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 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.
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