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A simple net ecosystem productivity model for gap filling of tower-based fluxesZisheng, Xing January 2007 (has links)
In response to global climate change, many important earth-systems-oriented science
programs have been established in the past. One such program, the Fluxnet program, studies
the response of world forests and other natural ecosystems by measuring biospheric fluxes of
carbon dioxide (CO2), water vapour, and energy with eddy-covariance (EC) techniques to
assess the role of world ecosystems in offsetting increases in CO2 emissions and related
impacts on global climate. The EC methodology has its limitations particularly when
weather is inclement and during system stoppages. These limitations create non-trivial
problems by creating data gaps in the monitored data stream, diminishing the integrity of the
dataset and increasing uncertainty with data interpretation.
This Thesis deals with the development of a parsimonious, semi-empirical approach
for gap filling of net ecosystem productivity (NEP) data. The approach integrates the effects
of environmental controls on diurnal NEP. The approach, because of its limited number of
parameters, can be rapidly optimized when appropriate meteorological, site, and NEP target
values are provided. The procedure is verified by applying it to several gap-filling case
studies, including timeseries collected over balsam fir (Abies Balsamea (L.) Mill.) forests in
New Brunswick (NB), Canada and several other forests along a north-south temperaturemoisture
gradient from northern Europe to the Middle East. The evaluation showed that the
model performed relatively well for most sites; i.e., r2 ranged from 0.68-0.83 and modelling
efficiencies, from 0.89-0.97, demonstrating the possibility of applying the model to forests
outside NB. Inferior model performance was associated with sites with less than complete
input datasets.
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Fluxes of carbon and water in a Pinus radiata plantation and a clear-cut, subject to soil water deficitArneth, Almut January 1998 (has links)
This thesis investigates the abiotic control of carbon (C) and water vapour fluxes (FCO₂ and E, respectively) in a New Zealand Pinus radiata D. Don plantation and a nearby clearcut. It concentrates on the limitation of these fluxes imposed by growing season soil water deficit. This results from low precipitation (658 mm a⁻¹) in combination with a limited root zone water storage capacity of the very stony soil (> 30% by volume). The thesis analyses results from seven eddy covariance flux measurement campaigns between November 1994 and March 1996. The study site was located in Balmoral Forest, 100 km north-west of Christchurch (42° 52' S, 172° 45' E), in a (in November 1994) 8-year-old stand. One set of measurements was conducted in an adjacent clearcut. Ecosystem flux measurements were accompanied by separate measurements of ground fluxes and of the associated environmental variables. Flux analysis focussed on the underlying processes of assimilation (Ac), canopy stomatal conductance (Gc) and respiration (Reco), using biophysical models coupled to soil water balance and temperature subroutines. Aiming to link time inegrated net ecosystem C (NEP) to tree growth, sequestration in tree biomass (NPP) was quantified by regular measurements of stem diameter using allometric relationships. Average rates of FCO₂ and E were highest in spring (324 mmol m⁻² d⁻¹ and 207 mol m⁻² d⁻¹, respectively) when the abiotic environment was most favourable for Gc and Ac. During summer, fluxes were impeded by the depletion of available soil water (θ) and the co-occurrence of high air saturation deficit (D) and temperature (T) and were equal or smaller than during winter (FCO₂ = 46 mmol m⁻² d⁻¹ in summer and 115 mmol m⁻² d⁻¹ in winter; E = 57 and 47 mol m⁻² d⁻¹, respectively). With increasingly dry soil, fluxes and their associated ratios became predominantly regulated by D rather than quantum irradiance, and on particularly hot days the ecosystem was a net C source. Interannually, forest C and water fluxes increased strongly with rainfall, and the simultaneously reduced D and T. For two succeeding years, the second having 3 % more rain, modelled NEP was 515 and 716 g C m⁻² a⁻¹, Ac 1690 and 1841 g C m⁻² a⁻¹ and Reco 1175 and 1125 g C m⁻² a⁻¹. NEP / E increased in wetter (and cooler) years (1.3 and 1.5 g kg⁻¹), reflecting a relatively larger gain in NEP. Responding mainly to increased rainfall during commonly dry parts of the year (ie summer), and reflecting the otherwise benign maritime climate of New Zealand, NEP during the winter months could exceed NEP during the middle of the notional tree growing season. Annual Ac, NEP, and NPP were strongly linearly related. This relation did not hold during bi-weekly periods when the processes of intermediate C storage were influential. Separate knowledge of tree growth and C fluxes allowed quantification of autotrophic, and heterotrophic respiration (Rhet≈ 0.4 NEP), as well as fine-root turnover (≈0.2 NEP). The ratio of NEP and stem volume growth was conservative (0.24 t C m⁻³) and allows a direct connection to be made between ecosystem carbon fluxes and forest yield tables. In the absence of living roots, the clearcut flux measurements demonstrated the expected limitation of Rhet by soil temperature (Ts) and θ. However, an additional 'pumping effect' was discovered at the open site whereby turbulence increased CO₂ efflux considerably when the soil surface was wet. Accounting for the combined effects of Ts, θ and turbulence, annual Rhet at the clear-cut site (loss to the atmosphere) was »50 % of NEP (C sequestered from the atmosphere) in the nearby forest. Clearly, there is an important contribution of C fluxes during early stages of ecosystem development to the total C sequestered over the lifetime of a plantation.
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