Conifer stem growth at the altitudinal treeline in response to four years of CO2 enrichment

Northern latitude and upper altitude climatic treelines have received increasing attention given their potential sensitivity to atmospheric and climate change. While greater radial stem growth at treeline sites in recent decades has been attributed largely to increasing temperature, rising atmospheric CO2 concentration may also be contributing to this growth stimulation. Tree ring increments of mature Larix decidua and Pinus uncinata were measured over 4 years in a free air CO2 enrichment experiment at treeline in the Swiss Central Alps (2180 m a.s.l.). In addition, a one-time defoliation treatment in the second year (2002) of the experiment was used to simulate one of the common natural insect outbreak events. In response to elevated atmospheric CO2, Larix showed a cumulative 4-year growth response of +41%, with particularly strong responses in the third and fourth year. This increase in radial stem wood growth was the result of more latewood production, in particular, the formation of larger tracheids, rather than a greater number of cells. In contrast, Pinus showed no change in ring width to elevated [CO2], neither in each of the treatment years, nor in the cumulative response over 4 years, although an increase in tracheid size was observed in the third year. Defoliation led to a pronounced decrease in annual ring width of both species, marked in particular by less latewood production, in the treatment, as well as subsequent years. There was no significant interaction between defoliation and CO2 enrichment. Although Pinus showed no growth response to CO2, the positive growth response observed in Larix after 4 years of CO2 enrichment implies that the sensitivity of treeline trees to global change may not be purely temperature driven. We conclude that the open sparse canopy in the treeline ecotone favours the indeterminate growth strategy of the early successional Larix when neither weather nor carbon are limiting, whereas the later successional Pinus does not show any indication of more vigorous growth under future higher atmospheric CO2 concentrations.