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A Treeline Ecosystem in a CO2-Enriched World

Dawes, M.A., Hagedorn, F., Handa, I.T., Streit, K., Ekblad, A., Rixen, C., Korner, C. and Hattenschwiler, S. 2013. An alpine treeline in a carbon dioxide-rich world: synthesis of a nine-year free-air carbon dioxide enrichment study. Oecologia 171: 623-637.
In an important study recently published in Oecologia, Dawes et al. (2013) write that "in situations where increased productivity occurs initially in response to elevated CO2, a negative biogeochemical feedback can occur over the longer term if greater N [nitrogen] sequestration into plant and soil organic matter is not met with sufficient replenishment via mineralization [progressive N limitation (PNL); Luo et al., 2004; Norby et al., 2010]." On the other hand, they note that "plant physiological adjustments such as increased plant N-use efficiency (Crous et al., 2008), accelerated N mineralization (Finzi et al., 2007; Phillips et al., 2011; Schleppi et al., 2012), and enhanced below-ground C [carbon] allocation to increase plant nutrient acquisition through stimulated root and mycorrhizal activity (Treseder, 2004), have been reported to prevent or at least delay the onset of PNL in some ecosystems with low N availability."

In a study designed to further explore this complex subject, the eight researchers - hailing from Canada, France, Sweden and Switzerland - conducted a long-term (2001-2009) free-air CO2 enrichment (FACE) study in the Swiss Central Alps that allowed them to evaluate the impacts of elevated CO2 (575 ppm vs. 380 ppm) on an undisturbed, multi-species treeline ecosystem in order to (1) compare the growth responses of the two dominant tree species (Larix decidua and Pinus uncinata), (2) assess changes in the growth and abundance of different understory plant species, and (3) determine how elevated CO2 influenced biochemical processes such as soil C and N cycling.

Focusing on the progressive nitrogen limitation hypothesis, Dawes et al. report that throughout the nine-year experimental period, they observed (1) no changes in soil C:N, inorganic N pool size, or dissolved organic C and N (DOC and DON) in the soil solution, and that (2) there was no long-term reduction of leaf N concentration in Vaccinium (an understory dwarf shrub) or Larix, and that a slight N reduction observed in Pinus needles was a dilution effect related to starch accumulation.

In a brief concluding statement related to the PNL hypothesis, the international research team states that "CO2 enrichment hardly affected the C-N balance in plants and soil, with unaltered soil total or mineral N concentrations and little impact on plant leaf N concentration," noting that "consistently increased C fixation, soil respiration and DOC production over nine years of CO2 enrichment provide clear evidence for accelerated C cycling with no apparent consequences on the N cycle in this treeline ecosystem."

So chalk up another win for the biosphere in its response to the ongoing rise in the air's CO2 content.

Additional References
Crous, K.Y., Walters, M.B. and Ellsworth, D.S. 2008. Elevated CO2 concentration affects leaf photosynthesis and nitrogen relationships in Pinus taeda over nine years in FACE. Tree Physiology 28: 607-614.

Finzi, A.C., Norby, R.J., Calfapietra, C., Gallet-Budynek, A., Gielen, B., Holmes, W.E., Hoosbeek, M.R., Iversen, C.M., Jackson, R.B., Kubiske, M.E., Ledford, J., Liberloo, M., Oren, R., Polle, A., Pritchard, S., Zak, D.R., Schlesinger, W.H. and Ceulemans, R. 2007. Increases in nitrogen uptake rather than nitrogen-use efficiency support higher rates of temperate forest productivity under elevated CO2. Proceedings of the National Academy of Sciences, USA 104: 14,014-14,019.

Luo, Y., Su, B., Currie, W.S., Dukes, J.S., Finzi, A., Hartwig, U., Hungate, B., McMurtrie, R.E., Oren, R., Parton, W.J., Pataki, D.E., Shaw, M.R., Zak, D.R. and Field, C.B. 2004. Progressive nitrogen limitation of ecosystem responses to rising atmospheric carbon dioxide. BioScience 54: 731-739.

Norby, R.J., Warren, J.M., Iverson, C.M., Medlyn, B.E. and McMurtrie, R.E. 2010. CO2 enhancement of forest productivity constrained by limited nitrogen availability. Proceedings of the National Academy of Sciences USA 107: 19,368-19,373.

Phillips, R.P., Finzi, A.C. and Bernhardt, E.S. 2011. Enhanced root exudation induces microbial feedbacks to N cycling in a pine forest under long-term CO2 fumigation. Ecology Letters 14: 187-194.

Schleppi, P., Bucher-Wallin, I., Hagedorn, F. and Korner, C. 2012. Increased nitrate availability in the soil of a mixed mature temperate forest subjected to elevated CO2 concentration (canopy FACE). Global Change Biology 18: 757-768.

Treseder, K.K. 2004. A meta-analysis of mycorrhizal responses to nitrogen, phosphorus, and atmospheric CO2 in field studies. New Phytologist 164: 347-355.

Archived 26 June 2013