CO₂ causes turf wars

The planet is warming, but Dr Marilyn Ball’s research group has found that rising carbon dioxide in the atmosphere makes cold-tolerant plants more susceptible to frost.

Drive south from Canberra on the Monaro Highway, through the rolling grasslands of the Monaro high plains, and you could almost be in the vast, treeless grasslands of the Central Asian steppes.

In 1909, renowned Australian surveyor and botanist Richard Cambadge wrote of the Monaro grasslands: “It is remarkable that such an extensive tract should naturally be so destitute of trees and shrubs … the country is made up of clear undulating plains, with only a few isolated tree-clad localities.”

It took nearly a century, but in 2002 plant ecophysiologist Professor Marilyn Ball’s research team at the Australian National University finally provided a convincing explanation about how the near-treeless grasslands of the Monaro have kept all but a few isolated pockets of woody invaders at bay.

In her address at the ComBio conference, Ball will describe her team’s recent research into how the hardy, cold-tolerant grasses and megaherbs of sub-Antarctic islands are responding to rapid global warming, and increasing concentrations of carbon dioxide (CO₂) and other greenhouse gases in the atmosphere - the work has not yet been published, but according to Ball, the news is not good.

The work is focused on understanding structure-function relations in plants and how plant species differ in their physiological capacity to respond to changes in climate.

“We study a range of plant communities from mangrove forests to subalpine eucalypt forests, to alpine vegetation in the subAntarctic,” she said. “We’re really interested in looking at temperature extremes - high and low. We restrict our studies to evergreen plants, because their leaves live for a long time and often have to cope with hostile extremes of temperature through the year.

“For example, we want to understand how the adaptations that enable tolerance of freezing temperatures in winter affect the capacity to cope with hot, dry conditions in summer. Alpine environments typically experience wetter conditions than lower altitudes, so they go from freezing conditions in winter to hot, humid days in summer.”

Defending its patch

A decade ago, Ball’s team made the surprising discovery that the native grasses of the Monaro plains rule by creating a hostile microenvironment that alternately freezes or scalds the emergent seedlings of would-be woody invaders.

Using a dry straw mulch to simulate a dense cover of live grass - their thermal properties are identical, said Ball - the ANU researchers compared the survival of Snow Gum (Eucalyptus pauciflora) seedlings on bare ground versus soil with a full grass cover, through summer and winter.

Snow Gums are one of Australia’s most cold-tolerant eucalypts. Near Charlotte Pass, in Kosciuszko National Park, they grow up to the snowline at an altitude of 1800 metres where winter temperatures can drop below minus 20°C in winter.

Ball’s team found that seedlings growing in bare soil had a longer growth season than those growing with their roots covered by grass. Competition for below-ground resources was only part of the story.

On bare ground, soil temperatures increase more rapidly in late winter and spring, giving the seedlings a longer growing season when soil moisture is abundant.

After a year, the seedlings on bare ground had accumulated four times the biomass of seedlings grown with a grass or straw cover. Their more vigorous root growth in later winter translated to an advantage in competing for soil moisture and nutrients in early summer.

Seedlings with a grass/straw cover experienced greater temperature extremes during the year.

Grass affects soil temperatures by reducing heat conduction into the soil in summer, so the seedlings entered winter with colder roots. Then, in winter, the insulating effect of the grass cover increases frost damage by lowering the temperature of the overlying air and extending the frost season. And in spring, grass cover retards soil warming, delaying the resumption of growth and reducing growth rates.

The ANU researchers suggest these microclimatic effects explain how tree seedlings grown on bare ground can accumulate four times more biomass over the course of a year.

“Once grass is established, it is a tenacious defender of its patch,” said Ball. “Trees have no chance to establish unless a disturbance creates a bare surface. We were stunned at just how potent those changes in microclimate can be.”

Ball suggests the effect of grasses on microclimate may explain why it can be difficult to re-establish eucalypt seedlings in frost-prone hollows after clear-felling. Self-sown or tube-grown seedlings initially grow well, but enter a phase known as ‘growth-check’ as a grassy ground cover develops.

Growth-check has been attributed to root competition, but Ball’s team suggests it may also be due to effects of the grass cover on microclimate.

Frost-tolerant species become vulnerable

In plant ecology, it seems nothing is as simple as it first appears (see The Kermit Effect). Ball, who comes from Florida, originally intended to become an environmental engineer because she was interested in incorporating native vegetation in engineered water-management systems.

But she changed tack after asking her teachers a simple question about mangroves: what’s the optimum mix of fresh and saltwater for a mangrove?

“Nobody knew,” she said. “How much flooding do the cypress swamps in the Florida Everglades require? Nobody could answer those questions.”

Ball revels in the complex problems that plant ecophysiology throws up, because they can yield surprising, even paradoxical answers.

Perhaps the most paradoxical finding to emerge from her team’s research is that, even as the planet warms, plants can become more vulnerable to frost injury, as the effects of frost are amplified by the rising carbon dioxide concentration in the atmosphere.

