Adapted from: Lenart, M.; C. Jones; and B. Kimball. 2006. Rising carbon dioxide levels and forest management. University of Arizona College of Agriculture and Life Sciences. Publication No. az1395
Plant tissue (including wood) is composed of about half carbon, all of which comes from carbon dioxide (CO2) in the atmosphere. Photosynthesis rates tend to increase as CO2 levels rise, leading to an increase in dry weight, or biomass, of plants grown under elevated carbon dioxide levels.
Compared to plants grown at existing carbon dioxide levels, plants “fertilized” by elevated levels of atmospheric carbon dioxide increase their photosynthesis rates (Norby et al. 1999, Kimball et al. 2002, Nowak et al. 2004). In addition, carbon dioxide-fertilized plants respond with increased biomass (dry weight), improved water use efficiency, and an increased tolerance of low light levels. Free-Air Carbon Dioxide Enrichment (FACE) research and other studies have focused on plant response to carbon dioxide levels of 550 parts per million or more, but it seems likely that plants in range and wildland environments are “fertilized” by modern levels (which are about one-third higher than pre-industrial levels of 280 parts per million). The FACE experiments involve exposing plants in the field to elevated carbon dioxide levels.
An important consideration in relation to climate change is that carbon dioxide fertilization raises the optimum temperature for photosynthesis (e.g., Norby et al. 1999). It also increases the water use efficiency of plants. However, other factors, such as deficient nitrogen or absence of moisture, can limit plant response even with increased CO2 levels.
Various plant species and genotypes express differences in their degree of responses to CO2 fertilization. Although most experiments have considered the effect of carbon dioxide fertilization on individual species and not interactions among species, woody species, such as trees and shrubs, appear to be more responsive to carbon dioxide fertilization than some grasses and crops. For instance, forest ecosystems tend to have a more pronounced response to CO2 fertilization than grassland ecosystems, based on a comparison of 18 FACE experiments by Nowak and colleagues (2004).
Woody Plants vs. Grasses
Woody plant biomass in southwestern forests and grasslands has increased in the past century, primarily due to fire suppression. Arguably, one should consider that carbon dioxide fertilization in recent decades could be contributing to the biomass increase. Response to CO2 fertilization may influence competition between species, including woody species versus grasses, and native grasses versus invasive species (Smith et al. 2000). Plants using the C4 photosynthetic pathway already boost their internal concentration of CO2, so they tend to be less affected by external increases than C3 plants. Therefore, C3 plants, which include all trees and shrubs as well as some cool-season invasive grasses like bromes (Bromus spp.) and cheatgrass (Bromus tectorum), may be more responsive to CO2 fertilization than C4 plants, which include most warm-season grasses and invasives, like lovegrass (Eragrostis spp.) and buffelgrass (Pennisetum ciliare).
Deserts vs. Forests
Meanwhile, desert ecosystems exhibited a greater response than both forests and grasslands in the FACE experiments after adjustments for differences in precipitation. Carbon dioxide fertilization increases plant water use efficiency, which may help explain why the desert ecosystems responded more dramatically than other ecosystems. In comparing all the ecosystems, Nowak and colleagues (2004) found that net primary productivity rose by an average of 12% in ecosystems exposed to elevated levels of CO2 compared to controls (typically 550 parts per million compared to ambient levels of 380 parts per million or below, depending on the year of the experiment).
There is some evidence that plants grown under conditions of carbon dioxide fertilization may increase the production of chemical compounds (phenols) that make them more resistant to some insects. On the other hand, it has been hypothesized that the increased growth will also promote herbivory, in part because the concentration of nitrogen in leaves tends to drop. However, declines observed among potted seedlings are not always apparent in field-grown trees (Norby et al. 1999).
Researchers continue to test whether the increased photosynthesis is a short-lived response or an ongoing one. Most experiments last only one or two years, so this is difficult to assess. There is no clear evidence that the response is short-lived for trees except when root containers (pots) limit their potential for root expansion (Curtis and Wang 1998). In a 15-year experiment in Maricopa, Arizona, the wood of sour orange(Citrus aurantium) trees exposed to CO2 levels about 300 parts per million higher than ambient levels initially yielded nearly three times as much biomass as control trees, with the rate slowing to less than double after a few years (Figure 2).
Adapted by Thomas DeGomez, University of Arizona
Curtis P.S. and X. Wang. 1998. A meta-analysis of elevated CO2 effects on woody plant mass, form, and physiology. Oecologia. 113: 299-313.
Kimball B.A., K. Kobayashi, and M. Bindi. 2002. Responses of agricultural crops to free-air CO2 enrichment. Advances in Agronomy. 77: 293-368.
Kimball B.A. and S.B. Idso. 2005. Long-term effects of elevated CO2 on sour orange trees, pages 73-80 in K. Omasa, I. Nouchi, and L.J. De Kok (eds.). Plant responses to air pollution and global change. Tokyo: Springer-Verlag.
Norby R.J. S.D. Wullschleger, C.A. Gunderson, D.W. Johnson, and R. Ceulemans. 1999. Tree responses to rising CO2 in field experiments: implications for the future forest. Plant, Cell and Environment. 22: 683-714.
Nowak R.S., D.S. Ellsworth, and S.D. Smith. 2004. Functional responses of plants to elevated atmospheric CO2 – Do photosynthetic and productivity data from FACE experiments support early predictions? New Phytologist. 162: 253-280.
Smith S.D., T.E. Huxman, S.F. Zitzer, D.C. Housman, J.C. Coleman, L.K. Fenstermaker, J.R. Seemann, and R.S. Nowak. 2000. Elevated CO2 increases productivity and invasive species success in an arid ecosystem. Nature. 408: 79-82.
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