Continuous and discontinuous variation in ecosystem carbon stocks with elevation across a treeline ecotone

Abstract

Abstract. Treelines differentiate vastly contrasting ecosystems: open tundra from closed forest. Treeline advance has implications for the climate system due to the impact of the transition from tundra to forest ecosystem on carbon (C) storage and albedo. Treeline advance has been seen to increase above-ground C stocks as low vegetation is replaced with trees but decrease organic soil C stocks as old carbon is decomposed. However, studies comparing across the treeline typically do not account for elevational variation within the ecotone. Here we sample ecosystem C stocks along an elevational gradient (970 to 1300 m), incorporating a large-scale and long-term livestock grazing experiment, in the southern Norwegian mountains. We investigate whether there are continuous or discontinuous changes in C storage across the treeline ecotone, and whether these are modulated by grazing. We find that vegetation C stock decreases with elevation, with a clear breakpoint between the forest line and treeline above which the vegetation C stock is constant. C stocks in organic surface horizons of the soil were higher above the treeline than in the forest, whereas C stocks in mineral soil horizons are unrelated to elevation. Total ecosystem C stocks also showed a discontinuous elevational pattern, increasing with elevation above the treeline (8 g m −2 per metre increase in elevation), but decreasing with elevation below the forest line (−15 g m −2 per metre increase in elevation), such that ecosystem C storage reaches a minimum between the forest line and treeline. We did not find any effect of short-term (12 years) grazing on the elevational patterns. Our findings demonstrate that patterns of C storage across the treeline are complex, and should be taken account of when estimating ecosystem C storage with shifting treelines.

How do soil properties affect alpine treelines? General principles in a global perspective and novel findings from Rolwaling Himal, Nepal

“…K and Mg also decrease in soil Ah horizons, however not significantly (Table 3). In foliage, similar results were found for tree species Abies spectabilis and Sorbus microphylla (not illustrated) which both occur in the closed forest and sparsely as krummholz above treeline, however here not in tree form as defined by Körner Speed et al, 2015;Zimmermann et al, 2010), whereas soil N concentrations (Of, Ah) significantly decrease ( Figure 6). Thus, significantly increasing C/N ratios ( Figure 6, Table 3; cf.…”

“…Soil C concentrations (Of, Ah) slightly decrease with elevation, however not significantly (see also Speed et al, 2015; Zimmermann et al, 2010), whereas soil N concentrations (Of, Ah) significantly decrease (Figure 6). Thus, significantly increasing C/N ratios (Figure 6, Table 3; cf.…”

Herbaceous Encroachment from Mountain Birch Forests to Alpine Tundra Plant Communities Through Above- and Belowground Competition

“…Tundra vegetation dominated by native shrubs sequesters a significant amount of atmospheric carbon [64]. Previous studies have shown that climate warming causes changes of carbon stocks in the vegetation of alpine tundra [11,[65][66][67][68]. A previous study of this system found that there were no significant changes in vegetative structure [29], indicating that climate warming would have limited impacts on the carbon stocks in the alpine tundra of Changbai Mountains.…”

Climate change evidence in tree growth and stand productivity at the upper treeline ecotone in the Polar Ural Mountains

“…Global warming is most pronounced at high latitudes where temperatures over the last 30 years have risen by 0.6°C per decade, which is twice as fast as the global average (Bekryaev et al 2010;Core Writing Team RKP and LAM 2014). Since it is largely considered that high altitude and latitude treelines are controlled by summer temperatures (Holtmeier and Broll 2007;Gehrig-Fasel et al 2008;MacDonald et al 2008) and growing season parameters (Körner and Paulsen 2004;Körner 2008), the ongoing warming trend is expected to cause important vegetation shifts and lead to significant changes in species composition and biomass production (Walther et al 2002;Epstein et al 2004;Gonzalez et al 2010;Paulsen and Körner 2014;Speed et al 2015). Despite the fact that treeline advances in Polar regions are mostly limited by temperature, other factors (e.g.…”