B32B-01 INVITED
The Effects of Disturbance on Forest Carbon Processes
We use data from flux sites to examine patterns of response to disturbance among forest types. Forest development is punctuated by disturbances associated with fire and harvest, which reduces photosynthesis and possibly increases microbial respiration. After stand-replacing disturbance, NPP tends to be low in early succession and heterotrophic respiration may be high, low, or relatively similar to that of undisturbed areas depending on the disturbance characteristics. Effects of wildfire differ from harvest in that wildfire leaves more standing dead trees that can take decades to fall, and charring reduces decomposition potential. High intensity fires can reduce soil respiration for several years due to root mortality and reduced microbial activity. Clearcut harvesting adds a large amount of dead plant material to the forest floor immediately, which may be left on-site or partially remove through management activities. These factors differentially affect the trajectory of respiration rates over time. Differences may also exist between productivity rates after disturbance associated with natural succession and tree planting. In addition, carbon allocation patterns change with time since disturbance, altering above- and belowground sources and sinks. The balance of these processes is net ecosystem production (NEP), and variation in regeneration, mortality, and decomposition affect NEP over time. Process models and LUE models should consider these disturbance effects when mapping regional carbon balances.
B32B-02
Unravelling nitrogen deposition effects on carbon cycling in forests
Nitrogen (N) limitation constrains forest productivity over large areas. Where N is suboptimal, provision of N through fertilization or atmospheric deposition stimulates gross primary productivity (GPP) directly or by enabling forests to benefit from increased atmospheric CO2. Nitrogen deposition also enhances carbon sequestration, but the mechanisms via which this stimulation occurs have so far not been unravelled. Here we analyze observations from 133 forests to decipher how it influences the fate of the absorbed carbon. We find that N-deposition stimulate GPP which, in turn, results in an increase in biomass production (NPP). Whilst the increase in woody biomass production is proportional to the increase in GPP, a disproportional high amount of carbon is used for the production short-lived tissues such as roots and foliage, thereby increasing carbon-inputs to the soil. Nitrogen deposition also strongly retarded heterotrophic respiration and the associated carbon-losses from the soil. Thus, N-deposition strengthened the CO2 sink in the boreal and temperate regions, which are typically N-limited. A future shift in N-deposition towards tropical regions will not necessarily result in a similar increased sink-strength, because these regions are typically not N-limited but suffer from phosphorus, potassium and molybdenum deficiencies.
B32B-03
Productivity, Respiration, CO2 Sink Potential, and Light-Response Parameters of World Grasslands Derived From Flux-Tower Data Partitioning
Net CO2 flux (Fc) data from 52 grassland flux tower sites in North America, Europe, and Asia representing 101 years of measurements were partitioned into gross primary productivity (Pg) and ecosystem respiration (Re) components using light-temperature-response functions method (Bas. Appl. Ecol. 2003, 4:167-183). This sample of sites encompasses a wide climatic range (mean annual temperatures 0.5 to 20° C and precipitation 190 to 1500 mm/yr) and includes unmanaged and extensively or intensively managed grasslands. Highest values of daily gross primary productivity (Pg,max = 64 g CO2/m2/d) were found in intensively managed grasslands of W. Europe with Atlantic climate, while lowest Pg,max (<10 g CO2/m2/d) were recorded in mixed prairies and shortgragss steppes of the Great Plains, shrubsteppes of the Intermountain West, and dry steppes of Mongolia. Maximum values of gross primary production (GPP) were achieved in intensively managed European grasslands with Atlantic climate (GPP=6900 g CO2/m2/yr), and lowest GPP values (<500 g CO2/m2/yr) were estimated for grazed mixed prairies under drought. Highest annual ecosystem respiration (RE) were recorded for warm-temperate and low montane W.European grasslands (RE > 5000 g CO2/m2/yr), and lowest RE were characteristic for shrubsteppes and grazed dry steppes (RE < 1000 g CO2/m2/yr). In 62 out of 101 site-years, the grasslands were net CO2 sinks, with average net annual CO2 exchange NEE = 250, maximum NEE = 2400 (uptake), and minimum NEE = –1350 g CO2/m2/yr (release). Maximum mean weekly values of the apparent quantum yield (α >70 mmol CO2/mol photons) were recorded in intensively managed Atlantic grasslands, the lowest weekly quantum efficiencies were observed in grazed mixed prairies of N.America and dry steppes of Central Asia (α <10 mmol/mol). The maximum values of the gross photosynthesis parameter (mean weekly Amax>2 mg CO2/m2/s) were estimated for intensively managed C3 grasslands of W.Europe and for unmanaged tallgrass (C3/C4) prairies. Maximum daytime respiration (rDay) were determined in intensively managed warm-temperate grasslands of W.Europe and E.North America (mean weekly rDay > 0.4 mg CO2/m2/s), while lowest rDay values are characteristic for semiarid mixed prairies and shrubsteppes (rDay<0.1 mg CO2/m2/s). Gross ecological light-use efficiency (ε=daily gross photosynthesis/daily income of photosynthetically active radiation) achieves maximum in intensively managed European grasslands with Atlantic climate (mean weekly ε >35 mmol/mol), and has lowest values in semiarid short- and mixed prairies and shrubsteppes of N. America and dry steppes of Central Asia (mean weekly ε <5 mmol/mol). Our data show significant allometric relationships between light-response parameters: α(Amax) and rDay(Amax), with allometry exponent <1 in both cases. Both the CO2 exchange characteristics (Pg,max, Re,max, GPP, RE, NEE), and the light-response parameters (α, Amax, rDay, ε) demonstrate significant relationships to climate (growing season length, sum of temperatures above 5° C, hydrologic year precipitation). These relationships become even stronger after stratification with respect to management (e.g., unmanaged vs. managed grasslands).
