Monodominance in C4 grasslands

A typical view in southern Australia. Here,
Kangaroo Grass dominates the structure
and cover of a native grassland.
(Photo: John Morgan)

One of the things that strikes you about the C4 grasslands in southern Australia is the complete dominance by Kangaroo Grass (Themeda triandra). The concept of monodominance is actully rather rare in grasslands. Only a minority of the world's grasses (around 600 out of 11,00 speces) are documented as being ecologically dominant. These dominant species, however, seem to share a common(ish) evolutionary history.

In an interesting paper on the origins of C4 grasslands by Erika Edwards and collegues, dominant grasses appear to be phylogenetically clustered, suggesting that certain clades of grasses are more prone than others to evolve traits that promote ecological dominance. But what might these traits be?

The answer to this question is not as straightforward as we might presume. While we accept the fact that Kangaroo Grass dominates the grasslands of southern Australia, there is still some uncertainty about why it does so. There's likely to be a few reasons. Some of them are evolutionary, while others are more ecological. Here, I outline a couple that I think are likely to be important.

Kangaroo Grass is a C4 species. "C4 photosynthesis" refers to a suite of biochemical and anatomical traits that increase photosynthetic efficiency in high light and high temperature environments. While C4 enhances the efficiency of photosynthesis, C4 plants only have an advantage over C3 plants in certain conditions - namely, high temperatures and low rainfall. Hence, C4 grasses are conspicuously absent from the world's cooler regions. This may, in part, explain the dominance of C4 grasses such as Kangaroo Grass in southern Australia.

Kangaroo Grass, in Grime's CSR plant strategy scheme, would comfortably be considered a competitive species. Such species are able to outcompete other plants by most efficiently tapping into available resources. Competitors do this through a combination of favorable characteristics, including rapid growth rate, high productivity (growth in height, lateral spread, and root mass), and high capacity for phenotypic plasticity. This last feature allows competitors to be highly flexible in morphology and adjust the allocation of resources throughout the various parts of the plant as needed over the course of the growing season.

Kangaroo Grass might be thought of as a pyrogladiator. It is fire-adapted, resprouting strongly from basal meristems with very little fire-induced mortality. By contrast, our Lab has shown that C3 grasses, such as Wallaby Grass and Spear Grass, can experience substantial levels of tussock mortality after fire (perhaps because of the fire event itself), further weakening their position in C4-dominated grasslands. Kangaroo Grass, by contrast, quickly accummulates biomass between fires, probably because the C4 pathway supports high photosynthetic rates and nitrogen use efficiencies, especially in the high-light environments after fire. The high water-use efficiency afforded by C4 metabolism probably also provides a competitive edge.

Beth Forrestel, from Yale University, admires
a C4-dominated grassland at Vite Vite on the
western plains of Victoria.  (Photo: John Morgan)

The concept of monodominance is not just of academic interest.

Dominant species shape communities and drive ecosystem processes. Our research has shown that healthy swards resist weed invasion. Hence, they should also be of key interest to restoration ecologists wanting to restore resilient ecosystems (see my last Blog as an example of this). Finding ways of returning dominant species (across large scales) might therefore be just as important as returning rare species to ecosystems.

And understanding how dominant species respond to climate change is a challenge we are only just starting to tackle.

Grassy White Box Woodland Restoration

Native grasslands and woodlands in Australia have been transformed since European settlement. Because they occur on the fertile soils (by Australian standards), and are dominated by palatable grasses, they were amongst the first ecosystems settled, and amongst the most intensively utilised. As a result, much of the original ecosystem has been lost - to cropping, to grazing, and to pasture improvement. Probably less than 15% of woodlands remain in eastern Australia, and grasslands occupy much less than 5% of their original range.


White Box woodland in a bush cemetery
(Photo: S. Prober)

But some remnants do survive - in areas that have escaped heavy utilisation such as bush cemeteries, travelling stock routes, town commons and railway lines. And they survive in relatively weed-free states with high native plant diversity. They provide a key insight into how degraded remnants might be restored.

Ian Lunt from Charles Sturt University and Suzanne Prober from CSIRO have been working for a long time now on the conservation and restoration of white box woodlands in southern Australia. Given the perilous state of these systems, their scientific studies are at the cutting edge of practical conservation biology.

