Obvious effects of drought in Australian alpine vegetation

When thinking about climate change impacts in alpine ecosystems, rising temperatures are thought to be one of the most important threats to their long-term persistence, particularly in places like Australia where there is very little elevation above treeline. It is generally accepted, and has even been demonstrated, that alpine species will need to migrate upslope to persist with climate warming. This hints that temperature is the most important determinant of alpine species distribution.

The Australian Alps - vulnerable to climate change because there is very
little capacity for upslope migration

But climate change is also likely to affect the frequency, intensity and amount of rainfall that occurs. And in Australian mountains that experience summer drought, the impacts of increasing drought severity and frequency are likely to also be very important to the persistence of alpine plants.

To illustrate this, I have shot a 11 min video about the drought impacts I observed recently in some Australian mountains. Interestingly, Mt Feathertop (which I point out in the video) was burning in a bushfire only 10 days after I shot this footage, hinting that drought also interacts strongly with fire in Australian mountains.


You can find my video on You Tube at
http://www.youtube.com/watch?v=ryn987ICBHs&feature=youtu.be

I focus on why I think drought is important in the Australian alps:

• because our mountains are covered in soil, vegetation grows across the entire mountain and therefore, is subject to water stress on shallow soils when there are dry spells
• how aspect affects moisture stress (remember, in the southern hemisphere, northerly aspects are the hot, dry slopes)
• show examples of some species showing signs of drought stress (note the effects on the dominant grass Poa hothamensis)
• think about the traits that may pre-dispose species to drought stress, and;
• conclude with the assertion that drought likely will be an important determinant of vegetation distribution in the coming century in mountains. This will likely be exaccerbated by rising temperatures.

Hope you enjoy my foray into 'film' making.
JOHN

Further reading on drought impacts in Australian alpine vegetation:

• Griffin & Hoffmann (2012) Mortality of Australian alpine grasses (Poa spp.) after drought: species differences and ecological paterns. Journal of Plant Ecology 5: 121-133.   
• Morgan (2004) Drought-related dieback in four subalpine shrub species, Bogong High Plains, Victoria. Cunninghamia 8: 326-330.

Below-ground ecology often gets ignored

One way a plant species will deal with the challenge of a changing climate is to track shifts in their climate envelope by dispersal and establishment in new locations that are climatically suitable for that species. Whilst this seems an obvious mechanism to deal with climate change, it fails to account for the fact that plants might be 'migrating' to very different soil types, and these soil types might be unsuitable for their establishment.
Differences in soil texture (which helps inform us about water holding capacity) and soil pH (which tells us something about nutrient availability) are two key ways in which soils might differ from Point A to Point B. So, plants that require free-drainage (like many Banksia in southern Australia that are found primarily on deep, acidic sands) probably won't do very well when they try to establish on poorly-draining clays.

But soils don't just provide nutrients and water. They are also the reservoir of soil biota that includes important groups such as mycorrhizal fungi.

Mycorrhizas are associations between fungi and plant roots that can be beneficial to both the plant and the fungi. The fungi link the plant with soil by acting as agents of nutrient exchange. The fungi receive carbohydrates as energy from the host plant root whilst nutrients such as phosphorus and zinc are passed back into the plant roots from the soil. Mycorrhizal associations may also reduce attack from root pathogens and increase the tolerance of the plant to adverse conditions such as heavy metals, drought, and salinity. In general, mycorrhizas play an important role in plant productivity.

It got me to thinking. How important are mycorrhizal associations when plant disperse to new soils? Could their absence (from the new soil types) inhibit colonisation potential of species trying to track their climate envelope shift?

While we know that mycorrhizal associations are important in plant communities (work in ecological restoration, for instance, has shown that some species are incredibly hard to re-establish if mycorrhizae are missing from the rehab site), it is hard to find much evidence for their existence in the plants I work with in southern Australia. In particular, what about the importance of mycorrhizal fungi for short-lived plants like annuals.



Hyalosperma praecox - one of the annual species
 I work with in grassy woodlands.

A feature of annual plants is their ability to grow rapidly after germination, establish one or more shoots that bear inflorescences, and then to flower and produce seed rapidly. The uptake of water, photosynthates and minerals into the developing flower heads and seeds occurs with some urgency because of the relatively short growing season. I got to wondering if mycorrhizae play a role here - after all, annual plants have very little time to grow roots to explore their soils for mineral nutrition and water.

