Based on theories of mire development and responses to a changing climate, the current role of mires as a net carbon sink has been questioned. A rigorous evaluation of the current net C-exchange in mires requires measurements of all relevant fluxes. Estimates of annual total carbon budgets in mires are still very limited. Here, we present a full carbon budget over 2 years for a boreal minerogenic oligotrophic mire in northern Sweden (64111 0 N, 19133 0 E). Data on the following fluxes were collected: land-atmosphere CO 2 exchange (continuous Eddy covariance measurements) and CH 4 exchange (static chambers during the snow free period); TOC (total organic carbon) in precipitation; loss of TOC, dissolved inorganic carbon (DIC) and CH 4 through stream water runoff (continuous discharge measurements and regular C-concentration measurements). The mire constituted a net sink of 27 AE 3.4 ( AE SD) g C m À2 yr À1 during 2004 and 20 AE 3.4 g C m À2 yr À1 during 2005. This could be partitioned into an annual surfaceatmosphere CO 2 net uptake of 55 AE 1.9 g C m À2 yr À1 during 2004 and 48 AE 1.6 g C m À2 yr À1 during 2005. The annual NEE was further separated into a net uptake season, with an uptake of 92 g C m À2 yr À1 during 2004 and 86 g C m À2 yr À1 during 2005, and a net loss season with a loss of 37 g C m À2 yr À1 during 2004 and 38 g C m À2 yr À1 during 2005. Of the annual net CO 2 -C uptake, 37% and 31% was lost through runoff (with runoff TOC4DIC ) CH 4 ) and 16% and 29% through methane emission during 2004 and 2005, respectively. This mire is still a significant C-sink, with carbon accumulation rates comparable to the long-term Holocene C-accumulation, and higher than the C-accumulation during the late Holocene in the region.
Forested histosols have been found in some cases to be major, and in other cases minor, sources of the greenhouse gas nitrous oxide (N 2 O). In order to estimate the total national or global emissions of N 2 O from histosols, scaling or mapping parameters that can separate low-and high-emitting sites are needed, and should be included in soil databases. Based on interannual measurements of N 2 O emissions from drained forested histosols in Sweden, we found a strong negative relationship between N 2 O emissions and soil CN ratios (r 2 adj 5 0.96, mean annual N 2 O emission 5 ae (Àb CN ratio) ). The same equation could be used to estimate the N 2 O emissions from Finnish and German sites based on CN ratios in published data. We envisage that the correlation between N 2 O emissions and CN ratios could be used to scale N 2 O emissions from histosols determined at sampled sites to national levels. However, at low CN ratios (i.e. below 15-20) other parameters such as climate, pH and groundwater tables increase in importance as regulating factors affecting N 2 O emissions.
Miniaturized thermal infrared (TIR) cameras that measure surface temperature are increasingly available for use with unmanned aerial vehicles (UAVs). However, deriving accurate temperature data from these cameras is non-trivialsince they are highly sensitive to changes in their internal temperature and low-cost models are often not radiometrically calibrated. We present the results of laboratory and field experiments that tested the extent of the temperature-dependency of a non-radiometric FLIR Vue Pro 640. We found that a simple empirical line calibration using at least three ground calibration points was sufficient to convert camera digital numbers to temperature values for images captured during UAV flight. Although the camera performed well under stable laboratory conditions (accuracy ±0.5 °C), the accuracy declined to ±5 °C under the changing ambient conditions experienced during UAV flight. The poor performance resulted from the non-linear relationship between camera output and sensor temperature, which was affected by wind and temperature-drift during flight. The camera’s automated non-uniformity correction (NUC) could not sufficiently correct for these effects. Prominent vignetting was also visible in images captured under both stable and changing ambient conditions. The inconsistencies in camera output over time and across the sensor will affect camera applications based on relative temperature differences as well as user-generated radiometric calibration. Based on our findings, we present a set of best practices for UAV TIR camera sampling to minimize the impacts of the temperature dependency of these systems.
Disturbance of ecosystems is a major factor in regional carbon budgets, and it is believed to be partly responsible for the large inter-annual variability of the terrestrial part of the carbon balance. Forest fires have so far been considered as the most important disturbance but also other forms of disturbance such as insect outbreaks or wind-throw might contribute significantly to the largely unexplained inter-annual variability, at least in specific regions. The effect of wind-throw has not yet been estimated because of lack of data on how carbon fluxes are affected. The Gudrun storm, which hit Sweden in January 2005, resulted in ca. 66 million m 3 of wind-thrown stem wood on an area of ca. 272 000 ha. Using a model (BIOME-BGC) calibrated to CO 2 flux measurements at two sites, the annual net ecosystem productivity during the first year after the storm was estimated to be in the range À897 to À1259 g C m À2 yr À1 . This is a much higher loss compared with harvested (clear-cut) forests in Europe, which ranged between ca. À420 and À100 g m À2 yr À1 . The reduction in the carbon sink scaled to the whole wind-thrown area was estimated at ca. 3 million tons C during the first year. By historical data on windthrow in Europe combined with modelling, we estimated that the large Lothar storm in 1999 reduced the European carbon balance by ca. 16 million tons C, this is ca. 30% of the net biome production in Europe. We conclude that the impact of increased forest damage by more frequent storms in future climate change scenarios must be considered and that intermittent large wind-throw events may explain a part of the large inter-annual variability in the terrestrial carbon sink.
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