Meteorological impacts of land use change in the maritime tropics

Vugts, H.F. [Promotor]Bruijnzeel, L.A. [Copromotor]Scatena, F.N. [Copromotor

A compact and stable eddy covariance set-up for methane measurements using off-axis integrated cavity output spectroscopy

A Fast Methane Analyzer (FMA) is assessed for its applicability in a closed path eddy covariance field set-up in a peat meadow. The FMA uses off-axis integrated cavity output spectroscopy combined with a highly specific narrow band laser for the detection of CH<sub>4</sub> and strongly reflective mirrors to obtain a laser path length of 2–20×10<sup>3</sup> m. Statistical testing and a calibration experiment showed high precision (7.8×10<sup>−3</sup> ppb) and accuracy (<0.30%) of the instrument, while no drift was observed. The instrument response time was determined to be 0.10 s. In the field set-up, the FMA is attached to a scroll pump and combined with a 3-axis ultrasonic anemometer and an open path infrared gas analyzer for measurements of carbon dioxide and water vapour. The power-spectra and co-spectra of the instruments were satisfactory for 10 Hz sampling rates. <br><br> Due to erroneous measurements, spikes and periods of low turbulence the data series consisted for 26% of gaps. Observed CH<sub>4</sub> fluxes consisted mainly of emission, showed a diurnal cycle, but were rather variable over. The average CH<sub>4</sub> emission was 29.7 nmol m<sup>−2</sup> s<sup>−1</sup>, while the typical maximum CH<sub>4</sub> emission was approximately 80.0 nmol m<sup>−2</sup> s<sup>−1</sup> and the typical minimum flux was approximately 0.0 nmol m<sup>−2</sup> s<sup>−1</sup>. The correspondence of the measurements with flux chamber measurements in the footprint was good and the observed CH<sub>4</sub> emission rates were comparable with eddy covariance CH<sub>4</sub> measurements in other peat areas. <br><br> Additionally, three measurement techniques with lower sampling frequencies were simulated, which might give the possibility to measure CH<sub>4</sub> fluxes without an external pump and save energy. Disjunct eddy covariance appeared to be the most reliable substitute for 10 Hz eddy covariance, while relaxed eddy accumulation gave reliable estimates of the fluxes over periods in the order of days or weeks

The full greenhouse gas balance of an abandoned peat meadow

International audienceGlobally, peat lands are considered to be a sink of CO2, but a source when drained. Additionally, wet peat lands are thought to emit considerable amounts of CH4 and N2O. Hitherto, reliable and integrated estimates of emissions and emission factors for this type of area have been lacking and the effects of wetland restoration on methane emissions have been poorly quantified. In this paper we estimate the full GHG balance of a restored natural peat land by determining the fluxes of CO2, CH4 and N2O through atmosphere and water, while accounting for the different GWP's. This site is an abandoned agricultural peat meadow, which has been converted into a wetland nature reserve ten years ago by raising the water level. GHG fluxes were measured continuously with an eddy-correlation system (CO2) and flux chamber measurements (CH4 and N2O). Meteorological and hydrological measurements were done as well. With growing seasons of respectively 192 and 155 days, the net annual CO2 uptake was 276±61 g C m?2 for 2004 and 311±58 g C m?2 for 2005. Ecosystem respiration was estimated as 887±668 g C m?2 for 2004 and 866±666 g C m?2 for 2005. CH4 fluxes from water, saturated land and relatively dry land varied: total annual CH4 fluxes are 10.4±19.2 g C m?2 yr?1, 101 g C m?2 yr?1±30 and 37.3±10.9 g C m?2 yr?1, respectively, and a annual weighed total CH4 emission of 31.27±20.44 g C m?2 yr?1. N2O fluxes were too low to be of significance. The carbon-balance consists for the largest part of CO2 uptake, CO2 respiration and CH4 emission from wet land and water. CO2 emission has decreased significantly as result of the raised water table, while CH4 fluxes have increased. In global warming potentials the area is a very small sink of 71 g CO2-equiv m?2 (over a 100-year period)

Longer growing seasons do not increase net carbon uptake in Northeastern Siberian tundra

