David Schimel, Britt Stephens, Stephan de Wekker, Jielun Sun, Bill Sacks, Steve Aulenbach, National Center for Atmospheric Research

Russ Monson, University of Colorado, Dennis Ojima, Colorado State University, Steve Running University of Montana, Chun-Ta Lai, University of Utah

ACME + CME: goals, methods , results

Carbon in the Mountains Experiment (CME)

Motivation: Carbon exchange budgets at regional scales serve to test and confirm top-down estimates made from continental-to-global scale observations at a scale where fluxes can be plausibly linked to land management, climate and other processes. A number of such studies are now in progress in the US, Europe, Australia and elsewhere. These studies share the common methodological characteristic that they rely on techniques whose accuracy is compromised in difficult terrain, such as highly fragmented landscapes and mountains. In North America, a significant fraction of ecosystem carbon uptake is located in mountainous regions. Figure 1 shows the distribution of modeled net primary production and net ecosystem exchange overlain on schematic topography for the entire (a) and Western US. Many prior estimates of NEE have been based on estimates of NPP. Figure 1 shows the fallacy in this for the US: while Fig. 1a shows NPP to be dominated by low elevations, NEE is nearly evenly distributed between high and low elevations. In addition, the efficiency of stabilization of carbon (NEE/NPP) is low at low elevations and high in the mountains. The explanation for this is simple: low elevations are dominated by herbaceous and agricultural elevation which is productive but does not produce long-lived tissue. Productivity in the mountains includes a high proportion of forest, which efficiently stores carbon as wood and high-lignin litter. The trends evident for the US as a whole (Fig. 1a) are exaggerated for the Western US (Fig. 1b) where, because of the arid and semi-arid climates of many of the lowlands, NPP and NEE are dominated by the high-elevation mountain regions where adequate water occurs, mainly as winter snowfall. Based on this analysis and our previous work, it is clear that any array of techniques that cannot make measurements in mountainous areas will leave approximately 50% of US net carbon uptake conjectural. While process models and inventory techniques provide constraints in the mountains, the translation of those techniques into actual land-atmosphere exchanges is poorly constrained without actual flux measurements.

The goals of the Airborne Carbon in the Mountains Experiment (ACME)-Carbon in the Mountains Experiment (CME) were to develop techniques for constraining fluxes in mountainous terrain, and to link those fluxes to underlying processes. Key problems in the mountains include large flux errors associated with horizontal topographic flows of air, difficulties in scaling over complex, heterogeneous terrain, complex hydrology due to seasonal snow cover, lateral flows and wind redistribution and challenges to remote sensing resulting from topographic effects on scene radiation.

ACME+CME combined ground based measurements of the advective component of fluxes, airborne measurements of regional NEE and Ecosystem Respiration fluxes, remote sensing and local and regional assimilation modeling. The study was conducted in the Colorado Rocky Mountains, anchored on the Niwot Ridge Long Term Ecological Research site (Niwot Ridge) and the affiliated Ameriflux site, in operation since 1998. ACME probed both mid-day NEE fluxes and the regional accumulation of nighttime respired CO2. CME focused on process-level understanding of the transport and accumulation of nighttime respired CO2. Assimilation models integrate the various aspects of the project. Simple assimilation models of local carbon fluxes (SIPnet) were used to partition the Niwot Ridge net exchange into photosynthesis, autotrophic and heterotrophic respiration and to estimate the seasonal and interannual sensitivity of those component fluxes to climate. Lagrangian inverse (STILT) and mesoscale variational assimilation models were used to translate airborne concentration profiles into regional flux “snapshots” to compare to bottom-up estimates. Finally, a comprehensive regional carbon modeling capability (the Integrated Regional Carbon model, IRC) is being developed, and can be parameterized and verified using the results from the two more focused assimilation exercises.

On the basis of preliminary results, a long-term measurement program “Regional Atmospheric Continuous CO2 Network in the Rocky Mountains” (Rocky RACCOON) has been initiated with two mountaintop stations and a valley bottom site currently operating, analyses contrasting site-level and regional carbon flux responses to seasonal climate forcing are in progress and planning has begun for a second Rocky Mountain intensive campaign, with coordinated airborne, ground-based and assimilation modeling components. The second campaign will interface with the Rocky Raccoon network, and more systematically sample the variations in daytime and nighttime fluxes at the regional scale.

Plant Carbon

Soil CarbonSoil Moisture

Drainage

Precip. Transpiration

Photosynthesis (Phenology,Soil Moisture,

Tair, VPD, PAR)

Plant Respiration(Plant C, Tair)

Litterfall(Plant C, Phenology)

Soil Respiration(Soil C, Soil Moisture,

Tsoil)

Figure 1 The motivation for ACME: modeled NPP and NEE over elevation for the US, b, as in a but for the West. The height of the red and green bars are proportional to the amount of total flux (NPP or NEE) in a given elevation bin. Note to get them on the same plot, NPP and NEE are scaled differently. The US as a whole has a significant amount of carbon uptake in low elevation regions, although a significant fraction is at high elevation, despite low NPP in those areas. This trend is exaggerated in the Western US, because of the aridity of low elevation regions. Note the dramatic differences in the efficiency in which the different elevation zones store carbon. High elevation areas are disproportionately important to NEE because of the abundance of mountain forests, which store more NPP than systems dominated by herbaceous vegetation.

