A simulation of the importance of length of growing season and canopy functional properties on the seasonal gross primary production of temperate alpine meadows

Background and Aims: Along snowmelt gradients, the canopies of temperate alpine meadowsdiffer strongly in their structural and biochemical properties.Here, a study is made of the effects of these canopy dissimilaritiescombined with the snow-induced changes in length of growingseason on seasonal gross primary production (GPP). Methods: Leaf area index (LAI) and community-aggregated values of leafangle and leaf nitrogen content were estimated for seven alpineplant canopies distributed along a marked snowmelt gradient,and these were used as input variables in a sun–shadecanopy bulk-photosynthesis model. The model was validated forplant communities of early and late snowmelt sites by measuringthe instantaneous CO₂ fluxes with a canopy closed-chamber technique.A sensitivity analysis was conducted to estimate the relativeimpact of canopy properties and environmental factors on thedaily and seasonal GPP. Key Results: Carbon uptake was primarily related to the LAI and total canopynitrogen content, but not to the leaf angle. For a given levelof photosynthetically active radiation, CO₂ assimilation washigher under overcast conditions. Sensitivity analysis revealedthat increase of the length of the growing season had a highereffect on the seasonal GPP than a similar increase of any otherfactor. It was also found that the observed greater nitrogencontent and larger LAI of canopies in late-snowmelt sites largelycompensated for the negative impact of the reduced growing season. Conclusions: The results emphasize the primary importance of snow-inducedchanges in length of growing season on carbon uptake in alpinetemperate meadows. It was also demonstrated how using leaf-traitvalues of the dominants is a useful approach for modelling ecosystemcarbon-cycle-related processes, particularly when continuousmeasurements of CO₂ fluxes are technically difficult. The studythus represents an important step in addressing the challengeof using a plant functional-trait approach for biogeochemicalmodelling.