Our research focuses on the population dynamics of plants and how they are influenced by impacts of natural disturbances and global environmental change. We are particularly interested in the interactive effects of fire, grazing and drought in grasslands and woodlands in southern Australia, and how climate change, fragmentation and shrub encroachment affect ecosystems.

Saturday, 30 April 2011

Annuals 101

Recently, Nathan Wong, Sera Cutler and I were looking at patterns of plant diversity in Yellow Gum woodlands in the Wimmera (Yellow Gum woodland species diversity). Two things struck us - (i) there were up to 104 species in an area only 32 m by 32 m (making them some of the most diverse plant communities in the world at this spatial scale) and (ii) annual plants contributed, on average, about 40% of the plant species were saw at most woodlands.
The 'inch flora" - hard to see!

So, why is the second observation of note? It would appear to me that the important contribution made by annuals to biodiversity in eucalypt woodlands in the 300-500 mm rainfall belt has been largely overlooked. This probably has something to do with their small size. There isn't much information on the biology and ecology of annuals from southern Australia at all. We know that there are many annuals in the Asteraceae, Centrolepidaceae, Crassulaceae and Portulacaceae, but we don't really know much else.

So, in this contribution, I thought I'd outline some of the basics about annual plant morphology and seed biology. Teri O'Brien, a cheerful retired plant physiologist who helps out in my lab, has started to unravel the secrets of the "inch flora" and I'm grateful to him for this information.

Siloxerus: an example of a 'strict' annual
(Photo: Pete Green)

1. Height: annual plants can be characterised as either 'strict' small plants (always <30 mm regardless of environmental conditions) or 'variable'. In the later group, height seems to depend on resource availability. In good rainfall years, for instance, heights of 100 mm or more are attained.

Gnephosis: an example of an 'erect' habit
(Photo: Pete Green)
2. Morphology: annuals develop either (i) a single shoot which bears just one terminal inflorescence (‘erect’ habit), or (ii) one or more additional shoots are produced from lateral buds on the primary axis (‘classic’ habit).  These lateral shoots appear to grow diageotropically and then curve upwards, bearing either inflorescences or further shoots that, in turn, bear inflorescences in a more or less erect position.

Evidence of myxospermy (foreground) compared
to a species which is not myxospermous
3. Seed biology: some annuals have myxospermous seeds. Here, the seed coat contains a mass of dry mucilage that hydrates and swells rapidly on contact with water, in some cases in as little as a few minutes. A number of theories have been suggested about its significance but I'm inclined to the idea that it assists the seed to attach to the soil, giving the radicle something to push against as it starts to elongate, minimizing the risk of pushing the root hair zone away from the soil surface.  

4. Seed germination:  the common seasonal pattern of high summer temperatures and a variable period of low rainfall combine to render the seed of many annual species initially dormant upon dispersal. As the summer progresses, the high temperatures that seeds experience at (or near) the soil surface appear likely to slowly reduce the dormancy of the embryo (this is called dry after-ripening) which eventually responds to the combination of cooler conditions and moisture by rapidly germinating. 

So, it's clear that annuals are a fascinating group of plants which warrant much better ecological understanding. Further quantitative investigation of the seed biology, seed germination requirements and developmental anatomy are clearly necessary.  Next time you head into your local bush, take the time to search for these much overlooked group of plants and marvel at their beauty.

Sunday, 24 April 2011

Flooding as an ecological disturbance in native grasslands

The native tussock grasslands in northern Victoria, unlike those on the Victorian volcanic plain, occur on alluvial floodplains and hence, flooding is a natural but rare disturbance. Indeed, the last time the plains were extensively flooded was 1974, so trying to study flood impacts on plant community dynamics isn't the easiest thing to do. In January 2011, northern Victoria received more than half its average rainfall in a single day with the result that large areas of the grassland plains flooded. What an opportunity to document an ecosystem's response!

The heavens are about to open on the plains near Echuca (Photo: M.Kohout)

Flooding is likely to be an important long-distance seed dispersal agent, but this has been under-appreciated in the literature on grassland dynamics (although I note that the Australian Centre for Biodiversity at Monash Uni is interested in similar events in terms of aquatic plant community organisation; see Aquatic Dispersal Project for more information).

Extensive flooding near Kerang (Photo: C. Moxham)
Tracking seed movements during flood events must be hard to do, but I reckon it's worth investigating for one simple reason. On the flat plains of northern Victoria, plant species with seeds that might normally fall in close proximity to maternal plants are potentially capable of moving many tens of kilometres during flood events, particularly if they have small or bouyant seeds. This has important implications for species distributions (perhaps allowing species to migrate in response to climate change), the transfer of genetic material amongst populations, and the re-assembly of plant communities by species that may rarely come into contact with one another.

Native grasslands were inundated for weeks (Photo: C. Moxham)
When the Loddon River broke its banks near Serpentine, native grasslands dominated by Spear Grass were inundated by tens of centimetres of water and the flooding lasted for several weeks. This included a research site I've been using as part of a global collaboration on factors that impact on grassland diversity (but that's for another day....). No doubt this has large effects on seed dispersal, but it's also likely to impact on plant survival (because of waterlogging), and nutrient enrichment (with the potential to lead to a dramatic increase in invasive species). And what impact it'll have on germination this winter is anyones guess!

