How Plants Respond to Changes in Atmospheric Carbon Dioxide

Written by Professor Jamal Munshi on September 2, 2020. Posted in Current News

IN THE CASE OF RISING CO2, photosynthesis will go up and there will be a greening of the earth as in the images above but this effect varies regionally. It is a strong function of the local ecosystem variables the most important being the availability of water.

Under ideal conditions, when there is plenty water {but not too much water where that partially submerges the plant}, plant productivity by way of photosynthesis will go up by a factor equal to 10% of the rise in atmospheric CO2 concentration in parts per million.

For example, all other factors being equal, a 100ppm increase in atmospheric CO2 concentration will increase plant productivity by 10% and a 200ppm increase will increase plant productivity by 20% and so on.

When sufficient water is available for plant survival but water availability is less than optimal, the gains from higher CO2 are constrained by the availability of water. Typically, in these water constrained situations, the rise in plant productivity is half the rise in productivity under ideal water conditions. So for example, a 100ppm rise in CO2 will result in a 5% increase in plant productivity, not 10% and a 200ppm rise in CO2 will result in a rise in plant productivity of only 10% and not 20%.

The third water constraint is extreme water scarcity as in drought conditions described in the literature as “lethal water shortage”. there will be no increase in plant productivity but higher CO2 will increase the probability that the plant will survive the drought. In that way, CO2 can still be credited with “greening” since it helps to preserve greenness that could otherwise have been lost.

A LIMIT TO THE EFFECT OF RISING CO2 is identified in the Van der Water 1994 paper cited above where we see a chart that identifies a limit to the rising CO2 effect. It shows that the CO2 effect on plant productivity under ideal water conditions is strong up to 300ppm, weaker from 300ppm to 500ppm, and then it flattens out implying that plants are unable to realize productivity benefits from atmospheric CO2 above 500ppm. At well above 400ppm we are now approaching the no greening condition.

IN THE CASE OF FALLING CO2: The Ward paper studied conditions during glaciation when the world cools and the ocean absorbs CO2 reducing atmospheric CO2 to well below 200ppm {180ppm to 190ppm have been reported}. In this range there is a strong response between plant productivity and atmospheric CO2 concentration as seen in the Van der Water chart above. The corresponding plant productivity is low. The effect may not appear as a loss of greenery as the life cycle of plants is not as readily replaced with new plants. It is for this reason that the transition from glaciation to interglacials is associated with significant greening of the earth.

THE FULL TEXT OF THE WARD PAPER IN PDF FORMAT IS AVAILABLE FOR DOWNLOAD FROM THIS SITE. https://chaamjamal.files.wordpress.com/2020/09/lowco2.pdf

RISING ATMOSPHERIC CO2

Idso, Sherwood B. “Three phases of plant response to atmospheric CO2 enrichment.” Plant Physiology 87.1 (1988): 5-7. Several years of research on seven different plants (five terrestrial and two aquatic species) suggest that the beneficial effects of atmospheric CO₂ enrichment may be divided into three distinct growth response phases. First is a (1) well-watered optimum-growth-rate phase where a 300 parts per million increase in the CO₂ content of the air generally increases plant productivity by approximately 30%. Next comes a (2) nonlethal water-stress phase where the same increase in atmospheric CO₂ is more than half again as effective in increasing plant productivity. Finally, there is a (3) lethal water-stress phase normally indicative of impending death, where atmospheric CO₂ enrichment may actually prevent plants from succumbing to the rigors of the environment and enable them to maintain essential life processes, as life ebbs from corresponding ambient-treatment plants.

FALLING ATMOSPHERIC CO2

Tansley review, Plant responses to low [CO2] of the past, 5 July 2010, Laci M. Gerhart and Joy K. Ward, Department of Ecology & Evolutionary Biology, University of Kansas, Lawrence, KS 66045, USA

During the Last Glacial Maximum (LGM; 18 000–20 000 yr ago) and previous
glacial periods, atmospheric [CO2] dropped to 180–190 ppm, which is among the lowest concentrations that occurred during the evolution of land plants. Modern atmospheric CO2 concentrations ([CO2]) are more than twice those of the LGM and 45% higher than pre-industrial concentrations. Since CO2 is the carbon source for photosynthesis, lower carbon availability during glacial periods likely had a major impact on plant productivity and evolution. From the studies highlighted here, it is clear that the influence of low [CO2] transcends several scales, ranging from physiological effects on individual plants to changes in ecosystem functioning, and may have even influenced the development of early human cultures (via the timing of agriculture). Through low-[CO2] studies, we have determined a baseline for plant response to minimal [CO2] that occurred during the evolution of land plants. Moreover, an increased understanding of plant responses to low [CO2] contributes to our knowledge of how natural global change factors in the past may continue to influence plant responses to future anthropogenic changes. Future work, however, should focus more on the evolutionary responses of plants to changing [CO2] in order to account for the potentially large effects of genetic change.

VAN DER WATER 1994: CITED BY WARD 2010

Van de Water, Peter K., Steven W. Leavitt, and J. L. Betancourt. “Trends in stomatal density and 13C/12C ratios of Pinus flexilis needles during last glacial-interglacial cycle.” Science 264.5156 (1994): 239-243.

Measurements of stomatal density and δ¹³C of limber pine (Pinus flexilis) needles (leaves) preserved in pack rat middens from the Great Basin reveal shifts in plant physiology and leaf morphology during the last 30,000 years. Sites were selected so as to offset glacial to Holocene climatic differences and thus to isolate the effects of changing atmospheric CO₂ levels. Stomatal density decreased ∼17 percent and δ¹³C decreased ∼1.5 per mil during deglaciation from 15,000 to 12,000 years ago, concomitant with a 30 percent increase in atmospheric CO₂. Water-use efficiency increased ∼15 percent during deglaciation, if temperature and humidity were held constant and the proxy values for CO₂ and δ¹³C of past atmospheres are accurate. The δ¹³C variations may help constrain hypotheses about the redistribution of carbon between the atmosphere and biosphere during the last glacial-interglacial cycle.

Read more at tambonthongchai.com

About the author: Jamal Munshi is a Chemical Engineer for Bechtel San Francisco and Professor Emeritus at Sonoma State University, Rohnert Park, CA

PRINCIPIA SCIENTIFIC INTERNATIONAL, legally registered in the UK as a company incorporated for charitable purposes. Head Office: 27 Old Gloucester Street, London WC1N 3AX. 

Please DONATE TODAY To Help Our Non-Profit Mission To Defend The Scientific Method.

Trackback from your site.