They have looked at why small remnants have maintained their diversity - and come to the conclusion that soil nutrients plays a key role. Where nutrients are high, exotic plants are favoured and these tend to outcompete the small native species that have evolved to survive on scant resources. Where nutrients are low, native species thrive because the exotic species basically have too few resources to survive.



Soil nutrients increase for a couple of reasons - the most obvious one is that they are applied by farmers to increase productivity. Less well known, however, is that when deep-rooted, long-lived perennial native grasses such as Kangaroo Grass are lost from ecosystems, lots of nutrients (including nitrate) are released into the soil and the elevated levels favour annual grasses. Annual grasses, of course, are short-lived - so they use soil nitrate to grow and flower, but because they die each year, that nitrate gets released back into the soil to be used again in the following year by even more annuals. And so the cycle continues.

Therefore, the groundlayer of grazed and degraded remnants rarely recovers well after fencing and livestock exclusion because these sites often have high soil nitrate levels that favour the exotics. So, how to overcome this problem.

Ian and Suzanne have found that it is absolutely imperative that soil nutrients be reduced if native species are to be re-established, but this is easier said than done. Indeed, until their work begun, few conservation biologists had really thought about this problem in Australia. They have trialled a number of techniques in small experimental plots - well replicated of course! Their treatments included i) re-establishing deep-rooted perennial native grasses (to lock up nutrients), ii) burning (this leads to some loss of nitrogen in smoke), and iii) adding sugar (to reduce nitrogen availablility due to microbial activity).

The results have been nothing short of stunning, and give hope that grassy woodlands and grasslands can be restored. It's an example of how really good science can inform practical conservation outcomes. Their work has just featured on the ABC's science program Catalyst - which I've included here so you can see what this long-term study has been able to achieve.

                    

2010 ISI Impact Factors are now out

You can't ignore the fact that Impact Factors have had a huge effect on publishing trends and the choices authors make about where to publish. This is somewaht unfortunate - I now hear scientists talking in the corridors of universities, or worse, at conferences, about where they published their most recent paper, not what they are publishing on!

Regardless of how much weight you put on Impact Factors (see this damning review as evidence that some don't rate IFs at all), all new PhD students and Post-Docs have to play a game of publishing in (perceived) high impact journals if they are to get that next job.

So what are Impact Factors and how are they calculated?

In a given year, the impact factor of a journal is the average number of citations received per paper published in that journal during the two preceding years. For example, if a journal has an impact factor of 3 in 2009, then its papers published in 2007 and 2008 received 3 citations each on average. The 2009 impact factor of a journal would be calculated as follows:

A = the number of times articles published in 2007 and 2008 were cited by indexed journals during 2009

B = the total number of "citable items" published by that journal in 2007 and 2008. ("Citable items" are research papers; not editorials, book reviews or Letters-to-the-Editor)

2009 impact factor = A/B.

I've included the 2010 Impact Factors for journals in conservation and plant ecology (and compared their "performance" to their 2009 rating). The big winners were Ecology Letters, Trends in Ecology and Evolution and Frontiers in Ecology and the Environment. Interestingly, the IFs of most journals rose over the last year.

Applied Vegetation Science: 1.802 (2010) versus 1.349 (2009)
Austral Ecology: 1.820 versus 1.578
Australian Journal of Botany: 1.681 versus 1.868
Biodiversity and Conservation: 2.146 versus 2.066
Biological Conservation: 3.498 versus 3.167
Conservation Biology: 4.894 versus 4.666
Diversity and Distributions: 4.248 versus 4.224
Ecography: 4.417 versus 4.385
Ecological Applications: 4.276 versus 3.672
Ecology: 5.073 versus 4.411
Ecology Letters: 15.253 versus 10.318
Frontiers in Ecology and Environment: 8.820 versus 6.922
Functional Ecology: 4.645 versus 4.546
Global Change Biology: 6.346 versus 5.561
Global Ecology and Biogeography: 5.273 versus 5.913
Journal of Applied Ecology: 4.970 versus 4.197
Journal of Biogeography: 4.273 versus 4.087
Journal of Vegetation Science: 2.457 versus 2.376
Molecular Ecology: 6.457 versus 5.96
Nature: 36.101 versus 34.480
Oecologia: 3.517 versus 3.192
PNAS: 9.771 versus 9.432
Polar Biology: 1.445 versus 0.582
Science: 31.364 versus 29.747
Trends in Ecology and Evolution: 14.448 versus 11.564

Why ecologists should know a little about geomorphology

I like rocks almost as much as I like plants!! There, I've said it!

