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

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 the C-limitation hypothesis, we applied a shade gradient ranging 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 three years (2014-2016). Total aboveground biomass did not significantly decrease under reduced PFD, with a confidence interval reaching from + 4% to -15% biomass in maximum shade. Belowground biomass, consisting to more than 80% of 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 lengths of the most abundant species increased with decreasing PFD. Foliar concentration of non-structural 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. Further, maximum shade lowered foliar δ¹³ C 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.