Shrubs in woodlands - directional or cyclic?

One of the things that ecologists have noticed in (some) grassy woodlands in southern Australia over the last two decades is that woody cover has been on the rise. This can take the form of increases in shrubs (such as Leptospermum spp., Acacia spp.), or dense regeneration of trees.

My own work (and that of my students and other scientists) show this increase might be due to changes in management (such as the removal of stock grazing from long-grazed forests upon reservation for nature conservation), altered fire regimes (usually a decline or exclusion of fire), overgrazing by native and exotic herbivores (that reduce competition from the ground-layer vegetation, as well as encouraging unpalatable species at the expense of palatable ones), and potentially even changes in atmospheric CO2 levels that favour woody plants over grasses.

Increases in woody plants have been recorded at several locations in southern Australia. This increase has generally been thought to be a uni-directional change that requires management intervention to alter this trajectory. So, at Wilsons Promontory National Park for instance, dense thickets of native Tea Tree that have established in swale grasslands and woodlands over the last 40 years are now being actively managed by carefully timed fire to open up the closed shrublands to benefit biodiversity and amenity.

Parks Victoria staff undertaking a burn to manage Coast Tea Tree (Leptospermum laevigatum) at Wilson's Promontory NP
(Photo: Greg McCarthy)

Just the other day, however, I saw evidence that woody plant encroachment isn't always uni-directional.

Herb-rich woodlands at Langi Ghiran
in mid-winter, before flowering.

At Langi Ghiran, there are fabulous herb-rich woodlands. Spring is a sight to behold - indeed, some of the highest species richness levels in Australia have been recorded there. Shrubs appear to be localised and generally sparse.

Hedge Wattle (Acacia paradoxa), a shrub native to the system, began spreading about a decade ago at this site and forming thickets. I'm not sure why it did this - was it climatically stimulated, was there a change in grazing pressure form native herbivores, was this a response to rising CO2?

It established prolifically in the inter-tree gaps, which are common in this woodland (and support much of the plant diversity), and establishes in the absence of fire. It also seems pretty unpalatable to the resident kangaroos and wallabies. I began to think that the end game here would be dense stands of wattle, a decline in diversity, and a landscape transformed visually.

However, when I last visited, the scene had changed a little bit.

Hedge Wattle - the dead looking stuff in the centre of the image - at
Langi Ghiran, 22nd June 2014.

Across 10's of hectares, Hedge Wattle had died, or was senescing. I'm pretty sure this was not because of old age - most shrubs are (and this is just a guess) less than 15 yrs old. We know from work by my Honours student Julia Franco that Hedge Wattles can live for five decades.

Where plants were not quite dead, the phyllodes were brown and desiccated and clearly the individuals looked like they were on the way out.

I first noticed this browning off in summer after a hot and dry spell. Now, in mid-winter, there seems no signs of recovery. This hints (to me) that drought has probably played a key role in the population dynamics of Hedge Wattle at this site. It hints that the population might be cyclical - assuming that the rise and fall of Hedge Wattle has occurred before, but just gone un-noticed.

If this is the case, it suggests a few important lessons:
- long-term observations can help decipher directional versus cyclical patterns in nature.
- if drought is at play here, was this drought 'extreme' and hence, might we expect its frequency (and potential effects) to increase over time?
- drought plays a key role in the dynamics of many woody plant populations but its effects are sometimes under-appreciated.

What else might be going on?? It's probably important to survey Hedge Wattle at Langi Ghiran to see if mortality is linked to particular aspects, slopes or soil types. Perhaps it is exacerbated in close proximity to century old trees. Might smaller plants be more susceptible than older ones (because they have less extensive root systems)? What about density? Are dense populations more or less susceptible to desiccation as individuals compete strongly for water?

Whatever the cause, this site illustrates that woody plant encroachment doesn't always end in permanent site occupation. Trying to figure out where it does would improve our understanding of both the process, and the management response.