Climate change effects on carbon and water fluxes in mountain ecosystems

Abstract

The spatial trade-off between elevation and latitude compresses many life zones into a few kilometres of elevation, and creates steep environmental gradients. This feature was used in this project to develop and test some ideas on ecosystem function, combining observations and robust models based on first principles. To reproduce some of the observed patterns of water and energy fluxes, the Simple Process-Led Algorithms for Simulating Habitats (SPLASH) model was extended, making use of first-principles concepts in order to reduce the need for free parameters to a minimum. Whenever empirical formulations were used (e.g. pedotransfer functions, albedo-snow cover functions), they were optimized/fitted using a combination of remote sensing data and globally distributed observational datasets. Simulations of soil water content, evapotranspiration and snow-water equivalent were compared against in-situ measurements using multiple data sources. The statistical performance of the extended model was tested with pooled measurements from multiple stations. The results were also compared to simulations from the Variable Infiltration Capacity (VIC-3L) model. To explore how different photosynthetic responses – well documented at leaf level – converge towards emergent spatial patterns at ecosystem scale, I considered how elevation shapes light use efficiency (LUE, the ratio of CO2 assimilated over light absorbed) and water use efficiency (WUE, the ratio of CO2 assimilated over water used in transpiration) in mountain regions worldwide. I used data from eddycovariance flux towers, from different networks, located in mountain regions around the world, adding up to 618 station-years of record. To complement my analysis, I included theoretical predictions using an optimality model (P-model) and evaluated changes in the spatial pattern with simulation experiments. Empirically I found a small but globally consistent effect of elevation on LUE andWUE. I propose a theoretical explanation for the observation that temperature differences have little impact on the biogeographical pattern of LUE and WUE, but I also find that different assumptions on the acclimation of the maximum rate of electron transport (Jmax ) and the intrinsic quantum yield of photosynthesis ('0) change this pattern. Finally to evaluate how environmental changes, particularly temperature enhancement at high elevations and CO2 increase, will alter carbon and water fluxes of mountain ecosystems, some hypotheses were formulated and tested with simulation experiments at high spatial resolution, using a climate projection to 2050 under the RCP4.5 climate-change scenario.