The starting place to even begin to answer this question has to be: do annual plants form mycorrhizal associations in soils in the short-time that they are active? Using some old fashioned techniques (germinating annuals in their 'native' soil, then extracting them 4 weeks later, preparing roots by staining and using a microscope), we've recently found that many annual plants do indeed form mycorrhizal associations. This is the first study of its kind on grassland annuals in Australia, so rather exciting news.



Vesicles and hyphae, shown here by the staining, in a root

of the annual daisy Triptilodiscus pygmaeus

(Photo: Rohan Ball)

My Research Student Rohan Ball focused on observing what are called vesicular-arbuscular mycorrhizas (or VAM). The fungal hyphae here penetrate root cells and form intricately branched, shrub-like arbuscles within the cells and, at times, bladder-like vesicles as well. They are relatively easy to see, so we thought this was a good place to start.

Of the eight species we surveyed (all in the Asteraceae), we can confidently say that the roots of six species were colonised by VAM within 4 weeks of germination. You can see VAM in the photos as dark, stained areas - both vesicles and hyphae stain here quite clearly.  We are not yet sure what growth advantage this gives these developing plants, but we think it points to an interesting set of experiments. For instance, would VAM form in plant roots grown on 'non-native' soils (i.e. soils that are very different to those the species currently occur on)? If we remove VAM from the 'native' soil, does this affect growth and reproduction? Such questions are important because they have an applied outcome: are VAM necessary for annuals to recolonise agricultural lands? Will the lack of VAM affect colonisation potential of native species that disperse to new areas?

Borrowing from others

I've been a little snowed under these last few weeks, so haven't been able to Blog as much as I'd like. But I have some interesting posts in the pipeline.......

For this post, I'd like to point you to a couple of fascinating bits of ecology that I've come across recently. I think they are worth sharing here. Both are challenging, but for different reasons.

Angela Moles, in a TedTalk, talks about invasive plant species in Australia. But not in the way many of you might expect. Ang takes a different approach to invasives. She talks about how, once introduced to Australia, exotic species should be expected to evolve into unique 'new' Australian species. Once species have been transported to Australia, they are isolated - so gene flow potentially stops. Or at the least, is mixed by the introduction of different genotypes from the source. Then, local adaptation kicks in. Exotic species in Australia often exist in a very different context to their point of origin. Soils will be poorer (mostly in N and P), climate is more variable (and extreme) in many cases, and interactions with herbivores and pollinators will likely be altered. Under such circumstances, plants are likely to 'evolve' to suit their new habitats. Given enough time, they should also evolve to be reproductively isolated from plants derived from their source of origin (i.e. new species). Just like Darwin's finches!

You may not agree with Ang here - particularly because of the impacts that exotic plants can have on native plant communities (although it should be noted that in many bits of bush, some exotic species do seem to coexist happily with native species, e.g. Aira spp in grasslands). You can see her talk at: http://www.youtube.com/watch?v=5EV3ZTzSzZE

The second piece of interesting ecology comes from Jeremy Fox. He has a terrific Blog called Dynamic Ecology. I highly recommend it. He covers a range of topics in ecology (almost daily) - from theoretical ecology, to doing experiments, to cool T-shirts for science nerds.

In a old Blog (October 2012), Fox outlines what he thinks is one of the most important contributions to ecology in recent years. He summarises (beautifully) an emerging topic in the field of community assembly: phylogenetic community ecology. If you don't know what this is, then you should read his Blog.

Put simply, co-occurrence of phenotypically-similar species indicates “habitat filtering”, meaning roughly that community membership reflects species’ abilities to tolerate the local abiotic environment. Conversely, co-occurrence of phenotypically-different species means that similar species are being competitively excluded (= limiting similarity). If, as is often the case, phenotypic traits are phylogenetically conserved, so that closely-related species tend to be phenotypically similar, co-occurrence of closely-related species (“phylogenetic clustering” or “attraction”) implies habitat filtering, while co-occurrence of distantly-related species (“phylogenetic repulsion” or “overdispersion”) implies competitive exclusion. This is a simple, novel, creative, and relatively easy-to-implement idea, and so it’s no surprise that it took off.

The problem is, this “simple logical framework” is wrong! That is why you need to read Fox's Blog: http://dynamicecology.wordpress.com/2012/10/09/can-the-phylogenetic-community-ecology-bandwagon-be-stopped-or-steered-a-case-study-of-contrarian-ecology/

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)

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.

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