Plants grown at elevated atmospheric CO2 concentrations exhibit many changes in their physical and chemical characteristics, as well as alterations in various physiological functions. Leaf senescence is one of the phenomena that may be included in this category of affected processes. Sometimes elevated CO2 shortens the time required to begin senescence (Miglietta et al., 1998), while at other times it may delay senescence (Li et al., 2000) or have no effect upon it at all (Lake and Hughes, 1999).
In the study of Miglietta et al. (1998), for example, a doubling of the CO2 concentration of the air in which potato plants were grown accelerated their life cycle and caused senescence to begin approximately one-week earlier than it did in potatoes exposed to ambient air. In contrast, Li et al. (2000) reported that the onset of senescence was delayed in leaves of the perennial scrub oak Quercus myrtifolia in CO2-enriched air having a concentration of 700 ppm. However, several studies indicate that rising atmospheric CO2 concentrations will have little or no effect on plant senescence. In one such study (Lake and Hughes, 1999), a CO2 concentration of 760 ppm had no effect on the timing of senescence in nasturtiums; while in another (Norby et al., 2000), no CO2-induced timing effects were observed in sugar and red maple seedlings. Thus, there is no clear indication as to how senescence in general will be affected by rising CO2 levels, for it appears to be a species-specific response.
In addition to potential differences in the timing of senescence, it is important to know whether atmospheric CO2 enrichment elicits any compositional changes in leaf litter. Hirschel et al. (1997), for example, reported that CO2-enriched litter derived from species representative of alpine grassland, calcareous grassland and wet tropical rainforest was not qualitatively different from litter produced by similar species maintained at ambient CO2 concentrations. Similarly, Schappi and Korner (1997) showed that a 330-ppm increase in the air's CO2 concentration did not cause any significant changes in the chemical composition of leaf litter in an alpine ecosystem, even though elevated CO2 did elicit such changes in intact green leaves. In contrast to these two studies, however, Norby et al. (2000) report that CO2-induced reductions in foliar nitrogen concentrations of intact green leaves of maple seedlings grown at 660 ppm CO2 are also reflected in the foliar nitrogen concentrations of their leaf litter. Thus, it is unclear as to whether there is any general statement that can be made about the effects of the rising atmospheric CO2 concentration on leaf litter quality.
In view of the wide ranges of results that have been obtained in these two research areas, we eagerly await the findings of further studies that may help us better understand what ramifications, if any, the ongoing rise in the air's CO2 content may have on the timing of plant senescence and the compositional quality of leaf litter.
References
Hirschel, G., Korner, C. and Arnone III, J.A. 1997. Will rising atmospheric CO2 affect leaf litter quality and in situ decomposition rates in native plant communities? Oecologia 110: 387-392.
Lake, J.C. and Hughes, L. 1999. Nectar production and floral characteristics of Tropaeolum majus L. grown in ambient and elevated carbon dioxide. Annals of Botany 84: 535-541.
Li, J.-H., Dijkstra, P., Hymus, G.J., Wheeler, R.M., Piastuchi, W.C., Hinkle, C.R. and Drake, B.G. 2000. Leaf senescence of Quercus myrtifolia as affected by long-term CO2 enrichment in its native environment. Global Change Biology 6: 727-733.
Miglietta, F., Magliulo, V., Bindi, M., Cerio, L., Vaccari, F.P., Loduca, V. and Peressotti, A. 1998. Free Air CO2 Enrichment of potato (Solanum tuberosum L.): development, growth and yield. Global Change Biology 4: 163-172.
Norby, R.J., Long, T.M., Hartz-Rubin, J.S. and O'Neill, E.G. 2000. Nitrogen resorption in senescing tree leaves in a warmer, CO2-enriched atmosphere. Plant and Soil 224: 15-29.
Schappi, B. and Korner, C. 1997. In situ effects of elevated CO2 on the carbon and nitrogen status of alpine plants. Functional Ecology 11: 290-299.