Matthias Peichl

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.

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)

Linking variability in soil solution dissolved organic carbon to climate, soil type, and vegetation type

Lateral transport of carbon plays an important role in linking the carbon cycles of terrestrial and aquatic ecosystems. There is, however, a lack of information on the factors controlling one of the main C sources of this lateral flux, i.e., the concentration of dissolved organic carbon (DOC) in soil solution across large spatial scales and under different soil, vegetation, and climate conditions. We compiled a database on DOC in soil solution down to 80 cm and analyzed it with the aim, first, to quantify the differences in DOC concentrations among terrestrial ecosystems, climate zones, soil, and vegetation types at global scale and second, to identify potential determinants of the site-to-site variability of DOC concentration in soil solution across European broadleaved and coniferous forests. We found that DOC concentrations were 75% lower in mineral than in organic soil, and temperate sites showed higher DOC concentrations than boreal and tropical sites. The majority of the variation (R2 = 0.67–0.99) in DOC concentrations in mineral European forest soils correlates with NH4+, C/N, Al, and Fe as the most important predictors. Overall, our results show that the magnitude (23% lower in broadleaved than in coniferous forests) and the controlling factors of DOC in soil solution differ between forest types, with site productivity being more important in broadleaved forests and water balance in coniferous stands.

Convergence of potential net ecosystem production among contrasting C3 grasslands

Metabolic theory and body size constraints on biomass production and decomposition suggest that differences in the intrinsic potential net ecosystem production (NEPPOT ) should be small among contrasting C3 grasslands and therefore unable to explain the wide range in the annual apparent net ecosystem production (NEPAPP ) reported by previous studies. We estimated NEPPOT for nine C3 grasslands under contrasting climate and management regimes using multiyear eddy covariance data. NEPPOT converged within a narrow range, suggesting little difference in the net carbon dioxide uptake capacity among C3 grasslands. Our results indicate a unique feature of C3 grasslands compared with other terrestrial ecosystems and suggest a state of stability in NEPPOT due to tightly coupled production and respiration processes. Consequently, the annual NEPAPP of C3 grasslands is primarily a function of seasonal and short-term environmental and management constraints, and therefore especially susceptible to changes in future climate patterns and associated adaptation of management practices.

Potential greenhouse gas mitigation through temperate tree-based intercropping systems.

Increasing awareness of global climate change has pressured agricultural producers to reduce greenhouse gas (GHG) emissions while at the same time encouraging them to maintain food production needed for an increasing popula- tion. Tree-based intercropping (TBI) systems are believed to be useful in climate change mitigation, especially in temper- ate regions, due to their potential to reduce GHG emissions from agricultural practices. The purpose of this paper is there- fore to review some of the research conducted on GHG mitigation in TBI in southern Ontario and Quebec, Canada. Re- search conducted at the University of Guelph Agroforestry Research Station (GARS) indicated that TBI systems had the potential to lower N2O emissions by 1.2 kg ha -1 y -1 compared to a conventional agricultural field cropping system. Trees can assimilate residual nitrate (NO3 - ) left from nitrogen (N) fertilizer applications, thereby leaving less NO3 - available for denitrification and subsequently reducing N2O losses. Carbon sequestration is also enhanced in TBI systems as carbon (C) is stored in both above and below ground tree components. Soil Organic Carbon (SOC) is higher in systems incorporating trees because tree litter decomposes slowly, therefore reducing CO2 loss to the atmosphere. The C sequestration potential of TBI systems and the possibility to include fast-growing tree species for bioenergy production in TBI systems make it a valid solution to mitigate climate change in temperate regions. The opportunity of C trading credits to offset the costs of implementing a TBI system and provide additional income to farmers could facilitate the adoption of TBI amidst agricul- tural producers in temperate regions.

How do disturbances and climate effects on carbon and water fluxes differ between multi-aged and even-aged coniferous forests?

