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

Abstract

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 CO 2 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 CO 2 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 CO 2 , Larix showed a cumulative 4-year growth response of 141%, 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 [CO 2 ], 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 CO 2 enrichment. Although Pinus showed no growth response to CO 2 , the positive growth response observed in Larix after 4 years of CO 2 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 CO 2 concentrations.

Free atmospheric CO<sub>2</sub> enrichment increased above ground biomass but did not affect symbiotic N<sub>2</sub>-fixation and soil carbon dynamics in a mixed deciduous stand in Wales

“…To date, existing experiments have demonstrated that rising atmospheric [CO 2 ] results in increases in net primary production (NPP) and C storage in forest vegetation, e.g. (Calfapietra et al, 2003;DeLucia et al, 1999;Gielen et al, 2005;Hamilton et al, 2002;Handa et al, 2006;Karnosky et al, 2003;Liberloo et al, 2009;Norby et al, 2002). Norby et al (2005) analyzed the response of NPP (g C m −2 y −1 ) to elevated [CO 2 ] in four forest FACE experiments and calculated the following regression NPP elev = 1.18×NPP amb + 55.4 (r 2 = 0.97).…”

Conifer growth and reproduction in urban forest fragments: Predictors of future responses to global change?

“…Climate envelope models and tree ring analyses imply suitable habitat is largely defined by climate variables (Littell et al 2010;Mckenzie et al 2003;Nitschke and Innes 2008). Additionally, growth and mortality of conifers are known to be sensitive to temperature, soil moisture (likely to decrease with warmer temperatures), and CO 2 (Fritts 1974;Gregg et al 2003;Handa et al 2006;Holman and Peterson 2006), particularly at seedling stages (Yin et al 2007). Recent studies demonstrate that tree mortality is increasing in western North America (van Mantgem et al 2009), raising the possibility of a sudden loss of adult trees.…”

Physiological responses of Abies faxoniana populations from different elevations to increased CO2 and N application

“…Some works conducted at some trees grown in Swiss Alps (2,180 m a.s.l.) have shown that the CO 2 -induced increase in photosynthesis has been maintained, and the initial increase in growth rate has also been sustained in Larix deciduas (Handa et al 2006;Dawes et al 2013;Streit et al 2014). Downward adjustment of photosynthetic capacity by -26 % on average under long-term exposure to elevated CO 2 has been observed in Swiss Alps (2,440 m a.s.l., Inauen et al 2012).…”

“…Our research found greater increases of NSC in HE population than in LE population. Excess C accumulates as NSC in leaves under elevated CO 2 (Handa et al 2006;Onoda et al 2009;Zhao et al 2011), but the level of increases in NSC concentrations depends on species and conditions (Stitt and Krapp 1999). A larger change in NSC detected in HE population than in LE population under elevated CO 2 suggests a large sink capacity in HE population (Handa et al 2006;Wu et al 2011).…”

“…Excess C accumulates as NSC in leaves under elevated CO 2 (Handa et al 2006;Onoda et al 2009;Zhao et al 2011), but the level of increases in NSC concentrations depends on species and conditions (Stitt and Krapp 1999). A larger change in NSC detected in HE population than in LE population under elevated CO 2 suggests a large sink capacity in HE population (Handa et al 2006;Wu et al 2011). Seedlings from HE have a large NSC, which may be advantageous for buffering extreme events, such as destructive hail or insect outbreaks (Körner 2003;Wu et al 2011).…”