The experimental layout for CME 04, showing flux towers, concentration towers and concentration transects for quantifying daytime turbulent and nighttime advective fluxes of CO2. The site is at Niwot Ridge, elevation 3050 m, in a subalpine evergreen forest. The site has several permanent (operated by CU and the USGS) towers, 3 NCAR facility towers (Willow, Pine and Aspen) installed for summer 2004, automated flask and continuous 13C measurements (operated by the University of Utah) and is co-located near the NOAA CMDL baseline monitoring site. The site has a gentle slope and is in the watershed of a small first-order stream, Como Creek. The lefthand false color image shows vegetation, while the right hand shows experimental detail and topography. The left-hand panel also shows sites from a 2002 pilot experiment.

ACME experimental approach. The airborne experiment combined semi-Lagrangian budgets aimed at estimating peak mid-day fluxes, early morning vertical profiles to estimate cumulative nocturnal accumulations and flux legs over subalpine and montane forest regions. 54 hours were flown in the NCAR C-130 at altitudes as low as 1000 ft agl. The aircraft was equiped with continuous carbon dioxide, carbon monoxide, ozone and fast water vapor sensors. Flask samples for 13CO2 and C18O2 were collected during flight, as was remote imagery and standard atmospheric state variables. The flight lines are shown over a modified version of the National Land Cover Database, adapted for carbon studies in montane regions by the Natural Resource Ecology Lab.

The SIPNET model was designed to simulate the most significant controls over variability in CO2 fluxes, including the diurnal cycle, while keeping the model as simple as possible. Using a simple model reduces the number of parameters, thus helping avoid the danger of over-fitting the observations during the optimization, and also reducing the computational intensity of the optimization. SIPNET describes carbon flux dynamics in two vegetation carbon pools and an aggregated soil carbon pool. There are six carbon fluxes in SIPNET: (1) gross ecosystem CO2 exchange (GEE; which is synonymous to gross primary productivity and gross photosynthesis) , (2) autotrophic respiration (RA), (3) heterotrophic respiration (RH), (4) new leaf growth, (5) formation of leaf litter, and (6) formation of wood litter. We use the same model structure for calculating GEE as is used for gross photosynthesis in PnET. Autotrophic respiration in SIPNET is represented as the sum of two respiratory terms: foliar and wood respiration. Heterotrophic respiration represents the respiration of microbes in the model’s single aggregated soil pool. Respiration rate is assumed to be proportional to the amount of soil carbon, and the temperature dependence is modeled with an exponential temperature function (similar to autotrophic respiration, but using soil temperature rather than air temperature). Heterotrophic respiration is also modified by a soil moisture function. During assimilation the parameters of the model are optimized using a Markov Chain Monte Carlo approach. The left panel shows the model structure and the right a typical multi-year fit.

Results from CME showing CO2 concentrations at several heights across the drainage and advective flowpath. The 4-m data show that gravitational flows concentrate CO2 strongly along the axis of the drainage, although at the lowest level, concentrations immediately above the stream are depressed. Similarly (data not shown) CO2 also concentrates going downslope along the line of towers. Gravitational flows occur in extremely shallow layers due to the cold nights associated with radiational cooling and efficiently transport CO2 downhill possibly leading to the large concentrations in the intermountain valleys shown below.

SIPNET partitions observed NEE (left, above) into GPP (right above) and respiration (not shown). Most of the variability in NEE is associated with either the timing of spring (left, above) or the mid-summer drought. Early springs lead to high early carbon uptake, due to enhanced early season GPP (above, right) but also tend to deplete soil moisture rapidly, leading to more severe summer drought. The panel to the left shows that longer growing seasons (due almost entirely to spring variability) actually lead to lower NEE (less negative), contrary to the trends observed at and across many sites. In the subalpine, the effects of spring temperature anomalies on NEE depend greatly on interactions with the water budget.

Local observations at Niwot Ridge and regional observations from boundary layer budgets show surprising convergence in mid-summer with the airborne snapshots of mid-day flux rates being quite similar to local fluxes at Niwot during the same hours. The continuous curves are the actual and long term average Niwot data for the year/day of the flight, and the average for Harvard Forest as a low altitude reference site. The horizontal bar indicates the average boundary layer budget flux and the vertical lines are the estimated uncertainty; The aircraft “footprint” includes a range of vegetation types and without additional data, the degree to which Niwot represents the region cannot be assessed. Future studies will improve and coordinate the ground and air sampling schemes.

Regional accumulation of respiration: In one of the most surprising findings of ACME, we found that “pools” of CO2 in large intermountain valleys caused by nighttime downslope flows were still evident up to 500-1000 m agl at 9-10 AM. If these pools or lakes of CO2 can be quantitatively related to regional fluxes, then they can provide a constraint on respiration rates at scales inaccessible in seemingly simpler landscapes.

Conclusions:• Regional mid-day fluxes can be measured in complex terrain using boundary layer budgeting techniques.•Small-scale drainage flows may be very predictable and in many situations will allow budgeting of the advective component of nighttime respiration.• Large-scale drainage flows may allow the use of intermountain catchments as a large-scale constraint on respiration, a possibility absent in “simpler” landscapes.• In this region, longer growing seasons result in reduced carbon uptake due to temperature-water interactions.• Generally, temperature responses of carbon exchange depend on moisture availability.

Elevation, cumulative %Elevation, cumulative %