Accurate estimation of terrestrial gross primary productivity (
Wetlands play an important role in regulating the atmospheric carbon dioxide (
- NSF-PAR ID:
- 10031337
- Publisher / Repository:
- Wiley-Blackwell
- Date Published:
- Journal Name:
- Global Change Biology
- Volume:
- 23
- Issue:
- 3
- ISSN:
- 1354-1013
- Page Range / eLocation ID:
- p. 1180-1198
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract GPP ) remains a challenge despite its importance in the global carbon cycle. Chlorophyll fluorescence (ChlF) has been recently adopted to understand photosynthesis and its response to the environment, particularly with remote sensing data. However, it remains unclear how ChlF and photosynthesis are linked at different spatial scales across the growing season. We examined seasonal relationships between ChlF and photosynthesis at the leaf, canopy, and ecosystem scales and explored how leaf‐level ChlF was linked with canopy‐scale solar‐induced chlorophyll fluorescence (SIF ) in a temperate deciduous forest at Harvard Forest, Massachusetts,USA . Our results show that ChlF captured the seasonal variations of photosynthesis with significant linear relationships between ChlF and photosynthesis across the growing season over different spatial scales (R 2 = 0.73, 0.77, and 0.86 at leaf, canopy, and satellite scales, respectively;P < 0.0001). We developed a model to estimateGPP from the tower‐based measurement ofSIF and leaf‐level ChlF parameters. The estimation ofGPP from this model agreed well with flux tower observations ofGPP (R 2 = 0.68;P < 0.0001), demonstrating the potential ofSIF for modelingGPP . At the leaf scale, we found that leafF q ’ /F m ’ , the fraction of absorbed photons that are used for photochemistry for a light‐adapted measurement from a pulse amplitude modulation fluorometer, was the best leaf fluorescence parameter to correlate with canopySIF yield (SIF /APAR ,R 2 = 0.79;P < 0.0001). We also found that canopySIF andSIF ‐derivedGPP (GPPSIF ) were strongly correlated to leaf‐level biochemistry and canopy structure, including chlorophyll content (R 2 = 0.65 for canopyGPPSIF and chlorophyll content;P < 0.0001), leaf area index (LAI ) (R 2 = 0.35 for canopyGPPSIF andLAI ;P < 0.0001), and normalized difference vegetation index (NDVI ) (R 2 = 0.36 for canopyGPPSIF andNDVI ;P < 0.0001). Our results suggest that ChlF can be a powerful tool to track photosynthetic rates at leaf, canopy, and ecosystem scales. -
Abstract Inland waters are increasingly recognized as critical sites of methane emissions to the atmosphere, but the biogeochemical reactions driving such fluxes are less well understood. The Prairie Pothole Region (
PPR ) of North America is one of the largest wetland complexes in the world, containing millions of small, shallow wetlands. The sediment pore waters ofPPR wetlands contain some of the highest concentrations of dissolved organic carbon (DOC ) and sulfur species ever recorded in terrestrial aquatic environments. Using a suite of geochemical and microbiological analyses, we measured the impact of sedimentary carbon and sulfur transformations in these wetlands on methane fluxes to the atmosphere. This research represents the first study of coupled geochemistry and microbiology within thePPR and demonstrates how the conversion of abundant labileDOC pools into methane results in some of the highest fluxes of this greenhouse gas to the atmosphere ever reported. AbundantDOC and sulfate additionally supported some of the highest sulfate reduction rates ever measured in terrestrial aquatic environments, which we infer to account for a large fraction of carbon mineralization in this system. Methane accumulations in zones of active sulfate reduction may be due to either the transport of free methane gas from deeper locations or the co‐occurrence of methanogenesis and sulfate reduction. If both respiratory processes are concurrent, any competitive inhibition of methanogenesis by sulfate‐reducing bacteria may be lessened by the presence of large labileDOC pools that yield noncompetitive substrates such as methanol. Our results reveal some of the underlying mechanisms that makePPR wetlands biogeochemical hotspots, which ultimately leads to their critical, but poorly recognized role in regional greenhouse gas emissions. -
Abstract Rich fens are common boreal ecosystems with distinct hydrology, biogeochemistry and ecology that influence their carbon (C) balance. We present growing season soil chamber methane emission (
FCH 4), ecosystem respiration (ER ), net ecosystem exchange (NEE ) and gross primary production (GPP ) fluxes from a 9‐years water table manipulation experiment in an Alaskan rich fen. The study included major flood and drought years, where wetting and drying treatments further modified the severity of droughts. Results support previous findings from peatlands that drought causes reduced magnitude of growing seasonFCH 4,GPP andNEE , thus reducing or reversing their C sink function. Experimentally exacerbated droughts further reduced the capacity for the fen to act as a C sink by causing shifts in vegetation and thus reducing magnitude of maximum growing seasonGPP in subsequent flood years by ~15% compared to control plots. Conversely, water table position had only a weak influence onER , but dominant contribution toER switched from autotrophic respiration in wet years to heterotrophic in dry years. Droughts did not cause inter‐annual lag effects onER in this rich fen, as has been observed in several nutrient‐poor peatlands. WhileER was dependent on soil temperatures at 2 cm depth,FCH 4was linked to soil temperatures at 25 cm. Inter‐annual variability of deep soil temperatures was in turn dependent on wetness rather than air temperature, and higherFCH 4in flooded years was thus equally due to increased methane production at depth and decreased methane oxidation near the surface. Short‐term fluctuations in wetness caused significant lag effects onFCH 4, but droughts caused no inter‐annual lag effects onFCH 4. Our results show that frequency and severity of droughts and floods can have characteristic effects on the exchange of greenhouse gases, and emphasize the need to project future hydrological regimes in rich fens. -
Abstract Earth system models (
ESM s) rely on the calculation of canopy conductance in land surface models (LSM s) to quantify the partitioning of land surface energy, water, andCO 2fluxes. This is achieved by scaling stomatal conductance,g w, determined from physiological models developed for leaves. Traditionally, models forg whave been semi‐empirical, combining physiological functions with empirically determined calibration constants. More recently, optimization theory has been applied to modelg winLSM s under the premise that it has a stronger grounding in physiological theory and might ultimately lead to improved predictive accuracy. However, this premise has not been thoroughly tested. Using original field data from contrasting forest systems, we compare a widely used empirical type and a more recently developed optimization‐typeg wmodel, termedBB andMED , respectively. Overall, we find no difference between the two models when used to simulateg wfrom photosynthesis data, or leaf gas exchange from a coupled photosynthesis‐conductance model, or gross primary productivity and evapotranspiration for aFLUXNET tower site with theCLM 5 communityLSM . Field measurements reveal that the key fitted parameters forBB andMED ,g 1Bandg 1M,exhibit strong species specificity in magnitude and sensitivity toCO 2, andCLM 5 simulations reveal that failure to include this sensitivity can result in significant overestimates of evapotranspiration for high‐CO 2scenarios. Further, we show thatg 1Bandg 1Mcan be determined from meanc i/c a(ratio of leaf intercellular to ambientCO 2concentration). Applying this relationship withc i/c avalues derived from a leaf δ13C database, we obtain a global distribution ofg 1Bandg 1M, and these values correlate significantly with mean annual precipitation. This provides a new methodology for global parameterization of theBB andMED models inLSM s, tied directly to leaf physiology but unconstrained by spatial boundaries separating designated biomes or plant functional types. -
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