Abstract
In high mountains, the effects of climate change are manifesting most rapidly. This is especially critical for the high-altitude carbon cycle, for which new feedbacks could be triggered. However, mountain carbon dynamics is only partially known. In particular, models of the processes driving carbon fluxes in high-altitude grasslands and Alpine tundra need to be improved. Here, we propose a comparison of three empirical approaches using systematic statistical analysis, to identify the environmental variables controlling CO2 fluxes. The methods were applied to a complete dataset of simultaneous in situ measurements of the net CO2 exchange, ecosystem respiration and basic environmental variables in three sampling sites in the same catchment. Large year-to-year variations in the Gross Primary Production (GPP) and Ecosystem Respiration (ER) dependences on solar irradiance and temperature were observed. We thus implemented a multi regression model in which additional variables were introduced as perturbations of the standard exponential and rectangular hyperbolic functions for ER and GPP, respectively. A comparison of this model with other common modelling strategies showed the benefits of this approach, resulting in large explained variances (83% to 94%). The optimum ensemble of variables explaining the inter- and intra-annual flux variability included solar irradiance, soil moisture and day of the year for GPP, and air temperature, soil moisture, air pressure and day of the year for ER, in agreement with other studies. The modelling approach discussed here provides a basis for selecting drivers of carbon fluxes and understanding their role in high-altitude Alpine ecosystems, also allowing for future short-range assessments of local trends.
Highlights
The carbon cycle plays a fundamental role in Earth’s climate, as it controls the amount of greenhouse gases in the atmosphere and the carbon stocks in the soils, biosphere, oceans and atmosphere
The carbon cycle spans a wide range of space and time scales, form the geological and slow dynamics associated with plate tectonics and surface rock weathering (e.g., Broecker, 2018; Orcutt et al, 2019) to the fast an much more intense fluxes generated by the action of living organisms
As discussed by Janssens et al (2001) and Law et al (2002) for a variety of ecosystems, stronger ecosystem respiration (ER) usually matches larger gross primary production (GPP); this was the case for the sites studied here. This feature may be induced by the enhanced canopy activity and the higher supply of fresh carbon to soil linked to vegetation physiology, for the sustenance of microbial communities (e.g. Balogh et al, 2011; Gavrichkova et al, 2018)
Summary
The carbon cycle plays a fundamental role in Earth’s climate, as it controls the amount of greenhouse gases in the atmosphere and the carbon stocks in the soils, biosphere, oceans and atmosphere. The heterogeneous terrestrial environment at the Earth’s surface, where “rock meets life”, is tightly coupled with the water, carbon and energy cycles, providing essential ecosystem services such as water regulation and carbon sequestration. This part of the Earth System has recently been called the “Critical Zone” (CZ, NRC, 2001; Giardino and Houser, 2015). A dense network of CZ Observatories (CZO) was globally established (www.czen.org) Such observatories are often located in climate-change hotspots and along strategic gradients of altitude, latitude, climate, biome and habitat. In many of these sites, researchers pursue the integrated exploration of the complex processes linking geosphere, biosphere and climate (Rietkerk et al, 2011)
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