B32B-04
What is the seasonality of photosynthesis across the Amazon basin?: a cross-site analysis of eddy flux tower measurements from BrasilFlux network
Amazonian forests play an important and complex role in the global carbon cycle, contributing substantially to increases (via land use change emissions) and possibly to net sequestration (in intact forests) of atmospheric CO2. Predicting these processes of net carbon uptake and release depends crucially on understanding ecosystem response to both seasonal and interannual variations. To better understand these discrepancies, we investigated the seasonal and spatial patterns of Amazonian forest photosynthetic activity, and the effects of land-use conversion thereon, by integrating data from a network of ground-based eddy flux towers in Brazil (the new "BrasilFlux" network). These flux towers, a legacy of the ‘Large- Scale Biosphere Atmosphere Experiment in Amazonia' (LBA) project, have now produced multi-year datasets. The pattern of photosynthesis at towers across the central Amazon (5°N-5°S) shows little evidence of seasonal water limitation, as they all sustain high -- or even increasing -- levels of GEP as the dry season progresses. Towers along the southern flank of the Amazon, in converted pasture sites, and in savanna, by contrast, exhibit seasonal patterns consistent with varying degrees of water stress.
B32B-05
Functional convergence of tundra vegetation simplifies the interpretation of flux observations at larger spatial scales
A central challenge in terrestrial carbon cycle research is upscaling measurements of vegetation function to larger spatial and temporal scales. A solution is required to, for example, make chamber-based measurements relevant at larger spatial scales and to make eddy covariance measurements applicable to leaf or chamber- based studies. Here we demonstrate that a simple model for photosynthesis and ecosystem respiration parameterized using pan-arctic chamber flux measurements closely matches eddy covariance flux observations in a tundra ecosystem near Abisko, Sweden. The agreement holds when using a generic parameter set that does not account for vegetation type or measurement location. Inverting the model to predict leaf area using eddy covariance-measured net ecosystem exchange closely approximates tower-based LAI estimates across seasons and during periods of drought stress. Thus, recent findings documenting functional convergence of arctic vegetation holds at multiple spatial as well as temporal scales using both chamber and tower measurements. After validating the model using the eddy covariance measurements, we integrate the model with meteorological and LAI observations using a simple data assimilation scheme. The reduction of error achieved via data assimilation is compared to standard techniques of estimating eddy covariance error. Our analysis demonstrates that accurate estimates of C flux at multiple spatial scales across the tundra biome are possible given accurate estimates of photosynthetically active radiation, temperature, and leaf area index given the observed functional convergence of tundra vegetation.
B32B-06
The importance of intrinsic factors in driving interannual Net Ecosystem Production (NEP) variability
Interannual Net Ecosystem Production (NEP) variability is the result of external forcing from the environment (i.e. extrinsic) and internal forcing from biological lags, carryovers, and feedbacks (i.e. intrinsic). Attribution of interannual NEP variability to intrinsic factors has been difficult partly because of the challenge of separating interactions between extrinsic and intrinsic factors from a single measure of ecosystem function. We elucidated the importance of intrinsic controls in driving interannual NEP variations by comparing long term records of eddy covariance data to other independent measures of ecosystem function (i.e. ring width chronologies, remote sensing indices, and biomass harvests) for a late successional Boreal Forest (BF) and also a Temperate Freshwater Marsh (TFM). Analyses indicated that intrinsic controls on NEP were strong in both ecosystems. Carryover effects associated with carbohydrate pools resulted in poor relationships between annual gross carbon uptake (GPP) and ring width at the BF. Feedbacks and carryover effects associated with the accumulation of standing litter and subsequent shading of future plant generations resulted in large interannual differences between remotely sensed vegetation indices, NEP, GPP, and carbon uptake per unit leaf area at the TFM. Small scale manipulations of standing litter coupled with measurements of NEE and surface reflectance further supported the importance of standing litter as an intrinsic control at the TFM. A water table drawdown at the TFM suppressed leaf area production and carbohydrate translocation, and delayed the seasonal cycle of Net Ecosystem Exchange (NEE) in the following year. Our analysis demonstrates that interannual variability in NEP involves both intrinsic and extrinsic factors and that attributing these factors to interannual variability in NEP requires a multifaceted approach.