And I particularly like the processes that give rise to the landforms that we see - geomorphology. Being able to read a landscape - such as the rock types and the way that they affect topography and drainage, as well as their impact on base level nutrient availability - is one of the most important skills that a plant ecologist can acquire. Indeed, an understanding of geomorphology broadly helps explain the distribution of native vegetation types in southern Australia.

Native grasslands, for example, occur predominantly on plains of low elevation, both in northern and southern Victoria. One might logically think that there is a similar underlying reason for their distribution and the absence of trees. Nothing could be further from the truth! These land surfaces have very different geological and geomorphological histories that have shaped these systems in different ways.

In western Victoria, volcanoes (more than 350 of them) have spewed out lava over the last 20,000 yrs to 5 M yrs, producing the third largest larva plain in the world, exceeded only by the Deccan in western India, and the Snake River Plateau in the United States! The volcanic activity was probably similar to that now active in Hawaii, with the dominant volcanic product being fluid basalt lava with only a small component of pyroclastic material (mainly scoria). Lavas of this type can spread rapidly across the landscape, and in places extend over 50 km from the volcano.

Lava flows must have produced an initially barren surface that required extensive denudation (i.e. modification by weathering) to be a suitable plant habitat, with primary succession proceeding from species derived in the surrounding landscape - this possibly explains why the western plains flora consists of many generalist species, and few endemics have evolved in the relatively short timeframes since volcanism. The soils that developed in situ are fine-textured cracking clays and are very nutrient-rich (indeed they are amongst the most productive in Australia; it also probably explains why weed invasions are so pronounced here too). As a consequence, trees are restricted to stony rises and cinder cones where drainage is best and soil cracking least.


Mt Elephant, as seen from Dundonnell, is one of the larger of the volcanoes found
 on the western plains. It is an example of a steep-sided scoria volcano - true 'fire mountains'
when they erupted. They formed when magma interacted explosively with groundwater,
blasting molten rock high into the air. The ejected material cooled before it hit the ground,
forming fragments of frothy red or black rock called scoria. These fragments quickly
settled around the vent, building cones with deep central craters. (Photo: John Morgan)



The Volcanic Plains of western Victoria - this geological map is a

good approximation of the distribution of the native grasslands(source: http://home.iprimus.com.au/foo7/volcmap.html)

By contrast, the vast native grasslands of northern Victoria are the product of an entirely different land forming process. Here, the landscapes were formed by river flooding spreading coarse alluvium. The alluvial plains are built of sediment derived from the erodible sandstones, mudrocks and igneous rocks of the Victorian Highlands and spread by rivers down the mountain flanks. These sedimentary surfaces are quite unlike the volcanic surfaces of western Victoria and formed under a completely different geological regime. These sediments, deposited and redistributed by rivers and wind, buried the older bedrock surfaces and produced a complex landscape of low relief and gentle slope. Like the volcanic plain, it is a mosaic of materials, ages and forms. And like the volcanic plains, the soils are fine-textured clays that easily waterlog in winter, preventing the growth of trees.

Riverine plains grassland - formed by alluvial processes.
(Photo: Eris O'Brien)

Grasslands also occur up in the highlands on mountain summits, plateaus and high plains, but these are not generally not due to the underlying rocks. Rather, they occur where low temperature or cold air drainage - the so called 'frost hollows' - suppresses tree growth. See Wearne & Morgan for a description of these interesting grasslands in the Mt Hotham region. At the moment, that low temperature envelope is very narrow and occurs generally above 1600 m, but in colder Pleistocene times the tree line may have been as low as 1000 m.