With global warming, snowmelt is occurring earlier and growing seasons are becoming longer around the Arctic. It has been suggested that this would lead to more uptake of carbon due to a lengthening of the period in which plants photosynthesize. To investigate this suggestion, 8 consecutive years of eddy covariance measurements at a northeastern Siberian graminoid tundra site were investigated for patterns in net ecosystem exchange, gross primary production (GPP) and ecosystem respiration (Reco). While GPP showed no clear increase with longer growing seasons, it was significantly increased in warmer summers. Due to these warmer temperatures however, the increase in uptake was mostly offset by an increase in Reco. Therefore, overall variability in net carbon uptake was low, and no relationship with growing season length was found. Furthermore, the highest net uptake of carbon occurred with the shortest and the coldest growing season. Low uptake of carbon mostly occurred with longer or warmer growing seasons. We thus conclude that the net carbon uptake of this ecosystem is more likely to decrease rather than to increase under a warmer climate. These results contradict previous research that has showed more net carbon uptake with longer growing seasons. We hypothesize that this difference is due to site-specific differences, such as climate type and soil, and that changes in the carbon cycle with longer growing seasons will not be uniform around the Arcti

Assessing the spatial variability in peak season CO2exchange characteristics across the Arctic tundra using a light response curve parameterization

This paper aims to assess the spatial variability in the response of CO2exchange to irradiance across the Arctic tundra during peak season using light response curve (LRC) parameters. This investigation allows us to better understand the future response of Arctic tundra under climatic change. Peak season data were collected during different years (between 1998 and 2010) using the micrometeorological eddy covariance technique from 12 circumpolar Arctic tundra sites, in the range of 64-74° N. The LRCs were generated for 14 days with peak net ecosystem exchange (NEE) using an NEE-irradiance model. Parameters from LRCs represent site-specific traits and characteristics describing the following: (a) NEE at light saturation (Fcsat), (b) dark respiration (Rd), (c) light use efficiency (a), (d) NEE when light is at 1000 µmol m-2s-1(Fc1000), (e) potential photosynthesis at light saturation (Psat) and (f) the light compensation point (LCP). Parameterization of LRCs was successful in predicting CO2flux dynamics across the Arctic tundra. We did not find any trends in LRC parameters across the whole Arctic tundra but there were indications for temperature and latitudinal differences within sub-regions like Russia and Greenland. Together, leaf area index (LAI) and July temperature had a high explanatory power of the variance in assimilation parameters (Fcsat, Fc1000and Psat, thus illustrating the potential for upscaling CO2exchange for the whole Arctic tundra. Dark respiration was more variable and less correlated to environmental drivers than were assimilation parameters. This indicates the inherent need to include other parameters such as nutrient availability, substrate quantity and quality in flux monitoring activities

Net ecosystem exchange of carbon dioxide and water of far eastern Siberian Larch (Larix cajanderii) on permafrost.

Observations of the net ecosystem exchange of water and CO₂ were made during two seasons in 2000 and 2001 above a Larch forest in Far East Siberia (Yakutsk). The measurements were obtained by eddy correlation. There is a very sharply pronounced growing season of 100 days when the forest is leaved. Maximum half hourly uptake rates are 18 &micro;mol m^-2 s^-1; maximum respiration rates are 5 &micro;mol m^-2 s^-1. Net annual sequestration of carbon was estimated at 160 gCm^-2 in 2001. Applying no correction for low friction velocities added 60 g C m^-2. The net carbon exchange of the forest was extremely sensitive to small changes in weather that may switch the forest easily from a sink to a source, even in summer. June was the month with highest uptake in 2001. <P style='line-height: 20px;'> The average evaporation rate of the forest approached 1.46 mm day^-1 during the growing season, with peak values of 3 mm day^-1 with an estimated annual evaporation of 213 mm, closely approaching the average annual rainfall amount. 2001 was a drier year than 2000 and this is reflected in lower evaporation rates in 2001 than in 2000. <P style='line-height: 20px;'> The surface conductance of the forest shows a marked response to increasing atmospheric humidity deficits. This affects the CO₂ uptake and evaporation in a different manner, with the CO₂ uptake being more affected. There appears to be no change in the relation between surface conductance and net ecosystem uptake normalized by the atmospheric humidity deficit at the monthly time scale. The response to atmospheric humidity deficit is an efficient mechanism to prevent severe water loss during the short intense growing season. The associated cost to the sequestration of carbon may be another explanation for the slow growth of these forests in this environment

The growing season greenhouse gas balance of a continental tundra site in the Indigirka lowlands, NE Siberia.

