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Global Climate Change Digest
A Guide to Information on Greenhouse Gases and Ozone Depletion
Published July 1988 through June 1999

FROM VOLUME 9, NUMBER 12, DECEMBER 1996

PROFESSIONAL PUBLICATIONS...
IMPACTS OF ELEVATED CO₂

Item #d96dec26

"System-Level Adjustments to Elevated CO₂ in Model Spruce Ecosystems," S. Hättenschwiler (Botanisches Inst., Univ. Basel, Schönbeinstr. 6, CH-4056 Basel, Switz.), C. Körner, Global Change Biology, 2(4), 377-387, Aug. 1996.

Predictions that atmospheric CO₂ enrichment and increasing N deposition will increase forest productivity are often based on data for isolated forest tree seedlings or their leaves. This study looked at the effects of these gases on whole stands of 4-yr-old spruce trees (Picea abies). One tree from each of six clones, together with two herbaceous understory species, were established in each of nine 0.7 m2 model ecosystems in nutrient-poor soil and grown in a simulated montane climate, with increasing CO₂ and N, for two years. The observed lack of above-ground growth response to elevated CO₂ is due to several effects, including increasing strength of below-ground carbon sinks. The non-linearity of treatment suggests that major responses of coniferous forests may already be under way and future responses may be smaller.

Item #d96dec27

"Forest Canopy Productivity Index," R.J. Norby (Environ. Sci. Div., Oak Ridge Natl. Lab., Oak Ridge TN 37831), Nature, 381(6583), 564, June 13, 1996.

Results of the few field experiments that have investigated tree responses to increased CO₂ are difficult to generalize and extrapolate, possibly because plant dry mass is used as the primary indicator of response. Proposed here is a more robust and conceptually more useful measure: the annual production of wood per unit of leaf area—the canopy productivity index (CPI), formerly called growth efficiency. It provides a useful construct for organizing existing data and future experiments.

Item #d96dec28

"Elevated CO₂ Decreases Seed Germination in Arabidopsis thaliana," C. Andalo,..I. Till-Bottraud (Biol. des Populations d'Altitude, URA 1946 Univ. Joseph Fourier, Bât 76, BP 53, F-38041 Grenoble Cedex 9, France; e-mail irene.till@ujf-grenoble.fr), Global Change Biology, 2(2), 129-135, Apr. 1996.

Studied two generations of seed from different genotypes of an annual herb. The maternal generation was produced in the greenhouse (present-day conditions); the offspring generation was produced in chambers where the CO₂ was either 350 ppm or 700 ppm. Elevated CO₂ during maturation of seeds on the mother plants decreased germination, but elevated CO₂ during germination had no effect on the proportion of germinated seeds. However, germination for seeds produced and germinated under elevated CO₂ was both low and slow. Since responses varied strongly among genotypes, there is ample genetic variance for selection.

Item #d96dec29

"Leaf Senescence and Decline of End-of-Season Gas Exchange in Five Temperate Deciduous Tree Species Grown in Elevated CO₂ Concentrations," K.D.M. McConnaughay (Dept. Biol., Bradley Univ., Peoria IL 61625; e-mail: kdm@bradley.bradley.edu), S.L. Bassow et al., ibid., 2(1), 25-33, Feb. 1996.

Seedlings from these species were studied: grey birch (Betula populifolia), ash (Fraxinus americana), red maple (Acer rubrum), yellow birch (Betula alleghaniensis), and striped maple (Acer pennsylvanicum). For current and elevated atmospheric CO₂ levels, parameters measured included leaf-level photosynthetic rates, canopy area per unit time, leaf level transpiration rates, and water use efficiency. Results suggest that whole canopy end-of-season gas exchange may be altered significantly under elevated CO₂, resulting in reduced carbon gain and water use efficiency. Seedling growth and survival, and ultimately temperate forest regeneration, could be reduced under higher levels of CO₂ in the future.

Item #d96dec30

"Effects of Elevated CO₂, Nitrogen Supply and Tropospheric Ozone on Spring Wheat. I. Growth and Yield," A. Fangmeier (Inst. Pflanzenökol., Heinrich-Buff-Ring 38, D-35392 Giessen, Ger.), U. Grüters et al., Environ. Pollut., 91(3), 381-390, 1996.

The wheat cultivar Minaret was exposed to three CO₂ levels, in combination with two N fertilizer levels and two levels of tropospheric O3. Plants were harvested at various growth stages to gain information about biomass partitioning. CO₂ enrichments drastically increased the biomass of organs serving as long-term carbohydrate pools; average yield increased 34%. Elevated N application was most effective on the biomass of green tissues, and yield was increased from 150 to 270 N ha-1. Significant interactions were observed between CO₂ enrichment and N application. The cultivar was insensitive to O3.

Item #d96dec31

"Growth Responses of an Alpine Grassland to Elevated CO₂," B. Schäppi, C. Körner (Botanisches Inst., Univ. Basel, Schönbeinstr. 6, CH-4056 Basel, Switz.), Oecologia, 105(1), 43-52, 1996.

Plots of narrow alpine grassland were exposed for three seasons to ambient and elevated CO₂; some plots also received moderate additions of N in a complete fertilizer mix. Results do not support the hypothesis that alpine plants, due to their higher carbon uptake efficiency, will increase biomass production under future atmospheric CO₂ enrichment, at least not in such late successional communities. However, because the subdominant generalistic species, Poa alpina, strongly increased shoot growth, altered community and perhaps ecosystem structure may occur in the longer term. Even under moderate climate warming or enhanced atmospheric nitrogen deposition, positive biomass responses to CO₂ enrichment of the currently dominating species are unlikely.

Item #d96dec32

"Comparison of Radiative and Physiological Effects of Doubled Atmospheric CO₂ on Climate," P.J. Sellers, (Biospheric Sci., MC 923, NASA-Goddard, Greenbelt MD 20771), L. Bounoua et al., Science, 271(5254), 1402-1406, Mar. 8, 1996.

A coupled biosphere-atmosphere model indicates that under doubled CO₂, evapotranspiration will drop and air temperature will increase over the tropical continents, amplifying the changes resulting from atmospheric radiative effects.

Item #d96dec33

Special Issue: Wheat Growth Under Global Environmental Change, J.I.L. Morrison, S.P. Long, Eds., Global Change Biology, 1(6), Dec. 1995. Request single issues from Subscription Dept., Marston Book Services, POB 87, Oxford OX2 0DT, UK (tel: 44 1865 791 155; fax: 44 1865 791 927).

Globally, as a food crop, wheat provides 19% of dietary energy and 20% of protein. The world wheat crop will respond to global change, and it has the potential to modify global change because it occupies such a large area of the globe. The latter could occur through changes in the fluxes of C, N, and water, in genotypes and agronomic practices, or through crop substitution. This special issue brings together contributions from six of the leading international laboratories that have investigated the topic, first presented as invited lectures to a conference organized by the Society for Experimental Biology (St. Andrew's, Scotland, April 1995). Some of the studies include the impact of altered temperature as well as elevated CO₂.