Ecoregion Framework Facilitating Synthesis and Up-scaling of Tower CO2 Exchange Measurements

Abstract:

As the flux-tower studies of terrestrial CO2 exchange are currently approaching the stage of generalization and up-scaling, there is a growing need for an effective framework for these efforts facilitating comparative analysis and synthesis of flux tower data. Developed in the 1980s to denote a region with recurring patterns of ecosystems with similar climatic, vegetation, soil, and relief conditions, during the past decades the ecoregion concept was widely used as a tool combining modeling, GIS and remote sensing techniques in environmental and nature conservation project at the national and international scale. We describe application of the ecoregion approach to synthesis and up-scaling of the tower CO2 exchange measurements in the three major ecoregions of the Great Plains: Shortgrass steppe (SGS), Mixed prairie (MP), and Tallgrass prairie (TP). Original data of tower CO2 exchange (NEE) measurements during the 1996-2013 period covering 23 years at 6 sites in SGS, 86 years at 21 sites in MP, and 80 years at 24 sites in TP were partitioned into the gross photosynthesis (Pg) and ecosystem respiration (Re) components using the light-soil temperature-VPD response method. We present ecoregion-specific multivariate models relating gross photosynthesis, ecosystem respiration and major ecosystem-scale ecophysiological parameters (photosynthetic capacity, apparent quantum yield, light-use efficiency, daytime respiration intensity) to on-site (climatic, plant, soil) and remotely sensed (NDVI) drivers of CO2 exchange. Superposition of these models on GIS layers of appropriate drivers was used to describe geographic patterns (maps) of photosynthesis, respiration, net CO2 exchange and ecophysiological parameters across the Great Plains. The source-sink activity at the ecoregion-scale was quantified by calculating spatial integrals of CO2 exchange characteristic over GIS pixels represented by the tower measurements.