It'll be interesting to see just how much change occurs in the grasslands we've been following during the last decade of drought. Stay tuned.....revisiting our permanent plots this spring should be really interesting. Perhaps we have seen the 'event' that'll determine the structure and function of these grasslands for the coming years.

Wednesday, 20 April 2011

Back to the Future......

Vegetation changes!

This might seem like an obvious statement to anyone who has more than a passing interest in natural ecosystems. Climate variation (such as the annual timing and amount of rainfall, and extreme events like frost), as well as disturbance (such as fire, landslips, animal activity and flooding) continually shape vegetation from small- to large spatial scales.

But to find good, long-term evidence of such changes is remarkably hard for ecosystems in southern Australia. The slow nature of long-term change requires monitoring techniques to capture this, but also requires ecologists to cast their observations with questions in mind. Otherwise, why would we bother to document changes?

There are many ways to document change in natural ecosystems but I'll present just two examples here to illustrate their utility. These methods are a way of 'looking back to inform the future'. Corny, but not far off the mark.

Aerial photos were first flown for the whole of Australia in the 1940s. Many of these are readily available (e.g. DSE has burnt to CD aerial photo mosaics of the whole state of Victoria from original photo runs in the 1930s and 40s) and hence, they provide an excellent baseline from which to assess multi-decade changes in vegetation cover. In the image below, you can see how tree cover in the Barmah forest has changed between 1945 and 1985. Indeed, with Google Earth and NearMap, it's now relatively easy to collect up-to-date imagery and compare this to the historical photos. We've used aerial photographs to great effect to detect changes in shrub cover in landscapes after agricultural abandonment (Geddes et al) and in woodlands during their first few decades of conservation reservation (Price & Morgan).

While aerial photos can give broad, landscape-scale perspectives on vegetation change, they lack the resolution to detect local changes. Here, permanent photo-points, where the investigator takes a photo of local vegetation condition from a fixed point repeatedly through time, can provide great insights. It's pretty obvious, though, that photos need to be archived in way that makes them valuable resources into the future.

Photo-points have been used to superb effect at Koonamore in South Australia  (see Recalling the past - there are a heap of photo-point videos worth looking at) to show how semi-arid vegetation responds to climate variation and grazing by stock/rabbits. Unbelieveably, these photo points have been regularly taken since 1926 and they now provide insights the original scientists could only have dreamed of. You can see the dramatic effects of low rainfall on productivity, when individual shrubs established and their growth rates, as well as the devastating effect that rabbits can have.

So when thinking about long-term vegetation dynamics, you really should explore the historical datasets that already exist. They may give you an insight into multi-decade changes that have already been occurring. What better way to place future changes into context.

Friday, 15 April 2011

From the ground up

Peter Grubb wrote a really important paper in 1977 (Biological Reviews 52: 107-145) on the 'regeneration niche' and how it might contribute to the maintenance of local species diversity in plant communities. It's one of those papers I think all new Post-Grads in my Lab should read, and has been central to my thinking about plant community dynamics for a long time.

Few studies in the eucalypt woodlands of south-eastern Australia have examined whether species have fundamentally different regeneration niches. Hence, it remains uncertain whether species coexistence is promoted by differences in regeneration requirements created by substrate heterogeneity.

Recently, Amber Briggs and I published a paper (Briggs & Morgan) examining this question in semi-arid woodlands in Terrick Terrick National Park. Here, biological soil crusts (a mix of lichens, liverworts and mosses) are common and have a very patchy distribution. We wondered whether groundlayer species would germinate differently in areas with different crust components.

We sowed the seeds of five herbaceous species with contrasting seed morphology on top of four patch types(foliose lichen, short-turf moss, tree leaf litter, disturbed crust) and followed their emergence. Germination varied between patch types and, for the largest-seeded species (Maireana excavata), final germination was significantly lower on the biological soil crust and litter patch types because they strongly acted as a physical barrier to seed penetration into the soil. Germination time courses showed that biological soil crusts delayed the timing of germination of these species.

Hence, the patchiness in the environment created by soil crusts might differentially affect the spatial patterning of plant species in semi-arid woodlands by their subtle influence on seedling emergence. Grubb's regeneration niche theory would, therefore, have some support in semi-arid woodlands.

 Figure: The small-scale heterogeneity that biological soil crusts create in semi-arid woodlands. Different plant species respond differently to biological soil crust components, potentially enhancing small-scale species coexistence.

Wednesday, 13 April 2011

Welcome to the Morgan Plant Ecology Lab

Welcome to my Blog. It replaces the out-of-date website hosted by my University and will be my main Research News page from now on. My aim is to welcome you into my Lab and keep you updated about the interesting research we conduct on plant ecology, vegetation dynamics and conservation biology in south-eastern Australia. I hope to regularly update my Blog with news from the Lab and with plant ecology news more generally.