Hot moments in ecosystem fluxes: High GPP anomalies exert outsized influence on the carbon cycle and are differentially driven by moisture availability across biomes

Abstract

The 'hot spot-hot moment' concept is a long-standing and popular framework often invoked to explain spatially or temporally variable rates of biogeochemical cycling. However, this concept has been rarely extended to ecosystem fluxes such as gross primary productivity (GPP), in part due to the lack of a quantitative definition of hot moments that can be applied to large flux datasets. Here, we develop a general statistical framework for quantifying hot moments in GPP and identify their spatial patterns and climatic drivers. Using 308 site-years of eddy covariance data from the FLUXNET2015 dataset spanning 32 U.S. sites, we found hot moments in GPP to comprise a disproportionate percentage of annual carbon (C) uptake relative to the frequency of their occurrence. For example, at five sites over 12% of annual C uptake occurred during the~2% most extreme half-hourly or hourly observations of GPP. Hot moments were most quantitatively important for the C cycle in short-stature, arid ecosystem such as grasslands, woody savannas, and open shrublands, where these positive anomalies in GPP were caused by increases in moisture availability. In contrast, hot moments were less important for annual C uptake in more mesic ecosystems, where their occurrence was largely determined by high temperature and light availability. Our results point to a need to consider how short-term spikes in environmental conditions exert an outsized influence on annual GPP, and how future shifts in these conditions could impact the terrestrial C cycle.

The Diurnal Dynamics of Gross Primary Productivity Using Observations From the Advanced Baseline Imager on the Geostationary Operational Environmental Satellite‐R Series at an Oak Savanna Ecosystem

“…Remotely sensed observations of the Earth have provided critical inputs for global carbon cycle studies, provided observation‐based GPP estimates for comparisons with Earth System Models and terrestrial carbon cycle models, and have revolutionized our understanding of the carbon cycle (Anav et al., 2015; Chen et al., 2017; Cramer et al., 1999; Field et al., 1995; Jung et al., 2020; Keenan et al., 2012; O’Sullivan et al., 2020; Prince & Goward, 1995; Ruimy et al., 1996; Running et al., 2004; Xiao et al., 2019; Zhang et al., 2016; Zscheischler et al., 2014). The diurnal to interannual variability of GPP is determined by limiting resources, climate, weather conditions, disturbance, phenology, and extreme events (Beer et al., 2010; Gu et al., 2002; Kannenberg et al., 2020; Randazzo et al., 2020; Roby et al., 2020; Stoy et al., 2005; Zscheischler et al., 2014). However, with existing polar‐orbiting satellites we have been largely limited to studying the multiday to interannual variability of GPP rather than its dynamic response to environmental variability across the course of a day.…”

High Vapor Pressure Deficit Decreases the Productivity and Water Use Efficiency of Rain‐Induced Pulses in Semiarid Ecosystems

“…Pulses are transient periods characterized by high rates of gross ecosystem photosynthesis ( GEP ), evapotranspiration ( ET ), and ecosystem respiration ( R e ) that occur when rain events temporarily mitigate water stress effects on plant and microbial activity (Huxman, Snyder, et al, 2004; Noy‐Meir, 1973; Schwinning et al, 2004). With high rates of biogeochemical cycling, pulses have a disproportionately large impact on annual GEP (Kannenberg et al, 2020), which is a key driver of variation in the net ecosystem production ( NEP = GEP − R e ) and ecosystem‐scale water use efficiency ( WUE = GEP / ET ) of semiarid regions (Baldocchi et al, 2018; Biederman et al, 2016; Jia et al, 2016; Liu et al, 2019; Scott et al, 2015). Therefore, it is necessary to examine controls on ecosystem carbon and water fluxes during these important subannual periods (Barnes et al, 2016; Jenerette et al, 2012; Jung et al, 2017).…”

Ten years of Gulf Coast ecosystem restoration projects since the Deepwater Horizon oil spill

“…Spatiotemporal scales of both restoration and background environmental conditions factor into the expression of unexpectedly high rates of ecosystem function, termed hot spots (places) or hot moments (periods) (HSHM) ( 15 ). Combined “spot–moments,” termed “ecosystem control points” by ref.…”