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Effects of Elevated CO2 and Warming on Photosynthesis in a Major Desert Biocrust Moss
Coe, K.K., Belnap, J., Grote, E.E. and Sparks, J.P. 2012. Physiological ecology of desert biocrust moss following 10 years exposure to elevated CO2: evidence for enhanced photosynthetic thermotolerance. Physiologia Plantarum 144: 346-356.

The authors write that non-vascular plants, such as mosses and their relatives, "are an important component of global net primary productivity (Campioli et al., 2009; Lindo and Gonzalez, 2010), biogeochemistry (Turetsky, 2003) and biodiversity (Molau and Alatalo, 1998; Shaw et al., 2005)," and they note in this regard that in earth's arid regions, which host biomes that are especially responsive to climate change, mosses in particular play an important role as key components of biological soil crusts or biocrusts. See also, in this regard, Deserts (Biological Soil Crusts), in our Subject Index.

What was done
Coe et al. examined the effects of long-term (10 years) elevated CO2 exposure (550 ppm vs. 360 ppm) on various photosynthetic properties and processes in the common desert moss Syntrichia caninervis, which they collected from the Nevada Free Air CO2 Enrichment (FACE) facility in the Mojave Desert (USA). And being especially concerned about the increase in temperature that is often predicted to result from continued anthropogenic CO2 emissions, the four scientists measured photosynthetic rates of the two CO2-treatment mosses across a set of atmospheric CO2 concentrations that ranged from 50 to 1000 ppm at air temperatures of 20 and 30°C - which they describe as being "common during the growing season" - as well as 40°C, which is considerably warmer.

What was learned
Across the entire 50-1000 ppm range of atmospheric CO2 concentrations, there were virtually no differences in the gross CO2 exchange rates of the 10-year ambient-CO2 and CO2-enriched plants when measured at either 20 or 30°C, as shown in the top two segments of the figure below. When measured at 40°C, however, the two CO2-treatment plants exhibited a huge difference, as also may be seen in the right-hand segment of the figure below.

Figure 1. Photosynthetic rates of the 10-year ambient-CO2 plants (360 ppm, filled circles) and elevated-CO2 plants (550 ppm, open circles) vs. short-term atmospheric CO2 concentration. Adapted from Coe et al. (2012).

In viewing this figure, it can be seen that although the extra 10-20°C of the 40°C air temperature treatment significantly depressed the photosynthetic rates of both the long-term ambient-CO2 and CO2-enriched-plants, the latter plants fared much better than the ambient-treatment plants under this high-temperature regime, photosynthesizing at essentially twice the rate of the ambient-treatment plants at earth's current atmospheric CO2 concentration (392 ppm for AD 2011), and better still at higher concentrations.

What it means
Quoting Coe et al., "the potential for sustained photosynthesis under high temperature for S. caninervis grown under elevated CO2 suggests the opportunity for higher rates of survival than would have been predicted under future conditions of simultaneous warming and CO2 enrichment." And they say that this "CO2-induced photosynthetic thermotolerance" may "enable individuals to perform better than expected under these conditions," which potentiality is extremely significant, in that they note that "S. caninervis is the dominant moss in low elevation Mojave Desert biocrusts (Stark et al., 1998; Bowker et al., 2000)," and that "the distribution of this species extends throughout western North America (Flowers, 1973) and in dry regions of Africa, Asia, the Middle East and Europe (Kramer, 1980)."

Bowker, M.A., Stark, L.R., McLetchie, D.N. and Mishler, B.D. 2000. Sex expression, skewed sex ratios, and microhabitat distribution in the dioecious desert moss Syntrichia caninervis (Pottiaceae). American Journal of Botany 87: 5217-526.

Campioli, M., Samson, R., Michelsen, A., Jonasson, S., Baxter, R. and Lemeur, R. 2009. Nonvascular contribution to ecosystem NPP in a subarctic heath during early and late growing season. Plant Ecology 202: 41-53.

Flowers, S. 1973. Mosses of Utah new to science. Bryologist 76: 286-292.

Kramer, W. 1980. Tortula Hedw. sect. Rurales De Not. (Pottiaceae, Musci) in der ostlichen Holarktis. Bryophyt. Bibl. 21: 1-165.

Lindo, Z. and Gonzalez, A. 2010. The bryosphere: an integral and influential component of the Earth's biosphere. Ecosystems 13: 612-627.

Molau, U. and Alatalo, J.M. 1998. Responses of subarctic-alpine plant communities to simulated environmental change: biodiversity of bryophytes, lichens, and vascular plants. Ambio 27: 322-329.

Shaw, A.J., Cox, C.J. and Goffinet, B. 2005. Global patterns of moss diversity: taxonomic and molecular inferences. Taxon 54: 337-352.

Stark, L.R., Mishler, B.D. and McLetchie, D.N. 1998. Sex expression and growth rates in natural populations of the desert soil crustal moss Syntrichia caninervis. Journal of Arid Environments 40: 401-416.

Turetsky, M.R. 2003. The role of bryophytes in carbon and nitrogen cycling. Bryologist 106: 395-409.

Reviewed 15 August 2012