Submitted to: Tree Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/28/2001
Publication Date: 3/1/2002
Citation: Polley, H.W., Tischler, C.R., Johnson, H.B., Derner, J.D. Growth rate and survivorship of drought: CO2 effects on the presumed tradeoff in seedlings of five woody legumes. Tree Physiology. 2002. v. 22. p. 383-391. Interpretive Summary: Grasslands worldwide have been invaded by woody plants during the last two centuries. Causes of woody invasion include overgrazing, suppression of grassland fires, and the recent increase in concentration of carbon dioxide gas in Earth's atmosphere because these changes favor woody plants over grasses. Carbon dioxide concentration is expected to continue to rise as a result of human activities, possibly to levels that are twice those today. If woody plants are further favored by this change, remaining grasslands may be threatened and management of grasslands will be complicated. Drought acts as a barrier to woody invasion of grasslands by killing woody seedlings. We measured effects of increasing carbon dioxide to levels predicted during the next century on survival of drought by seedlings of five invasive shrubs. Doubling carbon dioxide improved drought survival of all shrubs during at least some period as soil water declined, but higher carbon dioxide most consistently increased survival of shrubs that already are most tolerant of drought. On average, doubling today's carbon dioxide level prolonged survival of droughted seedlings by 7-11 days. Woody seedling displayed slightly lower water stress during drought at the high than current carbon dioxide level, no other trait consistently explained the benefit of high carbon dioxide to droughted seedlings. These results indicate that the continuing rise in carbon dioxide concentration will contribute to still greater woody invasion on dry grasslands. Woody plants that already are most tolerant of drought will benefit the most from the future increase in carbon dioxide concentration.
Technical Abstract: Plant traits like rapid growth rate that promote seedling recruitment when water is plentiful may become a liability when seedlings encounter drought. We tested the hypothesis that CO2 enrichment reinforces a tradeoff between growth rate and drought tolerance by exaggerating interspecific differences in maximum relative growth rate (RGR) and survivorship of drought among seedlings of five woody legumes. Species studied are invasive of grasslands and differ in distribution along a rainfall gradient. During each of two experiments, maximum response of RGR to CO2 enrichment occurred in one of the faster-growing species of those studied. Doubling CO2 concentration improved seedling survivorship of the most drought-tolerant species throughout the period of soil water depletion, but did not consistently affect survivorship of more drought-sensitive species. Nevertheless, CO2 enrichment increased percentage survival of all species during at least some period of drought. Across species, doubling CO2 concentration increased the maximum period of drought survival by 7-11 days. Mid-day xylem pressure potentials of droughted seedlings were less negative at elevated than ambient CO2 concentration, but no other measured trait was consistently correlated with improved survivorship at high CO2. To the extent that seedling persistence in grasslands correlates with drought survivorship, our results indicate a positive effect of CO2 enrichment on recruitment of five woody legumes, with greatest benefit to species that currently are most tolerant of drought.