Carbon dioxide fluxes were examined over the growing seasons of 2002 and 2003 from 14 different sites in Upper Midwest (USA) to assess spatial variability of ecosystem–atmosphere CO
2 exchange. These ...sites were exposed to similar temperature/precipitation regimes and spanned a range of vegetation types typical of the region (northern hardwood, mixed forest, red pine, jack pine, pine barrens and shrub wetland). The hardwood and red pine sites also spanned a range of stand ages (young, intermediate, mature). While seasonal changes in net ecosystem exchange (NEE) and photosynthetic parameters were coherent across the 2 years at most sites, changes in ecosystem respiration (ER) and gross ecosystem production (GEP) were not. Canopy height and vegetation type were important variables for explaining spatial variability of CO
2 fluxes across the region. Light-use efficiency (LUE) was not as strongly correlated to GEP as maximum assimilation capacity (Amax). A bottom-up multi-tower land cover aggregated scaling of CO
2 flux to a 2000
km
2 regional flux estimate found June to August 2003 NEE, ER and GEP to be −290
±
89, 408
±
48, and 698
±
73
gC
m
−2, respectively. Aggregated NEE, ER and GEP were 280% larger, 32% smaller and 3% larger, respectively, than that observed from a regionally integrating 447
m tall flux tower. However, when the tall tower fluxes were decomposed using a footprint-weighted influence function and then re-aggregated to a regional estimate, the resulting NEE, ER and GEP were within 11% of the multi-tower aggregation. Excluding wetland and young stand age sites from the aggregation worsened the comparison to observed fluxes. These results provide insight on the range of spatial sampling, replication, measurement error and land cover accuracy needed for multi-tiered bottom-up scaling of CO
2 fluxes in heterogeneous regions such as the Upper Midwest, USA.
Soil respiration (SR) represents a major component of forest ecosystem respiration and is influenced seasonally by environmental factors such as temperature, soil moisture, root respiration, and ...litter fall. Changes in these environmental factors correspond with shifts in plant phenology. In this study, we examined the relationship between canopy phenophases (pre-growth, growth, pre-dormancy, and dormancy) and SR sensitivity to changes in soil temperature (TS). SR was measured 53 times over 550 days within an oak forest in northwest Ohio, USA. Annual estimates of SR were calculated with a Q₁₀ model based on TS on a phenological (PT), or annual timescale (AT), or TS and soil volumetric water content (VWC) on a phenological (PTM) or annual (ATM) timescale. We found significant (p<0.01) difference in apparent Q₁₀ from year 2004 (1.23) and year 2005 (2.76) during the growth phenophase. Accounting for moisture-sensitivity increased model performance compared to temperature-only models: the error was -17% for the ATM model and -6% for the PTM model. The annual models consistently underestimated SR in summer and overestimated it in winter. These biases were reduced by delineating SR by tree phenophases and accounting for variation in soil moisture. Even though the bias of annual models in winter SR was small in absolute scale, the relative error was about 91%, and may thus have significant implications for regional and continental C balance estimates.
