Jörgen Sagerfors

Energy exchange and water budget partitioning in a boreal minerogenic mire

This study investigated patterns and controls of the seasonal and inter-annual variations in energy fluxes (i.e., sensible heat, H, and latent heat, lambda E) and partitioning of the water budget (i.e., precipitation, P; evapotranspiration, ET; discharge, Q; and soil water storage, Delta S) over five years (2001-2005) in a boreal oligotrophic fen in northern Sweden based on continuous eddy covariance, water table level (WTL), and weir measurements. For the growing season (May 1 to September 31), the 5 year averages (+/- standard deviation) of the midday (10:00 to 14:00 h) Bowen ratio (beta, i.e., H/lambda E) was 0.86 +/- 0.08. Seasonal and inter-annual variability of beta was mainly driven by lambda E which itself was strongly controlled by both weather (i.e., vapor pressure deficit, D, and net radiation, R-n) and physiological parameters (i.e., surface resistance). During the growing season, surface resistance largely exceeded aerodynamic resistance, which together with low mean values of the actual ET to potential ET ratio (0.55 +/- 0.05) and Priestley-Taylor alpha (0.89) suggests significant physiological constrains on ET in this well-watered fen. Among the water budget components, the inter-annual variability of ET was lower (199 to 298 mm) compared to Q (225 to 752 mm), with each accounting on average for 34 and 65% of the ecosystem water loss, respectively. The fraction of P expended into ET was negatively correlated to P and positively to R-n. Although a decrease in WTL caused a reduction of the surface conductance, the overall effect of WTL on ET was limited. Non-growing season (October 1 to April 30) fluxes of H, lambda E, and Q were significant representing on average -67%, 13%, and 61%, respectively, of their growing season sums (negative sign indicates opposite flux direction between the two seasons). Overall, our findings suggest that plant functional type composition, P and R-n dynamics (i.e., amount and timing) were the major controls on the partitioning of the mire energy and water budgets. This has important implications for the regional climate as well as for ecosystem development, nutrient, and carbon dynamics. Citation: Peichl, M., J. Sagerfors, A. Lindroth, I. Buffam, A. Grelle, L. Klemedtsson, H. Laudon, and M. B. Nilsson (2013), Energy exchange and water budget partitioning in a boreal minerogenic mire, J. Geophys. Res. Biogeosci., 118, 1-13, doi:10.1029/2012JG002073.

Variations in net ecosystem exchange of carbon dioxide in a boreal mire: Modeling mechanisms linked to water table position

[1] In mires, which occupy large areas of the boreal region, net ecosystem CO2 exchange ( NEE) rates vary significantly over various timescales. In order to examine the effect of one of the most influencing variables, the water table depth, on NEE the general ecosystem model GUESS-ROMUL was modified to predict mire daily CO2 exchange rates. A simulation was conducted for a lawn, the most common microtopographical feature of boreal oligotrophic minerotrophic mires. The results were validated against eddy covariance CO2 flux measurements from Degero Stormyr, northern Sweden, obtained during the period 2001 - 2003. Both measurements and model simulations revealed that CO2 uptake was clearly controlled by interactions between water table depth and temperature. Maximum uptake occurred when the water table level was between 10 and 20 cm and the air temperature was above 15 degrees C. When the water table was higher, the CO2 uptake rate was lower, owing to reduced rates of photosynthetic carbon fixation. When the water table was lower, NEE decreased owing to the increased rate of decomposition of organic matter. When the water table level was between 10 and 20 cm, the NEE was quite stable and relatively insensitive to both changes within this range and any air temperature changes above + 15 degrees C. The optimal water table level range for NEE corresponds to that characteristic of mire lawn plant communities, indicating that the annual NEE will not change dramatically if climatic conditions remain within the optimal range for the current plant community.

Environmental controls on the CO2 exchange in north Europea mires

Net CO2 exchange measured under well-mixed atmospheric conditions in four different mires in Sweden and Finland were used to analyse which factors were controlling photosynthesis and respiration. The parameters of a light response function showed strong seasonal variations with similar behaviour for all mires. The half-monthly nighttime respiration rates in the central part of the growing season were about two times higher in the southernmost, warmest site, Fåje, as compared to the northernmost, coldest site, Kaamanen. However, Kaamanen had high photosynthesis rates, and this in combination with the long daylight periods in the middle of the summer caused Kaamanen to have the largest net ecosystem exchange (NEE) during the summer period. Fåje that showed the highest productivity had also the highest respiration and therefore, the lowest NEE during summer. Correlation between half-monthly components and different environmental variables showed the highest correlation between the components themselves. Thereafter came temperature except for Fåje where water table depth (WTD) explained most of the variance both for detrended and temperature-normalized components. All sites showed dependencies between WTD and the respective components during drying up periods. Temperature sensitivity was higher for productivity than for respiration indicating that CO2 uptake would increase during global warming.

A 12-year record reveals pre-growing season temperature and water table level threshold effects on the net carbon dioxide exchange in a boreal fen

This study uses a 12-year time series (2001-2012) of eddy covariance measurements to investigate the long-term net ecosystem exchange (NEE) of carbon dioxide (CO2) and inter-annual variations in relation to abiotic drivers in a boreal fen in northern Sweden. The peatland was a sink for atmospheric CO2 in each of the twelve study years with a 12-year average (+/- standard deviation) NEE of -58 +/- 21 g C m(-2) yr(-1). For ten out of twelve years, the cumulative annual NEE was within a range of -42 to -79 g C m(-2) yr(-1) suggesting a general state of resilience of NEE to moderate inter-annual climate variations. However, the annual NEE of -18 and -106 g C m(-2) yr(-1) in 2006 and 2008, respectively, diverged considerably from this common range. The lower annual CO2 uptake in 2006 was mainly due to late summer emissions related to an exceptional drop in water table level (WTL). A positive relationship (R-2 = 0.65) between pre-growing season (January to April) air temperature (Ta) and summer (June to July) gross ecosystem production (GEP) was observed. We suggest that enhanced GEP due to mild pre-growing season air temperature in combination with air temperature constraints on ecosystem respiration (ER) during the following cooler summer explained most of the greater net CO2 uptake in 2008. Differences in the annual and growing season means of other abiotic variables (e.g. radiation, vapor pressure deficit, precipitation) and growing season properties (i.e. start date, end date, length) were unable to explain the inter-annual variations of NEE. Overall, our findings suggest that this boreal fen acts as a persistent contemporary sink for atmospheric CO2 that is, however, susceptible to severe anomalies in WTL and pre-growing season air temperature associated with predicted changes in climate patterns for the boreal region. (Less)