, the Big Wet provided the worst conditions possible for regenerating trees, by promoting fast-growing grasses that out-competed the small tree seedlings. It’s hard to please a small seedling isn’t it!

* * * * *

It was wonderful to receive so many observations of regeneration from across south-east Australia. In addition to the comments summarized above, many readers also observed that many trees died in recent years. In the next blog I’ll summarize the observations about die-back, which is perhaps the most perplexing of the ecological changes triggered by the recent Big Wet and Big Dry years. Thanks again to everyone who sent in their observations. Please leave a comment below if these records remind you of other changes in your area.

Related posts

Further reading

Dorrough J, Moxham C (2005) Eucalypt establishment in agricultural landscapes and implications for landscape-scale restoration. Biological Conservation 123(1), 55-66.

Fischer J, Stott J, Zerger A, Warren G, Sherren K, Forrester RI (2009) Reversing a tree regeneration crisis in an endangered ecoregion. Proceedings of the National Academy of Sciences of the United States of America 106(25), 10386-10391.

11 thoughts

    1. Hello WS, great to hear from you again. Drought? We did have a little one recently, but that’s the subject of next week’s post – stay tuned! Best wishes Ian

  1. And the great observations of regeneration keep rolling in… In response to this blog post, Bindi Vanzella (@bidgeewidgee) from Greening Australia sent the following observations from south-west NSW, complete with pictures, via twitter. It’s great to see Boree and Hakea regenerating. This is a fantastic way to share photos isn’t it. Thanks once again for all for the great observations, Ian

  2. My experience working with landholders in the Mernda/Wollert district (between Epping and Whittlesea, just north of Melbourne) in 2011-2012 was that many are very keen to implement a succession plan for their veteran paddock River Red Gums, but wanted advice about the best way to manage the dense seedlings that were springing up. They understood ecological thinning was an option, but were reluctant to intervene without specific advice about how much to thin, and how to best balance the regeneration with their rotational grazing. In most cases, I suspect the seedlings were left for nature to run its course.
    I understand the QAECO team at Melbourne Uni are running a project examining the issue.

    1. Hello Anna, thanks for your comment, It’s great to hear that landholders are keen to encourage regeneration of the old paddock trees. The challenge of how to encourage ‘enough’ regeneration without promoting very dense stands is a big one, especially for red gums. I’ll send a message to QAECO staff and see if someone there can add a comment to describe their work. Thanks again, best wishes Ian

  3. Hi Anna and Ian, it is so great to read posts and comments about this stuff. A few of us at QAECO have been investigating dense stands in Box-Ironbark country (we didn’t look specifically at Red Gums). There is a lot research out there on how overly-dense stands result in slow (poor) growth of the trees and suppression of the understorey. Our work has been focused on the understorey vegetation, which found that dense stands do suppress the understorey whereas thinning can result in a dramatic increase – but not in all cases. The amount of thinning required is a tricky one but guidance of ‘benchmark’ stem density can be found in papers such as Gibbons et al. 2010 on ‘Benchmarking stem densities for forests and woodlands…’. Direction on when thinning is appropriate and how much to thin is still unclear but hopefully we will know more soon.

    1. Hi Chris, thanks very much for the update, it’ll be great to see the results from your work when it’s finished. Best wishes Ian
      The abstract for the great paper by Phil Gibbons and colleagues can be read at this link: http://goo.gl/KGpeXO

  4. Great post Ian, interesting to read all the different responses. Did anyone mention floods suppressing river red gums? My understanding is that dense stands of young saplings prevent the establishment of a healthy, boxier distribution of larger red gums. This is an issue in the larger river red gum forests (Barmah etc.) and is being used as an argument for ecological thinning in the absence of adequate flooding. Did anyone report suppression of saplings? What do you think of this? Cheers, Pat

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