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.

Tuesday, 9 April 2013

A year in the life of a temperate grassland - Autumn

April 2013 - the end of the dry season

This year, as part of my Blog updates, I plan to document the changes that occur in the Kangaroo Grass grasslands that dominate the western plains near my home town of Melbourne. Ecologists have long-known that grasslands are dynamic ecosystems but I've not captured this very well in my own photos, so I thought I better change that!

Following the seasonal changes that occur in ecosystems is one way to understand the dynamism of nature.

Grasslands are brilliant for illustrating such changes - they respond to the changes of the season quite dramatically. The onset of autumn rains will see a greening up of the grassland, mostly by C3 grasses (including the many exotics that now co-exist with the native flora). As spring approaches, a riot of colour etches its way into the grassland (first whites and yellow, then pink, and finally blue) - this is the flowering of the forbs that contribute most of the diversity. As temperatures rise and rainfall declines, the C4 grass Kangaroo Grass greens up and flowers, before dying back in the heat of summer. In mid to late summer, it is very likely that the grassland will get burnt as part of its ongoing management. I look forward to capturing these changes by using photos to trace the changes of the seasons.

Temperate grasslands at the end of summer look pretty lifeless.
This grassland is found on the Mt Mercer Rd at Shelford. I'll be returning to this same spot throughout the year to illustrate the changes that occur.
(Photo: John Morgan)
 Currently, the grassland is straw-coloured and dry. Kangaroo Grass leaves have died back to the very base of the tillers and no green is evident. This reflects the fact that the summer has been particularly dry this year, perhaps a sign of things to come? All forbs are now dormant, waiting for the rains to release dormant buds.

The dead grass has formed a thatch, smothering the inter-tussock spaces. Without fire, it'll be interesting to see what forbs germinate, if any, and how the biological soil crust is represented this year.

Kangaroo Grass has died back with the long summer drought. The dead grass crowds out the inter-tussock space.
(Photo: John Morgan)
On the basalt soils that are high in clay content, deep cracks have formed. These are important for two reasons. First, cracks provide refuges for animals such as Legless Lizards to avoid late summer fires. I'm guessing they are also important as refuges per se - places to avoid the effects of heat or cold. Second, the cracks are one of the reasons that trees don't grow here.  The cracking is thought to sheer their woody roots, disrupting water uptake. These cracks won't close until soils swell with the onset of autumn rains.

An example of a soil crack that has formed as the basalt soils dry over summer. I can easily put my finger in this crack.
(Photo: John Morgan)

I'll return in about 10 weeks to see just how the grassland looks. It's highly likely that drought breaking rains will have arrived, but will this have cued germination, and will any species be in flower? Till next time.

Sunset on the plains, Truganina Cemetery
(Photo: John Morgan)

Monday, 1 April 2013

The best paper of the year!!!!

I've just come across what might be the best paper I will read all year. Big call, I know.......it is only April.

Andrew MacDougall and co-authors have just published a paper in Nature (494, 86-89) that potentially has implications for understanding disturbance ecology here in Australian temperate grasslands. It's about fire impacts in degraded savanna and ecosystem stability. I've admired Andrew's work in oak savanna in British Columbia for some time. He's written about whether exotic species are 'drivers' or 'passengers' of change in degraded systems, and how to define conservation strategies using historical perspectives. It's worth checking out his Lab Homepage.

In the original oak savanna of British Columbia, a high diversity of grasses and forbs was historically maintained by frequent fire. But with fire suppression, invasions and pastoralism, present day oak savanna are species-poor, and dominated by exotic grasses. These dynamics are captured in Supplemenatry Fig 1 of the paper:

Fig S1 from MacDougall et al (2013)

Present day oak savanna. Species-poor, and dominated by exotic grasses.
(Photo: http://www.uoguelph.ca/~amacdo02/MacDougall_Lab/Home%20Page.html)

Because of fire-prevention measures since the mid-nineteenth century, oak savanna have lost many of their plant species - including the fire-tolerant species that contribute most of the diversity. Ironically, present day savanna produces a relatively stable annual biomass (from the remaining exotic grasses) and remains resilient to climate fluctuation.

MacDougall et al. conducted a 10 yr study in which the low diversity savanna plots were periodically burned after a very long period without fire. Savanna were able to recover from burning only in areas that had a relatively high diversity of native plants. (i.e. the system had maintained native species from the original flora that were capable of regenerating after fire). While these native species were rare and mostly functionally redundant, they proliferated after burning and rapidly recover the structure and function of the savanna, as well as preventing invasion by woody species. The exotic grasses, by contrast, were not well-adapted to fire and the system subsequently crashed after fire. They concluded that the study demonstrates how persistent human activity can homogenize both structure and function of an ecological system and this can weaken the diversity-related mechanisms needed to compensate for sudden disturbance.

So, why did I get excited?

Well, in a similar vein here in southern Australia, many native grasslands exist as species-poor systems due to fire exclusion and grazing. Exotic grasses are common in many grazed grasslands, many of the daisies and lilies that characterise the grasslands have been lost, and in many cases, native C4 grasses have been replaced over vast areas by C3 species.

And it is exactly these types of grasslands that are being acquired to improved the conservation status of grasslands near Melbourne and offset losses to urban expansion. What has me most interested is the idea that managers will need to manage new grassland reserves for biodiversity and one key way to do this is to remove biomass by burning. This is exactly what scientists (like me) have been saying is necessary to manage for grassland diversity.

But, if fire is introduced into a grassland that perhaps has had a century of fire exclusion, and occurs in a system where many of the fire-tolerant native herbs have been lost because of grazing, what might we expect as the outcome?

From work that Ian Lunt, Andrew Scott and others have conducted, we know there is likely to be little soil seed bank of native forbs in grasslands. Hence, fire won't promote a flush of native species to bring about a miracle cure for the grasslands. And the most fire-tolerant grasses (like Kangaroo Grass) are now absent. So, might fire actually de-stabilise grassland structure and function as a result of one hundred years of land use and the loss of native species? Might ecosystem stability be compromised by the re-introduction of fire because there is a hidden vulnerability to sudden environmental change in ecosystems that have had the buffering effects of high species diversity eliminated?

These are crucial questions for which we do not yet have answers. But it is work like that of MacDougall et al. that point to the real, perhaps even urgent, need to examine these effects, least we assume that species-poor systems function as if they were still species-rich.

MacDougall, McCann, Gellner & Turkington (2013) Diversity loss with persistent human disturbance increases vulnerability to ecosystem collapse. Nature 494: 86-89.