Halving sunlight reveals no carbon limitation of aboveground biomass production in alpine grassland

Abstract

In temperate alpine environments, the short growing season, low temperature and a slow nutrient cycle may restrict plant growth more than carbon (C) assimilation does. To test whether C is a limiting resource, we applied a shade gradient from ambient light to 44% (maximum shade) of incident photon flux density (PFD) in late successional, Carex curvula‐dominated alpine grassland at 2,580 m elevation in the Swiss central Alps for 3 years (2014–2016). Total aboveground biomass did not significantly decrease under reduced PFD, with a confidence interval ranging from +4% to −15% biomass in maximum shade. Belowground biomass, of which more than 80% were fine roots, was significantly reduced by a mean of 17.9 ± 4.6% (±SE), corresponding to 228 g/m2, in maximum shade in 2015 and 2016. This suggests reduced investments into water and nutrient acquisition according to the functional equilibrium concept. Specific leaf area (SLA) and maximum leaf length of the most abundant species increased with decreasing PFD. Foliar concentration of nonstructural carbohydrates (NSC) was reduced by 12.5 ± 4.3% under maximum shade (mean of eight tested species), while NSC concentration of belowground storage organs were unchanged in the four most abundant forbs. Furthermore, maximum shade lowered foliar δ13C by 1.56 ± 0.35‰ and increased foliar nitrogen concentrations per unit dry mass by 18.8 ± 4.1% across six species in 2015. However, based on unit leaf area, N concentrations were lower in shade (effect of higher SLA). Thus, while we found typical morphological and physiological plant responses to lower light, shading did not considerably affect seasonal aboveground biomass production of this alpine plant community within a broad range of PFD. This suggests that C is not a growth‐limiting resource, matching the unresponsiveness to in situ CO2 enrichment previously reported for this type of grassland.

Growth of alpine grassland will start and stop earlier under climate warming

“…Beside its signalling effect, a short photoperiod also encompasses lower levels of photon fluxes, possibly limiting carbon uptake. However, perennial alpine plants have large belowground reserves 33 and are not carbon-limited 34 , even under shade 35 .…”

“…The sedge Carex curvula (Fig. 1B) is the dominant species, contributing around one third to total annual biomass production 35,59 . Grasses like Helictotrichon versicolor Vill.…”

Biomass allocation and seasonal non-structural carbohydrate dynamics do not explain the success of tall forbs in short alpine grassland

“…Our seasonal harvests under common alpine life conditions revealed little influence of aboveground growth -irrespective of the plant stature- on NSC concentrations in the belowground organs, so, NSC storage has an overwhelming priority for these alpine plants. Moreover, CO 2 enrichment and shading experiments in situ revealed that carbon is not a limiting resource for alpine plants (Körner et al 1997 ; Inauen et al 2012 ; Möhl et al 2020 ) and tall alpine forbs are no exception in this respect.…”

How do different functional crop groups perform in temperate silvoarable agroforestry systems? A Swiss case study

“…Young trees with a smaller canopy also have a smaller rooting zone, less litter fall and nutrient cycling, less contribution to water redistribution and so forth. What is more, the artificial shade constructions are expected to have altered aboveground microclimatic conditions such as changes in precipitation distribution, canopy temperature, wind speed and evaporative force (Friday & Fownes, 2002; Möhl et al, 2020; Siemannand Rogers, 2003). Nonetheless, field experiments that approximate natural conditions as closely as possible while meeting the scientific requirements of controlled, replicable experimental conditions are of high informative value (Sollen‐Norrlin et al, 2020).…”