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Arctic warming is affecting snow cover and soil hydrology, with consequences for carbon sequestration in tundra ecosystems. The scarcity of observations in the Arctic has limited our understanding of the impact of covarying environmental drivers on the carbon balance of tundra ecosystems. In this study, we address some of these uncertainties through a novel record of 119 site-years of summer data from eddy covariance towers representing dominant tundra vegetation types located on continuous permafrost in the Arctic. Here we found that earlier snowmelt was associated with more tundra net CO2 sequestration and higher gross primary productivity (GPP) only in June and July, but with lower net carbon sequestration and lower GPP in August. Although higher evapotranspiration (ET) can result in soil drying with the progression of the summer, we did not find significantly lower soil moisture with earlier snowmelt, nor evidence that water stress affected GPP in the late growing season. Our results suggest that the expected increased CO2 sequestration arising from Arctic warming and the associated increase in growing season length may not materialize if tundra ecosystems are not able to continue sequestering CO2 later in the season.

Peatlands store substantial amounts of carbon and are vulnerable to climate change. We present a modified version of the Organising Carbon and Hydrology In Dynamic Ecosystems (ORCHIDEE) land surface model for simulating the hydrology, surface energy, and CO2 fluxes of peatlands on daily to annual timescales. The model includes a separate soil tile in each 0.5° grid cell, defined from a global peatland map and identified with peat-specific soil hydraulic properties. Runoff from non-peat vegetation within a grid cell containing a fraction of peat is routed to this peat soil tile, which maintains shallow water tables. The water table position separates oxic from anoxic decomposition. The model was evaluated against eddy-covariance (EC) observations from 30 northern peatland sites, with the maximum rate of carboxylation (Vcmax) being optimized at each site. Regarding short-term day-to-day variations, the model performance was good for gross primary production (GPP) (r2 Combining double low line 0.76; Nash-Sutcliffe modeling efficiency, MEF Combining double low line 0.76) and ecosystem respiration (ER, r2 Combining double low line 0.78, MEF Combining double low line 0.75), with lesser accuracy for latent heat fluxes (LE, r2 Combining double low line 0.42, MEF Combining double low line 0.14) and and net ecosystem CO2 exchange (NEE, r2 Combining double low line 0.38, MEF Combining double low line 0.26). Seasonal variations in GPP, ER, NEE, and energy fluxes on monthly scales showed moderate to high r2 values (0.57-0.86). For spatial across-site gradients of annual mean GPP, ER, NEE, and LE, r2 values of 0.93, 0.89, 0.27, and 0.71 were achieved, respectively. Water table (WT) variation was not well predicted (r2

Peatlands store disproportionally large amounts of carbon per unit area, a function that is dependent on maintaining high and stable water tables. Climate change is likely to negatively impact carbon storage in peatlands, in part due to increases in vapour pressure deficit (VPD) driving higher evaporation (E) rates. However, the response of E to increasing VPD depends on the dominant vegetation type within peatlands. In this study, we used multiple years of eddy covariance (EC) measurements to compare E regimes at two peatlands with contrasting vegetation types – Kopuatai bog in Aotearoa New Zealand, dominated by the vascular jointed wire rush Empodisma robustum , and Mer Bleue bog in Canada, a 'typical' shrub and moss-dominated Northern Hemisphere peatland. We examined seasonal variability in E and equilibrium E (E eq), energy balance partitioning, and the response of E, evaporative fraction (EF), and canopy conductance (g c) to VPD. Mean annual E was 45 % lower than mean annual E eq at Kopuatai, but only 16 % lower at Mer Bleue, demonstrating much greater limitations on E at Kopuatai. In addition, the mean midday (10:00–14:30) dry canopy Bowen ratio (β) at Kopuatai was 1.96, compared to 0.77 at Mer Bleue; therefore, the sensible heat flux (H) dominated over the latent heat flux (LE) at Kopuatai, and vice versa at Mer Bleue. The responses of E , EF, and g c to increasing VPD at Kopuatai differed from those at Mer Bleue in a way that resulted in stronger limitations on E above ~0.7 kPa at the former bog. The observed limitations at Kopuatai were attributed to strong stomatal control by E. robustum due to the rapid decrease in g c with increasing VPD, however surface E could also be limited by its dense standing litter. At Mer Bleue, however, E was only weakly limited at VPD > 2 kPa, likely due to weak stomatal control over transpiration by the sparse shrub canopy and relatively large surface E from Sphagnum carpets. As such, the results of this study suggest that E. robustum drives a greater "hydrological resistance" to increasing VPD than the vegetation at Mer Bleue, leading to greater water retention at Kopuatai. This may enable greater resilience of the carbon sink function at Kopuatai to climatic warming and drying than at Mer Bleue. [ABSTRACT FROM AUTHOR]