Disturbances and climatic changes significantly affect forest ecosystem productivity, water use efficiency (WUE) and carbon (C) flux dynamics. A deep understanding of terrestrial feedbacks to such effects and recovery mechanisms in forests across contrasting climatic regimes is essential to predict future regional/global C and water budgets, which are also closely related to the potential forest management decisions. However, the resilience of multi-aged and even-aged forests to disturbances has been debated for >60years because of technical measurement constraints. Here we evaluated 62site-years of eddy covariance measurements of net ecosystem production (NEP), evapotranspiration (ET), the estimates of gross primary productivity (GPP), ecosystem respiration (Re) and ecosystem-level WUE, as well as the relationships with environmental controls in three chronosequences of multi- and even-aged coniferous forests covering the Mediterranean, temperate and boreal regions. Age-specific dynamics in multi-year mean annual NEP and WUE revealed that forest age is a key variable that determines the sign and magnitude of recovering forest C source-sink strength from disturbances. However, the trends of annual NEP and WUE across succession stages between two stand structures differed substantially. The successional patterns of NEP exhibited an inverted-U trend with age at the two even-aged chronosequences, whereas NEP of the multi-aged chronosequence increased steadily through time. Meanwhile, site-level WUE of even-aged forests decreased gradually from young to mature, whereas an apparent increase occurred for the same forest age in multi-aged stands. Compared with even-aged forests, multi-aged forests sequestered more CO2 with forest age and maintained a relatively higher WUE in the later succession periods. With regard to the available flux measurements in this study, these behaviors are independent of tree species, stand ages and climate conditions. We also found that distinctly different environmental factors controlled forest C and water fluxes under three climatic regimes. Typical weather events such as temperature anomalies or drying-wetting cycles severely affected forest functions. Particularly, a summer drought in the boreal forest resulted in an increased NEP owing to a considerable decrease in Re, but at the cost of greater water loss from deeper groundwater resources. These findings will provide important implications for forest management strategies to mitigate global climate change.

Gross primary production controls the subsequent winter CO2 exchange in a boreal peatland

In high-latitude regions, carbon dioxide (CO2 ) emissions during the winter represent an important component of the annual ecosystem carbon budget; however, the mechanisms that control the winter CO2 emissions are currently not well understood. It has been suggested that substrate availability from soil labile carbon pools is a main driver of winter CO2 emissions. In ecosystems that are dominated by annual herbaceous plants, much of the biomass produced during the summer is likely to contribute to the soil labile carbon pool through litter fall and root senescence in the autumn. Thus, the summer carbon uptake in the ecosystem may have a significant influence on the subsequent winter CO2 emissions. To test this hypothesis, we conducted a plot-scale shading experiment in a boreal peatland to reduce the gross primary production (GPP) during the growing season. At the growing season peak, vascular plant biomass in the shaded plots was half that in the control plots. During the subsequent winter, the mean CO2 emission rates were 21% lower in the shaded plots than in the control plots. In addition, long-term (2001-2012) eddy covariance data from the same site showed a strong correlation between the GPP (particularly the late summer and autumn GPP) and the subsequent winter net ecosystem CO2 exchange (NEE). In contrast, abiotic factors during the winter could not explain the interannual variation in the cumulative winter NEE. Our study demonstrates the presence of a cross-seasonal link between the growing season biotic processes and winter CO2 emissions, which has important implications for predicting winter CO2 emission dynamics in response to future climate change.

Enhanced winter soil frost reduces methane emission during the subsequent growing season in a boreal peatland

Winter climate change may result in reduced snow cover and could, consequently, alter the soil frost regime and biogeochemical processes underlying the exchange of methane (CH4 ) in boreal peatlands. In this study, we investigated the short-term (1-3 years) vs. long-term (11 years) effects of intensified winter soil frost (induced by experimental snow exclusion) on CH4 exchange during the following growing season in a boreal peatland. In the first 3 years (2004-2006), lower CH4 emissions in the treatment plots relative to the control coincided with delayed soil temperature increase in the treatment plots at the beginning of the growing season (May). After 11 treatment years (in 2014), CH4 emissions were lower in the treatment plots relative to the control over the entire growing season, resulting in a reduction in total growing season CH4 emission by 27%. From May to July 2014, reduced sedge leaf area coincided with lower CH4 emissions in the treatment plots compared to the control. From July to August, lower dissolved organic carbon concentrations in the pore water of the treatment plots explained 72% of the differences in CH4 emission between control and treatment. In addition, greater Sphagnum moss growth in the treatment plots resulted in a larger distance between the moss surface and the water table (i.e., increasing the oxic layer) which may have enhanced the CH4 oxidation potential in the treatment plots relative to the control in 2014. The differences in vegetation might also explain the lower temperature sensitivity of CH4 emission observed in the treatment plots relative to the control. Overall, this study suggests that greater soil frost, associated with future winter climate change, might substantially reduce the growing season CH4 emission in boreal peatlands through altering vegetation dynamics and subsequently causing vegetation-mediated effects on CH4 exchange.

