The dark side of global warming

Global warming is making nights warmer around the globe and from pole to pole, and will have major effects on plant photosynthesis and productivity, according to Columbia University plant physiologist Dr Kevin Griffin.

Griffin told the recent ComBio 2003 conference in Melbourne that average night temperatures are rising faster than day temperatures, because rising concentrations of greenhouse-gas levels -- particularly carbon dioxide -- are absorbing increasing amounts of heat radiation from the Earth's surface at night.

Griffin's research is focused on the effects of nocturnal warming on plant growth. By day, plants photosynthesise, trapping the energy of sunlight to make carbon-rich sugars. But after the sun goes down, plants continue to respire, depleting the sugar reserves they have accumulated during the day.

The balance between the two processes determines the productivity of the plant, and Griffin's research suggests not all plants -- including crop and forest species plants - will respond in the same way.

Since 1999 Griffin and his colleagues have been exploring the effect of increasing night temperatures on plant growth in the world's largest greenhouse, the 1.3 hectare Biosphere 2 facility in the Sonora Desert in the US south-west.

While global surface temperatures have increased by an average of 0.8 degrees in the past 100 years, average night temperatures have risen more than twice as much -- 1.8 degrees.

Some regions are warming much faster than others. Griffin says average day temperatures in the tall prairie grasslands of the Colorado Plateau have risen by 7.1 degrees in the past century, while nocturnal temperatures have warmed by a phenomenal 16 degrees.

In the Black Rock Forest of Griffin's home state, New York, average minimum temperatures in February, in the heart of winter, have been increasing at a rate of around 5 degrees per century, yet the average maximum temperature in June has hardly changed.

But global average temperatures are predicted to increase by somewhere between 1 and 6 degrees during the next century, with the upper end of this range looking increasingly likely at current warming rates.

The enormous Biosphere 2 greenhouse allows researchers to carefully control day and night temperatures, and modify carbon dioxide levels to simulate the likely composition the atmosphere later this century.

As a model plant, Griffin's team chose the fast-growing cottonwood, Populus deltoides. In their first experiment in 1999, they raised normal night temperatures by 10 degrees, to a constant 28 degrees.

As expected, the trees' nocturnal respiration increased, depleting their reserves of plant sugars, but unexpectedly, they compensated by slightly increasing their photosynthetic activity during the day.

In 2000, Griffin's team returned to Biosphere 2 to explore this effect further. They ramped up night temperatures from 15 to 20 to 25 degrees on successive nights, while keeping day temperatures steady. Again, the trees depleted their energy reserves at night, but increased their photosynthesis by day in compensation.

But greenhouse warming will increase both night and day temperatures, so in 2001 Griffin's team ran two more experiments. In one, they left the night temperature at a constant 20 degrees, while lifting the day temperature from 25 to 31 degrees. There was no change in night respiration, but a significant increase in photosynthesis -- the trees grew faster.

In the second, they raised night temperatures by 10 degrees, and day temperatures by 6 degrees. Night respiration increased significantly, but they also measured a very large increase in photosynthesis during the day.

The sugars formed by photosynthesis are transformed into starches, and eventually into cellulose and lignin, the complex polymers that make up the cell walls and wood.

Plant scientists have been concerned that the more rapid increase in night temperatures with global warming, and the consequent rise in nocturnal respiration, would reduce photosynthesis.

Griffin's experiments show that, at least in cottonwood, this is not the case -- photosynthesis actually increases, and the trees grow faster they can adapt, or acclimate, to warmer temperatures.

The increase in photosynthesis means that the plants accumulate more biomass, and take more carbon from the atmosphere. If the effect occurs globally, the carbon dioxide draw-down effect would slow the rate of greenhouse warming.

But when Griffin's team obtained a different result when they applied similar warming regime to an annual plant, a shrubby Texas native, Xanthium strumarium. It did not increase its photosynthesis at higher day temperatures, but its nocturnal respiration increased. It grew much larger, thinner leaves, and while its total biomass did not change significantly, its seed production decreased by a massive 52 per cent -- with ominous implications for the effect of global warming on crop productivity.

The long-term effect of global warming on seed production and leaf size and thickness in perennial plants like cottonwoods and deciduous trees is unclear, according to Griffin.

But when a species' seed production drops by nearly half, it inevitably will become less abundant. And changes in leaf morphology could also have profound ecological effects -- larger, thinner leaves in dominant canopy trees will cast more shade, restricting the amount of light penetrating to ground level, and reducing the growth of understorey species, perhaps even causing annual species to drop out.

Plants are the primary source of energy for complex food chains; any change in productivity will have ripple-down effects on both plant and animal communities. Griffin's research hints that global warming will have unpredictable but potentially wide-ranging impacts on plant and animal communities.