 

So, I've provided a simple example of how land forming processes are responsible for the landscapes we see today. Unfortunately, there's no textbook that adequately introduces geomorphology of Victoria that I can recommend. Rather, you'll need to observe the landscape and ask: what are the rocks that underlay this area? when did this occur? how have aeolian and fluvial processes shaped the landform? Are there obvious associations of vegetation when the above change?

Experimenting with Fire

My PhD primarily revolved around the effects of fire frequency on regeneration dynamics and species coexistence in the endangered temperate grasslands of western Victoria. Here, fire plays an indirect role - frequent fire prevents competitive exclusion of the intertussock forbs from the dominant C4 tussock grasses. In this case, it was the frequency of fire, not the type of fire that seemed most important to the conservation of plant diversity.

In 2003, landscape-scale fires burnt the alpine
vegetation of Victoria. But it was clearly very patchy.
(Photo: John Morgan)

But fires can come in many guises - fires ain't fires - you only need to look at a wildfire to see that it can burn thoroughly or patchily. It's clear to me, however, that we don't understand very much about plant community responses to difference in fire "type". Rather, much of our knowledge (and research) is from the standpoint of the time-since-last fire and, perhaps, the fire frequency (and these assume that fires are much the same). Yet, it is the type of fire that might ultimately affect mortality of established plants, germination cues, and resource levels. Not to mention how much C is returned to the atmosphere.

To learn more about fire and how to measure it, I've just spent a week burning tropical savanna in the Northern Territory Wildlife Park with Dick Williams from CSIRO. This was excellent fun, but also highly informative. The Burning for Biodiversity experiment is an amazing field study examining the effect of fire frequency and timing on a variety of taxa and the dynamics of carbon. Importantly, it relates these outcomes to aspects of fire behaviour. So, what better way to learn about fires than to visit one of the few experiments in Australia that is quantifying fire!

A common measure of fire behaviour is fire intensity – the amount of energy released per unit length of fire front (kW m^-1). It is defined as the product of rate of spread (ROS), fuel load, and the heat released from the fuel during combustion. The higher the fuel load and the ROS, the higher the fire intensity.

Fuel loads are easy to calculate - the amount of fine (<6 mm diameter) fuel is sampled in quadrats pre-fire and weighed. It is the fine fuel that will rapidly combust (flamming combustion) and affect properties of the fire front. Larger diameter fuels burn more slowly (in a process called smouldering combustion), typically after the fire front has passed. These fuels are important to quantify, as they will release much more C into the atmosphere.




Harvesting fuels prior to ignition
(Photo: James Camac)

Savanna in the Northern Territory - awaiting burning
(Photo: John Morgan)



ROS is a little harder to quantify because it is much more dynamic, but plays a critical role on fire intensity. In Darwin, we used two techniques to estimate ROS from our contolled burns. First, we measured the time the fire front takes to reach pre-defined points in the landscape - using points marked with numbered poles (we used six) and a stopwatch, the average rate of spread of the fire between the points can be calculated. As a  backup to the estimates by eye (it can get quite hairy when the fire front is moving quickly), we also used specially designed automatic timers buried in the soil with a small thermocouple left exposed above-ground - these timers record the time at which the thermocouple heats to >200 deg C and, somewhat ingeniously, the residence time (the time that the temperature stayed above 200 deg C).



Lighting the fireline with a drip torch
(Photo: James Camac)

 


Timers, attached to thermocouples, are buried in the soil.

These record the time at which fire passes, and how long
flaming continues at the point. (Photo: James Camac)







Flamming combustion

(Photo: John Morgan)





Smouldering combustion
(Photo: John Morgan)



These are very simple measures that can address fundamental research questions - they should be in the toolbox of all fire ecologists because, if measured, they allow quantification of how fire intensity might affect biodiversity.

We certainly saw large differences in fire intensity in hectare-scale plots burnt on the same day! I look forward to coming back to Darwin in 2012 to observe just why these differences might matter. At this point, I'm not sure when, where, and why fire intensity affects biodiversity - there are simply too few examples in the literature to provide a coherent review. But I know that in future, I'll be quantifying fire intensity in my research (both within and across different fires) to get a better understanding of this primary aspect of fire behaviour.