CO₂ as Antifreeze

By Robert C. Balling Jr., Ph.D.
Arizona State University

Plants grow faster. Photosynthesis increases. Root systems improve. Yields jump. Water-use efficiency rises. Drought resistance becomes stronger. Countless stresses are minimized.

An ideal biosphere? Maybe. An attainable one? You bet.

Elevated atmospheric carbon dioxide (CO₂) concentrations have netted these benefits again and again, as thousands of articles in major scientific journals attest.

Given the increase in water-use efficiency and tolerance to drier conditions with higher levels of atmospheric CO₂, many plant species will be able to survive in marginal drylands in the future where today, various stresses inhibit their survival.

Many of our past essays have dealt with this expected greening up of today's deserts thereby sequestering additional carbon, improving rangelands in semi-arid areas, and increasing agricultural productivity in arid areas. But as plants extend their habitat into desert areas, they will be forced to cope with a stress often overlooked in dryland areas—freezing temperatures.

Clear desert skies in the winter months may keep the temperatures high in the afternoon, but those same clear and dry conditions allow substantial radiative cooling at night; subfreezing temperatures in deserts are common in winter, and plants in those environments must protect themselves from the cold nights.

An article in a recent issue of the Journal of Arid Environments brings us news of yet another biological benefit of elevated CO₂.

A team of biologists from throughout America grew seedlings of three yucca species all native to the Southwest in controlled-environment glasshouses. Some glasshouses had natural, or ambient, levels of atmospheric CO₂ maintained at 360 parts per million (ppm); others at 700 ppm.

They grew plants in various temperature regimes based in part on an analysis of 30 years of temperature records from the yucca's natural range. But selected plants were placed in coolers, where temperatures were lowered from 20°C to –15°C at a rate of 3°C per hour.

Loik and colleagues discovered that "Plants maintained at elevated CO₂ had a greater low-temperature tolerance compared to controls," results that indicate "survival during episodic subzero temperature events will be enhanced" should CO₂ levels increase. They also found that higher "concentrations of CO₂ may allow seedlings to have a greater likelihood of surviving lower temperatures and thereby establishing at higher elevations and latitudes in the future."

We hear (and say)a lot about higher temperatures and CO₂; but isn't it nice to know we can add freeze protection to that long list of biological bonuses from elevated atmospheric CO₂?

Reference:

Loik, M.E., et al., 2000. Low temperature tolerance and cold accumulation for seedlings of three Mojave Desert Yucca species exposed to elevated CO₂. Journal of Arid Environments, 46, 43–56.

Grasslands Round-Up

Most studies of carbon dioxide's effect on plant life are fairly short in duration—a year or two at most. But one research team stayed with their experiment for six years. Swiss and Portuguese scientists grew ryegrass near Zurich in open fields with atmospheric carbon dioxide levels maintained at 350 parts per million, or ppm, and 600 ppm. They found that in the first year of the enrichment, the grass increased its dry weight by 7 percent; but by year six, the increase in dry weight had risen to 25 percent, thanks to higher carbon dioxide (CO₂) concentrations. The longer the experiment ran, the better the news about CO₂! Furthermore, their results suggest that the many one- and two- year studies may be grossly underestimating the goodness of increased atmospheric CO₂ levels.

Another team of scientists from Switzerland, some of whom were involved in the Zurich study, grew various grasses in an open field in 1994 and 1995 with atmospheric CO₂ concentrations maintained at normal and doubled levels. Under low levels of nitrogen, the grassland increased overall biomass by 13 percent; under high nitrogen levels, the grasses responded with a 30 percent increase.

Van Ginkel grew that same ryegrass specie for 115 days in growth chambers with atmospheric CO₂ concentrations of 350 ppm and 700 ppm, after which some chambers had the temperature increased by 2°C for 230 days. They observed that elevated CO₂ increased root biomass substantially, and the authors wrote "root biomass is the driving parameter for all subsequent below-ground processes in our plant-soil system." Further, the beneficial microbial biomass increased by 46 percent for the elevated CO₂ concentration, and the increased temperatures appeared to have little negative effect on the benefits of higher CO₂. Elevated CO₂? Great! Higher temperatures? No problem.

With all that grasslands stand to gain as CO₂ increases, the pessimists among us might start searching for reasons it's all just too good to be true. For example, won't some herbivore come along and ruin the green parade?

That question is so important that a research team headed by scientists from Cambridge and Harvard came together to investigate whether those slimy interlopers we call slugs will gain the upper hand in the pasturelands of generations to come.

Peters and colleagues grew a variety of grassland species at natural (356 ppm) and elevated (600 ppm to 650 ppm) CO₂ concentrations and fed the forage to some slugs.

As with hundreds of other studies, they found that elevated CO₂ significantly increased biomass of the pasture species. Even more important, they noted, "In terms of the total amount of plant material consumed, there was no evidence that consumption of plants grown at elevated CO₂ was any different than consumption on controls grown at ambient CO₂."

In the battle between plants and slugs, elevated CO₂ favored the plants.

Again, the message rings loud and clear: If you want a greener planet, increasing (not decreasing) atmospheric CO₂ concentrations is just what the biosphere ordered.

References:

Daepp, M., et al., 2000. Yield response of Lolium perenne swards to free air CO₂ enrichment increased over six years in a high N input system on fertile soil. Global Change Biology, 6, 805–816.

Lüscher, A., et al., 2000. Direct evidence that symbiotic N₂ fixation in fertile grassland is an important trait for a strong response of plants to elevated atmospheric CO₂. Global Change Biology, 6, 655–662.

Van Ginkel, J.H., et al., 2000. Elevated atmospheric carbon dioxide concentration: Effects of increased carbon input in a Lolium perenne soil on microorganisms and decomposition. Soil Biology and Biochemistry, 32, 449–456.