The age-dependent variability of ecosystem carbon (C) (fluxes was assessed by measuring the net ecosystem exchange of C (NEE) in five managed forest stands in northern Wisconsin, USA. The study sites ...ranged in age from 3-year-old clearcut to mature stands (65 years). All stands, except the clearcut, accumulated C over the study period from May to October 2002. Seasonal NEE estimates were -655 plus or minus 17.5 g C m super(-2) in the mature, hardwood (MHW), -648 plus or minus 16.8 in the mature red pine (MRP), -195 plus or minus 15.6 in the pine barrens (PB), +128 plus or minus 17.1 In the young hardwood clearcut (YHW), and -313 plus or minus 14.6 In the young red pine (YRP). The age-dependent differences were similar in the hardwood and conifer forests. Even though PB was not part of either the hardwood or conifer chronosequence, and had a different disturbance agent, it still fits the same general age relationship. Higher ecosystem respiration (ER) in the young than in the mature stands was the combined result of earlier soil warming in spring, and higher temperature and greater biological activity in summer, as Indicated by temperature-normalized respiration rates. The fire-generated PB had lower ER than the harvest-generated YHW and YRP, where high ER was sustained partly on account of logging residue. During the main growing season, the equivalent of 31 (MHW), 48 (MRP), 68 (PB), 114 (YHW) and 71% (YRP) of daily gross ecosystem production (GEP) was released in ER during the same day. The lower ER:GEP ratio in the mature stands was driven by greater age-dependent changes in ER than GEP. The magnitude of the increase in ER:GEP ratio In spring and fall was Interpreted as the extent of the decoupling of ER and GEP. Decoupling (sustained high ER despite decreasing GEP) was observed In YHW, PB and MHW, whereas in coniferous stands (MRP and YRP) the stable ER:GEP ratio suggested preferential use of new photosynthates in ER. The results indicate that a great part of the variation in landscape-level C fluxes can be accounted for by mean stand age and associated parameters, which highlights the need to consider this source of heterogeneity In regional C balance estimates.
Two aspen (Populus tremuloides Michx.) clones, differing in O^sub 3^ tolerance, were grown in a free-air CO^sub 2^ enrichment (FACE) facility near Rhinelander, Wisconsin, and exposed to ambient air, ...elevated CO^sub 2^, elevated O^sub 3^ and elevated CO2+O3. Leaf instantaneous light-saturated photosynthesis (P ^sub S^) and leaf areas (A) were measured for all leaves of the current terminal, upper (current year) and the current-year increment of lower (1-year-old) lateral branches. An average, representative branch was chosen from each branch class. In addition, the average photosynthetic rate was estimated for the short-shoot leaves. A summing approach was used to estimate potential whole-plant C gain. The results of this method indicated that treatment differences were more pronounced at the plant- than at the leaf- or branch-level, because minor effects within modules accrued in scaling to plant level. The whole-plant response in C gain was determined by the counteracting changes in P ^sub S^ and A. For example, in the O^sub 3^-sensitive clone (259), inhibition of P ^sub S^ in elevated O^sub 3^ (at both ambient and elevated CO^sub 2^) was partially ameliorated by an increase in total A. For the O^sub 3^-tolerant clone (216), on the other hand, stimulation of photosynthetic rates in elevated CO^sub 2^ was nullified by decreased total A.PUBLICATION ABSTRACT
Increased radiative forcing is an inevitable part of global climate change, yet little is known of its potential effects on the energy fluxes in natural ecosystems. To simulate the conditions of ...global warming, we exposed peat monoliths (depth, 0.6 m; surface area, 2.1 m2) from a bog and fen in northern Minnesota, USA, to three infrared (IR) loading (ambient, +45, and +90${\rm W}\ {\rm m}^{-2}$) and three water table (-16, -20, and -29 cm in bog and -1, -10 and -18 cm in fen) treatments, each replicated in three mesocosm plots. Net radiation (Rn) and soil energy fluxes at the top, bottom, and sides of the mesocosms were measured in 1999, 5 years after the treatments had begun. Soil heat flux (G) increased proportionately with IR loading, comprising about 3%-8% of Rn. In the fen, the effect of IR loading on G was modulated by water table depth, whereas in the bog it was not. Energy dissipation from the mesocosms occurred mainly via vertical exchange with air, as well as with deeper soil layers through the bottom of the mesocosms, whereas lateral fluxes were 10-20-fold smaller and independent of IR loading and water table depth. The exchange with deeper soil layers was sensitive to water table depth, in contrast to G, which responded primarily to IR loading. The qualitative responses in the bog and fen were similar, but the fen displayed wider seasonal variation and greater extremes in soil energy fluxes. The differences of G in the bog and fen are attributed to differences in the reflectance in the long waveband as a function of vegetation type, whereas the differences in soil heat storage may also depend on different soil properties and different water table depth at comparable treatments. These data suggest that the ecosystem-dependent controls over soil energy fluxes may provide an important constraint on biotic response to climate change.