Full carbon and greenhouse gas balances of fertilized and nonfertilized reed canary grass cultivations on an abandoned peat extraction area in a dry year

Bioenergy crop cultivation on former peat extraction areas is a potential after‐use option that provides a source of renewable energy while mitigating climate change through enhanced carbon (C) sequestration. This study investigated the full C and greenhouse gas (GHG) balances of fertilized (RCG‐F) and nonfertilized (RCG‐C) reed canary grass (RCG; Phalaris arundinacea) cultivation compared to bare peat (BP) soil within an abandoned peat extraction area in western Estonia during a dry year. Vegetation sampling, static chamber and lysimeter measurements were carried out to estimate above‐ and belowground biomass production and allocation, fluxes of carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) in cultivated strips and drainage ditches as well as the dissolved organic carbon (DOC) export, respectively. Heterotrophic respiration was determined from vegetation‐free trenched plots. Fertilization increased the above‐ to belowground biomass production ratio and the autotrophic to heterotrophic respiration ratio. The full C balance (incl. CO2, CH4 and DOC fluxes from strips and ditches) was 96, 215 and 180 g C m−2 yr−1 in RCG‐F, RCG‐C and BP, respectively, suggesting that all treatments acted as C sources during the dry year. The C balance was driven by variations in the net CO2 exchange, whereas the combined contribution of CH4 and DOC fluxes was <5%. The GHG balances were 3.6, 7.9 and 6.6 t CO2 eq ha−1 yr−1 in RCG‐F, RCG‐C and BP, respectively. The CO2 exchange was also the dominant component of the GHG balance, while the contributions of CH4 and N2O were <1% and 1–6%, respectively. Overall, this study suggests that maximizing plant growth and the associated CO2 uptake through adequate water and nutrient supply is a key prerequisite for ensuring sustainable high yields and climate benefits in RCG cultivations established on organic soils following drainage and peat extraction.

Including hydrological self-regulating processes in peatland models: Effects on peatmoss drought projections.

The water content of the topsoil is one of the key factors controlling biogeochemical processes, greenhouse gas emissions and biosphere - atmosphere interactions in many ecosystems, particularly in northern peatlands. In these wetland ecosystems, the water content of the photosynthetic active peatmoss layer is crucial for ecosystem functioning and carbon sequestration, and is sensitive to future shifts in rainfall and drought characteristics. Current peatland models differ in the degree in which hydrological feedbacks are included, but how this affects peatmoss drought projections is unknown. The aim of this paper was to systematically test whether the level of hydrological detail in models could bias projections of water content and drought stress for peatmoss in northern peatlands using downscaled projections for rainfall and potential evapotranspiration in the current (1991-2020) and future climate (2061-2090). We considered four model variants that either include or exclude moss (rain)water storage and peat volume change, as these are two central processes in the hydrological self-regulation of peatmoss carpets. Model performance was validated using field data of a peatland in northern Sweden. Including moss water storage as well as peat volume change resulted in a significant improvement of model performance, despite the extra parameters added. The best performance was achieved if both processes were included. Including moss water storage and peat volume change consistently reduced projected peatmoss drought frequency with >50%, relative to the model excluding both processes. Projected peatmoss drought frequency in the growing season was 17% smaller under future climate than current climate, but was unaffected by including the hydrological self-regulating processes. Our results suggest that ignoring these two fine-scale processes important in hydrological self-regulation of northern peatlands will have large consequences for projected climate change impact on ecosystem processes related to topsoil water content, such as greenhouse gas emissions.