Large interannual CO2 and energy exchange variability in a freshwater marsh under consistent environmental conditions

Adrian V Rocha,Michael L Goulden

doi:10.1029/2008jg000712

Adrian V Rocha, Michael L Goulden

Open Access

https://doi.org/10.1029/2008jg000712

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Abstract

We analyzed a 5‐year record of the CO2 and energy exchange, Aboveground Net Primary Production (ANPP), maximum Leaf Area Index (LAImax), and Enhanced Vegetation Index (EVI) for a Typha marsh in Southern California. The marsh was a net source of carbon over the study, despite high rates of ANPP. Interannual Net Ecosystem Production (NEP) variability was the largest that has been reported for any terrestrial ecosystem and was attributed to changes in maximum photosynthetic rates (GEEmax). The variation in energy and mass exchange was coupled between years; years with higher than average rates of carbon uptake were associated with lower than average sensible heat fluxes. Remotely sensed measures of surface greenness (EVI) were closely related to GEEmax variation, providing further evidence of interannual variability. We were unable to attribute the fluctuations in GEEmax to the direct effects of weather on ecosystem physiology, or to interannual variation in LAImax. GEE did not vary systematically with air temperature or the presence of standing water in the marsh; GEEmax did not vary with LAImax between years. Rather, interannual variation in carbon exchange at the SJFM resulted from shifts in the marsh's production efficiency (the rates of gross or net CO2 exchange per LAI) that were not caused by changes in the weather. Our findings challenge the assumptions that interannual variation of land‐atmosphere exchange is universally caused by the direct effect of weather on ecosystem physiology, and that an ecosystem's physiological response to the physical environment is consistent from year‐to‐year.

Highlights

[4] In this study, we describe the interannual dynamics of CO2 and energy exchanges, Aboveground Net Primary Production (ANPP), maximum Leaf Area Index (LAImax), and remotely sensed vegetation indices at the Typha-dominated San Joaquin Freshwater Marsh (SJFM) located in Southern California
We hypothesized that interannual variability in energy and mass exchanges would be small, and that the high productivity of the SJFM would result in a strong carbon sink
We address these hypotheses by documenting a long-term record of whole ecosystem CO2 and energy exchange, assessing the ability of the SJFM to sequester carbon, determining the relationships between interannual weather and ecosystem function, and comparing the interannual SJFM ecosystem function variability to that reported for other ecosystem types

Summary

Introduction

[2] Long-term records of whole ecosystem CO2, water and energy exchanges have provided valuable insight into the interannual dynamics of ecosystem function. Most models are highly parameterized such that ecosystem respiration is represented as a simple Q10 response with temperature, and photosynthesis is represented with a constant light use efficiency and carboxylation capacity [Hanson et al, 2004] These model parameterizations do not allow for temporal changes in biological factors associated with interannual changes in light use efficiency, carbon allocation, or substrate induced respiration [Davidson et al, 2006]. We hypothesized that interannual variability in energy and mass exchanges would be small, and that the high productivity of the SJFM would result in a strong carbon sink.

Methods

Results

Does High Plant Productivity at the SJFM Result in a Large Carbon Sink?

Discussion