[1] Accurately simulating gross primary productivity (GPP) in terrestrial ecosystem models is critical because errors in simulated GPP propagate through the model to introduce additional errors in simulated biomass and other fluxes. We evaluated simulated, daily average GPP from 26 models against estimated GPP at 39 eddy covariance flux tower sites across the United States and Canada. None of the models in this study match estimated GPP within observed uncertainty. On average, models overestimate GPP in winter, spring, and fall, and underestimate GPP in summer. Models overpredicted GPP under dry conditions and for temperatures below 0 C. Improvements in simulated soil moisture and ecosystem response to drought or humidity stress will improve simulated GPP under dry conditions. Adding a low-temperature response to shut down GPP for temperatures below 0 C will reduce the positive bias in winter, spring, and fall and improve simulated phenology. The negative bias in summer and poor overall performance resulted from mismatches between simulated and observed light use efficiency (LUE). Improving simulated GPP requires better leaf-to-canopy scaling and better values of model parameters that control the maximum potential GPP, such as ɛ max (LUE), V cmax (unstressed Rubisco catalytic capacity) or J max (the maximum electron transport rate).

[1] The ecosys model was applied to investigate the effects of water table and subsurface hydrology changes on carbon dioxide exchange at the ombrotrophic Mer Bleue peatland, Ontario, Canada. It was hypothesized that (1) water table drawdown would not affect vascular canopy water potential, hence vascular productivity, because roots would penetrate deeper to compensate for near-surface dryness, (2) moss canopy water potential and productivity would be severely reduced because rhizoids occupy the uppermost peat that is subject to desiccation with water table decline, and (3) given that in a previous study of Mer Bleue, ecosystem respiration showed little sensitivity to water table drawdown, gross primary productivity would mainly determine the net ecosystem productivity through these vegetation-subsurface hydrology linkages. Model output was compared with literature reports and hourly eddy-covariance measurements during 2000-2004. Our findings suggest that late-summer water table drawdown in 2001 had only a minor impact on vascular canopy water potential but greatly impacted hummock moss water potential, where midday values declined to −250 MPa on average in the model. As a result, simulated moss productivity was reduced by half, which largely explained a reduction of 2-3 mmol CO 2 m −2 s −1 in midday simulated and measurement-derived gross primary productivity and an equivalent reduction in simulated and measured net ecosystem productivity. The water content of the near-surface peat (top 5-10 cm) was found to be the most important driver of interannual variability of annual net ecosystem productivity through its effects on hummock moss productivity and on ecosystem respiration.

[1] The ecosys model was applied to examine the effects of peatland hydrology on soil respiration and ecosystem respiration at Mer Bleue peatland, Ontario, Canada. It was hypothesized that a decrease in near-surface microbial respiration in peat hummocks resulting from water table (WT) drawdown and subsequent desiccation of the uppermost peat would offset an increase of soil respiration at depth with improved aeration (respiration offsetting mechanism). In contrast, shallower water table in hollows would not allow near-surface desiccation to offset increased soil respiration at depth during drying. However, increased hollow soil respiration with WT drawdown would be offset by decreased aboveground moss respiration with near-surface desiccation in hummocks. Model results for microbial respiration were tested against independent laboratory experiments and ecosystem respiration against hourly eddy-covariance measurements of bog

In this study, the ecosys model was used to simulate the hydrology of the Mer 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, θ, 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 θ and Z modeled with the Hagen–Poiseuille equation for macropore flow and Richards' equation for peat matrix flow corresponded better to the measured θ and Z (regression slopes between 0.62 and 1.03, intercepts between −0.05 and 0.02 m3 m−3, and R2 between 0.40 and 0.56), than did the modeled θ and Z with Richards' equation alone (regression slopes between 0.33 and 1.43, intercepts between 0.11 and 0.22 m3 m−3, and R2 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.

[1] Changes in arctic C stocks with climate are thought to be caused by rising net primary productivity (NPP) during longer and warmer growing seasons, offset by rising heterotrophic respiration (R h ) in warmer and deeper soil active layers. In this study, we used the process model ecosys to test hypotheses for these changes with CO 2 and energy fluxes measured by eddy covariance over a mesic shrub tundra at Daring Lake, Canada, under varying growing seasons. These tests corroborated substantial rises in NPP, smaller rises in R h , and, hence, rises in net ecosystem productivity (NEP) from 17 to 45 g C m −2 yr −1(net C sink), modeled with higher T a and longer growing seasons. However, NEP was found to decline briefly during midsummer warming events (T a > 20°C). A model run under climate change predicted for Daring Lake indicated that rises in NPP would exceed those in R h during the first 100 years, causing NEP to rise. Rises in NPP were driven by more rapid net N mineralization from more rapid R h in warming soils. However, greater declines in NEP were modeled during more frequent and intense midsummer warming events as climate change progressed. Consequently, average annual NEP (± interannual variability) rose from 30 (±13) g C m −2 yr −1 under current climate to 57 (±40) g C m −2 yr −1 after 90 years but declined to 44 (±51) g C m −2 yr −1 after 150 years, indicating that gains in tundra NEP under climate change may not be indefinite.Citation: Grant, R. F., E. R. Humphreys, P. M. Lafleur, and D. D. Dimitrov (2011), Ecological controls on net ecosystem productivity of a mesic arctic tundra under current and future climates,