Micrometeorological measurements of nighttime ecosystem respiration can be systematically biased when stable atmospheric conditions lead to drainage flows associated with decoupling of air flow above ...and within plant canopies. The associated horizontal and vertical advective fluxes cannot be measured using instrumentation on the single towers typically used at micrometeorological sites. A common approach to minimize bias is to use a threshold in friction velocity, u sub(*) sub(,) to exclude periods when advection is assumed to be important, but this is problematic in situations when in-canopy flows are decoupled from the flow above. Using data from 25 flux stations in a wide variety of forest ecosystems globally, we examine the generality of a novel approach to estimating nocturnal respiration developed by van Gorsel et al. (van Gorsel, E., Leuning, R., Cleugh, H.A., Keith, H., Suni, T., 2007. Nocturnal carbon efflux: reconciliation of eddy covariance and chamber measurements using an alternative to the u sub(*)-threshold filtering technique. Tellus 59B, 397-403, Tellus, 59B, 307-403). The approach is based on the assumption that advection is small relative to the vertical turbulent flux (F sub(C)) and change in storage (F sub(S)) of CO sub(2) in the few hours after sundown. The sum of F sub(C) and F sub(S) reach a maximum during this period which is used to derive a temperature response function for ecosystem respiration. Measured hourly soil temperatures are then used with this function to estimate respiration R sub(R) sub(m) sub(a) sub(x). The new approach yielded excellent agreement with (1) independent measurements using respiration chambers, (2) with estimates using ecosystem light-response curves of F sub(c)+F sub(s) extrapolated to zero light, R sub(L) sub(R) sub(C), and (3) with a detailed process-based forest ecosystem model, R sub(c) sub(a) sub(s) sub(t). At most sites respiration rates estimated using the u sub(*)-filter, R sub(u) sub(s) sub(t), were smaller than R sub(R) sub(m) sub(a) sub(x) and R sub(L) sub(R) sub(C). Agreement of our approach with independent measurements indicates that R sub(R) sub(m) sub(a) sub(x) provides an excellent estimate of nighttime ecosystem respiration.
Micrometeorological measurements of nighttime ecosystem respiration can be systematically biased when stable atmospheric conditions lead to drainage flows associated with decoupling of air flow above ...and within plant canopies. The associated horizontal and vertical advective fluxes cannot be measured using instrumentation on the single towers typically used at micrometeorological sites. A common approach to minimize bias is to use a threshold in friction velocity, u*, to exclude periods when advection is assumed to be important, but this is problematic in situations when in-canopy flows are decoupled from the flow above. Using data from 25 flux stations in a wide variety of forest ecosystems globally, we examine the generality of a novel approach to estimating nocturnal respiration developed by van Gorsel et al. (van Gorsel, E., Leuning, R., Cleugh, H.A., Keith, H., Suni, T., 2007. Nocturnal carbon efflux: reconciliation of eddy covariance and chamber measurements using an alternative to the u*-threshold filtering technique. Tellus 59B, 397–403, Tellus, 59B, 307-403). The approach is based on the assumption that advection is small relative to the vertical turbulent flux (FC) and change in storage (FS) of CO2 in the few hours after sundown. The sum of FC and FS reach a maximum during this period which is used to derive a temperature response function for ecosystem respiration. Measured hourly soil temperatures are then used with this function to estimate respiration RRmax. The new approach yielded excellent agreement with (1) independent measurements using respiration chambers, (2) with estimates using ecosystem light-response curves of Fc + Fs extrapolated to zero light, RLRC, and (3) with a detailed process-based forest ecosystem model, Rcast. At most sites respiration rates estimated using the u*-filter, Rust, were smaller than RRmax and RLRC. Agreement of our approach with independent measurements indicates that RRmax provides an excellent estimate of nighttime ecosystem respiration.