“They get caught out, because they’re slower to acclimate to freezing temperatures in autumn, and they can also suffer damage because they de-acclimate more rapidly in late winter when grown under elevated CO₂,” she said.

Another complication emerged from their studies of woody plants in the high-altitude heathlands of the Blue Mountains, west of Sydney.

Ball explains that plants that have adapted to habitats with freezing winter temperatures typically have small xylem vessels. Xylem vessels are the conduits for transport of water and nutrients from the roots to the stems and leaves. Maintenance of conduit function is essential for evergreen species. Reducing the diameter of xylem vessels reduces the risk of a freeze-induced air embolism - a breakage in the flow of the water streaming through the vessels.

More than a decade ago, several overseas groups discovered that increasing CO₂ concentrations alone increased the risk of fluid freezing in the tissues of frost-tolerant species because the gas increases the temperature at which ice nucleation occurs in their tissues.

But this adaptation seriously affects the plant’s capacity to fix carbon by photosynthesis because of a physical law of diminishing returns that governs fluid flow through pipes: resistance to flow increases to the fourth power as the diameter of the vessel decreases. Halve the diameter of a vessel and resistance to flow increases by a factor of 16, so even a small reduction in xylem vessel diameter substantially reduces the amount of water the plant can draw up through its stem during photosynthesis.

Ball says the reduction in conduit size is a trade-off that minimises the risk of frost injury at the expense of the plant’s ability to supply water to its leaves.

Carbon cannot be absorbed through the plant’s open leaf stomata without water vapour escaping, so any reduction in water transport due to narrowed xylem vessels unavoidably reduces the capacity for carbon gain during warmer seasons.

“You don’t see much shoot growth in winter, but winter photosynthesis is very important for growing the roots that will power the plant’s growth in the warmer part of year. So what goes on in winter is really important.”

Ball says anything that reduces photosynthesis in winter - like the effect of elevated CO₂ levels on frost resistance - potentially has serious implications for vegetation in cold climates and may limit their capacity to respond with growth to climate warming.

Alpine vegetative communities

Preliminary results from the ANU team’s study of the alpine communities of the sub-Antarctic Macquarie Island - there are no woody plants on the island - indicate the plants are very sensitive to warmer, drier conditions.

“The pattern is similar to what we have predicted from our studies of woody plants in the Blue Mountains because the subAntarctic plants have limited capacities for water transport consistent with their adaptations to cold-wet environments,” she said.

“We installed heat lamps that gently warmed the plants by around 2°C. This reduced the photosynthetic capacity of a common megaherb, while the dominant grass, Poa foliosa, responded with extra growth.

“That’s a real concern for the iconic megaherbs, which have very large leaves but low capacity for water transport - they really suffer with an increase in temperature.”

Ball says subAntarctic islands like Macquarie Island have a relatively mild oceanic climate, where the surface temperature of the surrounding Southern Ocean keeps temperatures relatively constant through the year. “You get more temperature variation in a single day in Canberra than you see in a year on Macquarie Island,” she said.

Macquarie Island’s climate is constantly cool and wet, becoming cold and wet during winter. The island has no woody species, but some subAntarctic islands like Auckland Island, south of New Zealand, do have woody vegetation, and Ball says increasing temperatures will probably favour the growth of woody species and grasses, threatening major ecological changes.

She says that complex interactions make ecological changes under a greenhouse-warming regime difficult to predict.

“Nevertheless, our work has identified some basic ecophysiological traits that may make it easier to identify plants that might be advantaged by CO₂ enrichment, and predict their responses to warming, and to identify other plants that might be at a disadvantage,” she said.

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The Kermit Effect

Global warming sceptics recently took heart from the findings of a joint CSIRO-ANU analysis of satellite imagery, which showed that the world’s arid and semi-arid regions have become significantly greener since 1982, because of the CO₂ fertilisation effect.

The analysis, which found that Australia’s dry interior has become 11% greener during the study period, confirms a longstanding prediction about the response of woody plants to rising CO₂ levels.

Woody plants that employ the C-3 photosysnthesis system become more water efficient as CO₂ concentrations in the atmosphere increase. As plants take in CO₂ from the atmosphere, they lose water vapour through their open stomata - the tiny pores in the lower surface of their leaves. It could be called the ‘Kermit Effect’: it’s easier to be green when you’re getting more photosynthetic bang for your CO₂ buck.

But there’s a catch, according to one of the principals of the study - Dr Randall Donohue of CSIRO Land and Water. South African researchers have reported that the CO₂ boost for woody plants, by increasing foliage cover, could allow them to shade out grass and invade grasslands. Woody plants increase fuel loads, so the end result could be a more flammable landscape.

Another implication, says Dr Donohue, is that the increased foliage cover for woody plants makes them more sensitive to drought - their greater leaf area makes them more vulnerable to running out of water. “The foliage cover makes them more sensitive to changes in rainfall - logically, the changes means they will brown off more rapidly in drought, but green up quicker when the rain returns.”

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