Average Net Ecosystem Exchange Research Articles

Experimental manipulations of winter snow and summer rain influence ecosystem carbon cycling in a mixed‐grass prairie, Wyoming, USA

AbstractWater‐limited grasslands may be exceedingly vulnerable to changes in the timing or amount of precipitation which may result in shifts in the magnitudes and patterns of carbon cycling. Shifts in CO2 exchange may lead to alterations in carbon sequestration or net losses and could accentuate the rising CO2 concentrations in the atmosphere or ameliorate the increases. The objective of our project was to quantify how changes in winter, summer, and combined winter and summer precipitation may alter rates of ecosystem C cycling in the mixed‐grass prairie of the United States. Three replicated 50 m snow fences were installed to increase winter snow while summer precipitation was manipulated by either increasing (+50%) or decreasing (−50%) precipitation amounts. Measurements of net ecosystem exchange (NEE), gross ecosystem photosynthesis (GEP), and ecosystem respiration (ER) and plant biomass were conducted throughout the snow‐free period. Deeper snow in winter increased summer ER by an average of 27%, GEP by 45%, NEE by 90% and plant biomass by 50% compared to ambient snow conditions. Average plant biomass increased 44% with the addition of summer rain and decreased by 18% where rainfall was excluded under ambient snow conditions. Average NEE was also lower where rain was excluded. Our findings indicate that seasonality changes of precipitation may be important in regulating the future extent of C sequestration and C cycling in one of the most extensive, intact grasslands of North America. Copyright © 2010 John Wiley & Sons, Ltd.

Climate-Driven Interannual Variability in Net Ecosystem Exchange in the Northern Great Plains Grasslands

Abstract The Northern Great Plains grasslands respond differently under various climatic conditions; however, there have been no detailed studies investigating the interannual variability in carbon exchange across the entire Northern Great Plains grassland ecosystem. We developed a piecewise regression model to integrate flux tower data with remotely sensed data and mapped the 8-d and 500-m net ecosystem exchange (NEE) for the years from 2000 to 2006. We studied the interannual variability of NEE, characterized the interannual NEE difference in climatically different years, and identified the drought impact on NEE. The results showed that NEE was highly variable in space and time across the 7 yr. Specifically, NEE was consistently low (−35 to 322 g C·m −2 ·yr −1 ) with an average annual NEE of −2 ± 242 g C·m −2 ·yr −1 and a cumulative flux of −152 g C·m −2 . The Northern Great Plains grassland was a weak source for carbon during 2000–2006 because of frequent droughts, which strongly affected the carbon balance, especially in the Western High Plains and Northwestern Great Plains. Comparison of the NEE map with a drought monitor map confirmed a substantial correlation between drought and carbon dynamics. If drought severity or frequency increases in the future, the Northern Great Plains grasslands may become an even greater carbon source.

Influence of nocturnal low‐level jet on turbulence structure and CO2 flux measurements over a forest canopy

The present study analyzes features of nocturnal low‐level jets observed at the Florida AmeriFlux site and their influence on CO2 flux measurements over a tall forest canopy. At that location, two categories of nocturnal flow are commonly observed, one with a strong low‐level jet throughout the night and the other without. Jets of diverse speed and height are observed during nearly 70% of the nocturnal periods over a 3‐month campaign, of which almost 50% are strong jets with speed higher than 10 m s−1 and height in the range 200–400 m. Strong jet activity contributes to weak atmospheric stabilities with gradient Richardson numbers lower than 0.2 and higher friction velocities (0.2 to 0.6 m s−1) attributed to enhanced canopy turbulence. The canopy shear length scale exhibits a linear relationship with jet shear. Jet periods also show dominant downward transport of turbulent kinetic energy and turbulent CO2 fluxes in the range 2 to 8 μmol m−2 s−1. The difference between the net ecosystem exchange (NEE) at two levels above the canopy adds on average, flux contribution of 1.25 μmol m−2 s−1 (18% of the average NEE at z = 1.4h, h is the canopy height) to CO2 exchange during periods characterized by strong jets. A comparison of CO2 and wind velocity Fourier spectra and cospectra between periods with dissimilar jet activity shows larger low‐frequency spectral contributions in the strong jet case, supporting the possibility of variance and flux contributions at scales comparable to the jet height.

Net ecosystem CO 2 exchange in a temperate cattail marsh in relation to biophysical properties

Net ecosystem exchange (NEE) of carbon dioxide (CO 2) was measured at a temperate cattail marsh using the eddy covariance technique in order to examine the relationships between NEE, weather, and vegetation properties. Analyses of CO 2 fluxes for a complete year (May 9, 2005 to May 30, 2006) showed that the marsh wetland was a net CO 2 sink for each month from June to September (monthly 24-h averages of −0.1, −5.1, −4.8, and −2.2 g C m −2 day −1, respectively) and a source of CO 2 to the atmosphere for the remaining fall and winter months. Wintertime (November to April) average NEE was 0.5 g C m −2 day −1. The annual cumulative CO 2 balance shows a net uptake of 264 g C m −2 year −1 by the marsh. Ecosystem respiration (ER) and gross ecosystem production (GEP) were estimated to be 567 and 831 g C m −2 year −1, respectively. Respiration rates were associated with a Q 10 value of 2.8. Peak aboveground biomass (average of 1156 g m −2) and peak green LAI (average of 3.63) were reached in mid-August. Variations in growing season NEE were well correlated with variations in live biomass ( r = 0.92) and green LAI ( r = 0.94).

