The Big Wet regeneration pulse

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.

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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.

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.

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!

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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

• What did you get from the Big Wet?
• Wonders of the Big Wet
• The Big Grass years
• Drought, dieback and insect attack

Further reading