Long‐term atmospheric CO2 concentration records have suggested a reduction in the positive effect of warming on high‐latitude carbon uptake since the 1990s. A variety of mechanisms have been proposed to explain the reduced net carbon sink of northern ecosystems with increased air temperature, including water stress on vegetation and increased respiration over recent decades. However, the lack of consistent long‐term carbon flux and in situ soil moisture data has severely limited our ability to identify the mechanisms responsible for the recent reduced carbon sink strength. In this study, we used a record of nearly 100 site‐years of eddy covariance data from 11 continuous permafrost tundra sites distributed across the circumpolar Arctic to test the temperature (expressed as growing degree days, GDD) responses of gross primary production (GPP), net ecosystem exchange (NEE), and ecosystem respiration (ER) at different periods of the summer (early, peak, and late summer) including dominant tundra vegetation classes (graminoids and mosses, and shrubs). We further tested GPP, NEE, and ER relationships with soil moisture and vapor pressure deficit to identify potential moisture limitations on plant productivity and net carbon exchange. Our results show a decrease in GPP with rising GDD during the peak summer (July) for both vegetation classes, and a significant relationship between the peak summer GPP and soil moisture after statistically controlling for GDD in a partial correlation analysis. These results suggest that tundra ecosystems might not benefit from increased temperature as much as suggested by several terrestrial biosphere models, if decreased soil moisture limits the peak summer plant productivity, reducing the ability of these ecosystems to sequester carbon during the summer. [ABSTRACT FROM AUTHOR]

The theory of conductive heat transfer cannot explain different attenuations of the daily amplitude of peat temperatures (Ts) in hummocks (detectable below the 20-cm depth) and hollows (disappearing above the 10+ cm depth). Large readily drained macropores in the upper fibric peat determine a large air permeability and hence may enhance heat transfer by air convection in porous media, driven by temperature gradients between hummock sides and interiors. In this study, the ecosys model was used to simulate a peat thermal regime at Mer Bleue peatland, Ontario, Canada. It was hypothesized that adding the air-convective heat transfer to conductive plus water-convective heat transfers would improve simulations of [T.sub.S]. The results for [T.sub.S], ground heat fluxes, G, and sensible heat fluxes, H, modeled with and without air-convective heat transfer were tested with continuous hourly measurements from 2000 to 2004 using thermocouples, heat flux plates, and eddy covariance. Simulated air-convective heat transfer caused an average increase in 6' and a corresponding decrease in H of ~20 W [m.sup.-2] from the simulated conductive plus water-convective heat transfer. Hastened soil warming in hummocks resulted in better agreement between measured and simulated hummock [T.sub.S] values with (RMSD of 2.23[degrees]C) than without air-convective heat (RMSD of 2.54[degrees]C). Enhanced hummock [T.sub.S] caused an indirect increase in hollow Ts in the model with (RMSD of 1.68[degrees]C) compared to without airoconvective heat (RMSD of 1.82[degrees]C). Our results suggest that air convection is probably an important mechanism of heat transfer in peat hummocks and should be included in peatland biogeochemical models. doi: 10.2136/sssaj2009.0288

In this study, the ecosys model was used to simulate the hydrology of the Met Bleue bog, Ontario, Canada, with seasonally varying water tables in the upper 1 m. The soil profile was divided into three zones of peat (fibric, hemic, and sapric). In the model, large, readily drained macropore fractions in the fibric peat caused low water-holding capacity and high infiltration rates, in contrast to hemic and sapric peat, with small macropore fractions, high water-holding capacities, and low infiltration rates. Model results for peat water contents, [theta], and water table depths, Z, were tested with continuous hourly measurements from 2000 to 2004 using time domain reflectometry probes and piezometers. Within the zone of pronounced water table variation, the [theta] and Z modeled with the Hagen--Poiseuille equation (or macropore flow and Richards' equation for peat matrix flow corresponded better to the measured [theta] and Z (regression slopes between 0.62 and 1.03, intercepts between -0.05 and 0.02 [m.sup.3] [m.sup.- 3], and [R.sup.2] between 0.40 and 0.56), than did the modeled [theta] and Z with Richards' equation alone (regression slopes between 0.33 and 1.43, intercepts between 0.11 and 0.22 [m.sup.3] [m.sup.-3], and [R.sup.2] between 0.27 and 0.41). The Richards equation alone, even when parameterized with extremely high or low bulk saturated hydraulic conductivities of fibric peat, modeled slower infiltration, greater water retention, and lower Z than measured. The implications of macropore flow might be of great importance for peatland hydrology, therefore this experimental and modeling work should be extended to other wetlands as well. Abbreviations: DOY, day of the year; EC, eddy covariance; ET, evapotranspiration; TDR, time domain reflectometry; WT, water table. doi:10.2136/sssaj2009.0148

Abstract: We measured interaction of photosynthetically active radiation (PAR) at a staghorn sumac (Rhus typhina L.) canopy near Peterborough, Ontario, during summer 2006. Measurements included above-canopy and below-canopy incoming and [...]