B32B-07
The CEIP-ADVEX Campaigns: An Experimental Approach to Measure Advective CO2 Fluxes and Their Impact on NEE at Three European FLUXNET Forest Sites
This study presents mean characteristics of the advective CO2 fluxes in a first site-to-site comparison and evaluates the main problems for future investigation of advection mechanisms. Extensive field measurements have been performed at three European FLUXNET forest sites with different topography (Renon/Ritten, Italian Alps, Italy; Wetzstein, Thuringia, Germany; Norunda, Uppland, Sweden) to evaluate the relevant terms of the carbon balance by measuring CO2 concentrations and the wind field in a 3D multi-tower cube setup [1]. The same experimental setup (geometry and instrumentation) and the same methodology for the calculation of the advective fluxes were applied to all the three experiments in order to provide a reliable base for a comprehensive site to site comparison. We present mean diurnal/nocturnal courses of all relevant terms of the carbon balance equation, i.e. turbulent flux, storage change and horizontal and vertical advection. It is shown that all sites are affected by advection in different ways and strength. The size of the averaged non-turbulent advective fluxes was of the same order of magnitude as the turbulent flux measured by eddy-covariance technique, but with considerable scatter. This implies that it is not advisable to use directly measured quantities of the non-turbulent advective fluxes for the estimation of net ecosystem exchange (NEE) on e.g. a hourly basis, though there may be useful information for a better understanding of the steering mechanisms of advection in this noisy data. Nevertheless, situations with and without advection were closely related to local or synoptic meteorological conditions and the topography of the respective site. Thus, it is possible to separate advection affected NEE estimates from fluxes which are representative of the source term. The main goal of ongoing analysis of the ADVEX data set is the development of a robust correction scheme for advection, based on a more detailed site specific analysis of single events for the identification of the relevant processes. [1] Feigenwinter, C., Bernhofer, C., Eichelmann, U., Heinesch, B., Hertel, M., Janous, D., Kolle, O., Lagergren, F., Lindroth, A., Minerbi, S., Moderow, U., Moelder, M., Montagnani, L., Queck, R., Rebmann, C., Vestin, P., Yernaux, M., Zeri, M., Ziegler, W. and Aubinet, M., 2007, Comparison of horizontal and vertical advective CO2 fluxes at three forest sites. Agric. Forest Meteorol., in press
B32B-08
Carbon Balance Assessment of a Natural Steppe of Southern Siberia by Multiple Constraint Approach
Steppe ecosystems represent an interesting case in which the assessment of carbon balance may be performed through a cross validation of the eddy covariance measurements against ecological inventory estimates of carbon exchanges (Ehman et al., 2002; Curtis et al., 2002). Indeed, the widespread presence of ideal conditions for the applicability of the eddy covariance technique, as vast and homogeneous grass vegetation cover over flat terrains (Baldocchi, 2003), make steppes a suitable ground to ensure a constrain to flux estimates with independent methodological approaches. We report about the analysis of the carbon cycle of a true steppe ecosystem in southern Siberia during the growing season of 2004 in the framework of the TCOS-Siberia project activities performed by continuous monitoring of CO2 fluxes at ecosystem scale by the eddy covariance method, fortnightly samplings of phytomass, and ingrowth cores extractions for NPP assessment, and weekly measurements of heterotrophic component of soil CO2 effluxes obtained by an experiment of root exclusion. The carbon balance of the monitored natural steppe was, according to micrometeorological measurements, a sink of carbon of 151.7±36.9 gC m-2, cumulated during the growing season from May to September. This result was in agreement with the independent estimate through ecological inventory which yielded a sink of 150.1 gC m-2 although this method was characterized by a large uncertainty \(±130%\) considering the 95% confidence interval of the estimate. Uncertainties in belowground process estimates account for a large part of the error. Thus, in particular efforts to better quantify the dynamics of root biomass (growth and turnover) have to be undertaken in order to reduce the uncertainties in the assessment of NPP. This assessment should be preferably based on the application of multiple methods, each one characterized by its own merits and flaws.