Carbon dioxide and methane fluxes were measured at a tundra site near Chokurdakh, in the lowlands of the Indigirka river in north-east Siberia. This site is one of the few stations on Russian tundra and it is different from most other tundra flux stations in its continentality. A suite of methods was applied to determine the fluxes of NEE, GPP, &lt;i&gt;R&lt;/i&gt;&lt;sub&gt;eco&lt;/sub&gt; and methane, including eddy covariance, chambers and leaf cuvettes. Net carbon dioxide fluxes were high compared with other tundra sites, with NEE=&amp;minus;92 g C m&lt;sup&gt;&amp;minus;2&lt;/sup&gt; yr&lt;sup&gt;&amp;minus;1&lt;/sup&gt;, which is composed of an &lt;i&gt;R&lt;/i&gt;&lt;sub&gt;eco&lt;/sub&gt;=+141 g C m&lt;sup&gt;&amp;minus;2&lt;/sup&gt; yr&lt;sup&gt;&amp;minus;1&lt;/sup&gt; and GPP=&amp;minus;232 g C m&lt;sup&gt;&amp;minus;2&lt;/sup&gt; yr&lt;sup&gt;&amp;minus;1&lt;/sup&gt;. This large carbon dioxide sink may be explained by the continental climate, that is reflected in low winter soil temperatures (&amp;minus;14&amp;deg;C), reducing the respiration rates, and short, relatively warm summers, stimulating high photosynthesis rates. Interannual variability in GPP was dominated by the frequency of light limitation (&lt;i&gt;R&lt;sub&gt;g&lt;/sub&gt;&lt;/i&gt;&amp;lt;200 W m&lt;sup&gt;&amp;minus;2&lt;/sup&gt;), whereas &lt;i&gt;R&lt;/i&gt;&lt;sub&gt;eco&lt;/sub&gt; depends most directly on soil temperature and time in the growing season, which serves as a proxy of the combined effects of active layer depth, leaf area index, soil moisture and substrate availability. The methane flux, in units of global warming potential, was +28 g C-CO&lt;sub&gt;2&lt;/sub&gt;e m&lt;sup&gt;&amp;minus;2&lt;/sup&gt; yr&lt;sup&gt;&amp;minus;1&lt;/sup&gt;, so that the greenhouse gas balance was &amp;minus;64 g C-CO&lt;sub&gt;2&lt;/sub&gt;e m&lt;sup&gt;&amp;minus;2&lt;/sup&gt; yr&lt;sup&gt;&amp;minus;1&lt;/sup&gt;. Methane fluxes depended only slightly on soil temperature and were highly sensitive to hydrological conditions and vegetation composition

An estimate of the terrestrial carbon budget of Russia using inventory-based, eddy covariance and inversion methods

We determine the carbon balance of Russia, including Ukraine, Belarus and Kazakhstan using inventory based, eddy covariance, Dynamic Global Vegetation Models (DGVM), and inversion methods. Our current best estimate of the net biosphere to atmosphere flux is -0.66 Pg C yr-1. This sink is primarily caused by forests that using two independent methods are estimated to take up -0.69 Pg C yr-1. Using inverse models yields an average net biosphere to atmosphere flux of the same value with a interannual variability of 35%. The total estimated biosphere to atmosphere flux from eddy covariance observations over a limited number of sites amounts to -1 Pg C yr-1. Fires emit 137 to 121 Tg C yr-1 using two different methods. The interannual variability of fire emissions is large, up to a factor 0.5 to 3. Smaller fluxes to the ocean and inland lakes, trade are also accounted for. Our best estimate for the Russian net biosphere to atmosphere flux then amounts to -659 Tg C yr-1 as the average of the inverse models of -653 Tg C yr-1, bottom up -563 Tg C yr-1 and the independent landscape approach of -761 Tg C yr-1. These three methods agree well within their error bounds, so there is good consistency between bottom up and top down methods. The best estimate of the net land to atmosphere flux, including the fossil fuel emissions is -145 to -73 Tg C yr-1. Estimated methane emissions vary considerably with one inventory-based estimate providing a net land to atmosphere flux of 12.6 Tg C-CH4yr-1 and an independent model estimate for the boreal and Arctic zones of Eurasia of 27.6 Tg C-CH4yr-1