Closing the Carbon Budget of a Scots Pine forest in the Netherlands

The aim of this study was to close the carbon budget and reduce uncertainty in annual C balances for Scots pine (Pinus sylvestris) forests in The Netherlands. This was done by comparing estimates of the Net Ecosystem Exchange (NEE) as assessed by two different methods. The inventory based carbon budgeting method estimated the average NEE for 1997 – 2001 at 202 g C m−2 yr−1 (a sink) with a confidence interval of 138 – 271 g C m−2 yr−1. The estimate obtained by the eddy covariance method was 295 g C m−2 yr−1 on average for the same period, with a confidence interval of 224 – 366 g C m−2 yr−1. Uncertainties in the eddy covariance method were mostly related to gap filling of the data. Main uncertainties in the inventory-based method are related to the soil and the root compartment. The difference in NEE as obtained by two independent methods indicates that it is not straightforward to design a sound National System for monitoring and reporting of the total land area and for accounting of changes in forest area under the Kyoto Protocol, and that more effort is required in this field.

Net ecosystem CO 2 exchange of mixed forest in Belgium over 5 years

In this paper, we present and discuss the annual net ecosystem exchange (NEE) results from 5 years (1997–2001) of continuous eddy covariance measurements of CO 2 flux above a mixed temperate forest. The forest was a 70-year-old coniferous (Scots pine)—deciduous mixture, with slow growth rate and a leaf area index (LAI) of about 3, and was part of the European CARBOEUROFLUX research network. Effects of the data pre-treatment and the gap filling method on annual NEE estimates were analyzed. The u *-correction increased the annual NEE by +61 g C m −2 per year on average. The maximum difference in annual NEE estimates from different gap filling methods amounted up to 130 g C m −2 per year in a year with a large gap in the CO 2 flux series. The estimated average annual NEE over the 5 years was + 110 g C m −2 per year (ranging from −9 to 255 g C m −2 per year) when using the most defensible gap filling strategy. We also analyzed the inter-annual variability of carbon balance, which was found to be mainly dependent on the length of the growing season and on the annual temperature. The observation that this forest acted as a CO 2 source contrasts with previous results from most other temperate forests.

Reconstruction of the carbon balance for microsites in a boreal oligotrophic pine fen, Finland.

Carbon dioxide (CO2) exchange was studied at flark (minerotrophic hollow), lawn and hummock microsites in an oligotrophic boreal pine fen. Statistical response functions were constructed for the microsites in order to reconstruct the annual CO2 exchange balance from climate data. Carbon accumulation was estimated from the annual net CO2 exchange, methane (CH4) emissions and leaching of carbon. Due to high water tables in the year 1993, the average carbon accumulation at the flark, Eriophorum lawn, Carex lawn and hummock microsites was high, 2.91, 6.08, 2.83 and 2.66 mol C m-2, respectively, and for the whole peatland it was 5.66 mol m-2 year-1. During the maximum primary production period in midsummer, hummocks with low water tables emitted less methane than predicted from the average net ecosystem exchange (NEE), while the Carex lawns emitted slightly more. CH4 release during that period corresponded to 16% of the contemporary NEE. Annual C accumulation rate did not correlate with annual CH4 release in the microsites studied, but the total community CO2 release seemed to be related to CH4 emissions in the wet microsites, again excluding the hummocks. The dependence of CO2 exchange dynamics on weather events suggests that daily balances in C accumulation are labile and can change from net carbon uptake to net release, primarily in high hummocks on fens under warmer, drier climatic conditions.

Treating Cattle with Ivermectin: Effects on the Fauna and Decompsition of Dung Pats

The old-growth, multiple ages, multispecies natural forest has played an important role in terrestrial ecosystem dynamics model and the global carbon budget. However, carbon fluxes of old forests in subtropical regions are rarely reported in China. In the present study, the CO₂ flux of an old-growth subtropical evergreen and deciduous broad-leaved mixed forest was observed using eddy covariance technique in Tianmu Mountain of Zhejiang Province. Based on the data sets which were observed from July 2013 to June 2014, the variations of net ecosystem exchange (NEE), eco-system respiration (Re), and gross ecosystem exchange (GEE) were analyzed. The results showed that during the study period, the monthly NEE all had a negative value (acted as a carbon sink) except for December and February (acted as a carbon source). The average monthly NEE was -61.52 g C · m⁻², the monthly carbon sequestration showed a double-peak curve and the maximum carbon sink was -149.40 g C · m⁻², which occurred in June while the maximum carbon source was 23.45 g C · m⁻², which occurred in February. The maximum of monthly mean CO₂ flux occurred in June with a value of -0.98 mg · m⁻² · s⁻¹, while the minimum value occurred in December with a value of -0.35 mg · m⁻² · s⁻¹. The NEE at the time point of positive and negative conversion had typical seasonal characteristics. The yearly NEE, Re, and GEE were -738.18, 931.05 and -1669.23 g C · m⁻², respectively. Compared with other forest ecosystems located at the similar latitude, the carbon fixation of the old-growth forest was larger, likely due to its complicated structure within the canopy and the presence of young-growth regeneration and successional stands. This showed that other than in carbon neutral, old-growth forests of Tianmu Mountain in subtropical China had a strong capability in carbon sequestration.