55 research outputs found
Estimating Canopy Gap Fraction Using ICESat GLAS within Australian Forest Ecosystems
Spaceborne laser altimetry waveform estimates of canopy Gap Fraction (GF) vary withrespect to discrete return airborne equivalents due to their greater sensitivity to reflectance differencesbetween canopy and ground surfaces resulting from differences in footprint size, energy thresholding,noise characteristics and sampling geometry. Applying scaling factors to either the ground or canopyportions of waveforms has successfully circumvented this issue, but not at large scales. This studydevelops a method to scale spaceborne altimeter waveforms by identifying which remotely-sensedvegetation, terrain and environmental attributes are best suited to predicting scaling factors basedon an independent measure of importance. The most important attributes were identified as: soilphosphorus and nitrogen contents, vegetation height, MODIS vegetation continuous fields productand terrain slope. Unscaled and scaled estimates of GF are compared to corresponding ALS datafor all available data and an optimized subset, where the latter produced most encouraging results(R2 = 0.89, RMSE = 0.10). This methodology shows potential for successfully refining estimates ofGF at large scales and identifies the most suitable attributes for deriving appropriate scaling factors.Large-scale active sensor estimates of GF can establish a baseline from which future monitoringinvestigations can be initiated via upcoming Earth Observation missions
OzFlux data: network integration from collection to curation
Measurement of the exchange of energy and mass between the surface and the atmospheric boundary-layer by the eddy covariance technique has undergone great change in the last 2 decades. Early studies of these exchanges were
confined to brief field campaigns in carefully controlled conditions followed by months of data analysis. Current practice is to run tower-based eddy covariance systems continuously over several years due to the need for continuous monitoring as part of a global effort to develop local-, regional-, continental- and global-scale budgets of carbon, water and energy. Efficient methods of processing the increased quantities of data are needed to maximise the time available for analysis and interpretation. Standardised methods are needed to remove differences in data processing as possible contributors to observed spatial variability. Furthermore, public availability of these data sets assists with undertaking global research efforts. The OzFlux data path has been developed (i) to provide a standard set of quality control and post-processing tools across the network, thereby facilitating inter-site integration and spatial comparisons; (ii) to increase the time available to researchers for analysis and interpretation by reducing the time spent collecting and processing data; (iii) to propagate both data and metadata to the final product; and (iv) to facilitate the use of the OzFlux data by adopting a standard file format and making the data available from web-based portals. Discovery of the OzFlux data set is facilitated through incorporation in FLUXNET data syntheses and the publication of collection metadata via the RIFCS format. This paper serves two purposes. The first is to describe the data sets, along with their quality control and post-processing, for the other papers of this Special Issue. The second is to provide an example of one solution to the data collection and curation challenges that are encountered by similar flux tower networks worldwide.J. Beringer is funded under an
ARC FT (FT1110602)
Net ecosystem carbon exchange of a dry temperate eucalypt forest
Forest ecosystems play a crucial role in the global carbon cycle by
sequestering a considerable fraction of anthropogenic CO<sub>2</sub>, thereby
contributing to climate change mitigation. However, there is a gap in our
understanding about the carbon dynamics of eucalypt (broadleaf evergreen)
forests in temperate climates, which might differ from temperate evergreen
coniferous or deciduous broadleaved forests given their fundamental
differences in physiology, phenology and growth dynamics. To address this gap
we undertook a 3-year study (2010–2012) of eddy covariance measurements in a
dry temperate eucalypt forest in southeastern Australia. We determined the
annual net carbon balance and investigated the temporal (seasonal and
inter-annual) variability in and environmental controls of net ecosystem
carbon exchange (NEE), gross primary productivity (GPP) and ecosystem
respiration (ER). The forest was a large and constant carbon sink throughout
the study period, even in winter, with an overall mean NEE of
−1234 ± 109 (SE) g C m<sup>−2</sup> yr<sup>−1</sup>. Estimated annual ER was
similar for 2010 and 2011 but decreased in 2012 ranging from 1603 to
1346 g C m<sup>−2</sup> yr<sup>−1</sup>, whereas GPP showed no significant
inter-annual variability, with a mean annual estimate of
2728 ± 39 g C m<sup>−2</sup> yr<sup>−1</sup>. All ecosystem carbon fluxes had a
pronounced seasonality, with GPP being greatest during spring and summer and
ER being highest during summer, whereas peaks in NEE occurred in early spring
and again in summer. High NEE in spring was likely caused by a delayed
increase in ER due to low temperatures. A strong seasonal pattern in
environmental controls of daytime and night-time NEE was revealed. Daytime
NEE was equally explained by incoming solar radiation and air temperature,
whereas air temperature was the main environmental driver of night-time NEE.
The forest experienced unusual above-average annual rainfall during the first
2 years of this 3-year period so that soil water content remained relatively
high and the forest was not water limited. Our results show the potential of
temperate eucalypt forests to sequester large amounts of carbon when not
water limited. However, further studies using bottom-up approaches are needed
to validate measurements from the eddy covariance flux tower and to account
for a possible underestimation in ER due to advection fluxes
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Integration of MODIS land and atmosphere products with a coupled-process model to estimate gross primary productivity and evapotranspiration from 1 km to global scales
We propose the Breathing Earth System Simulator (BESS), an upscaling approach to quantify global gross primary productivity and evapotranspiration using MODIS with a spatial resolution of 1-5 km and a temporal resolution of 8 days. This effort is novel because it is the first system that harmonizes and utilizes MODIS Atmosphere and Land products on the same projection and spatial resolution over the global land. This enabled us to use the MODIS Atmosphere products to calculate atmospheric radiative transfer for visual and near infrared radiation wave bands. Then we coupled atmospheric and canopy radiative transfer processes, with models that computed leaf photosynthesis, stomatal conductance and transpiration on the sunlit and shaded portions of the vegetation and soil. At the annual time step, the mass and energy fluxes derived from BESS showed strong linear relations with measurements of solar irradiance (r(2) = 0.95, relative bias: 8%), gross primary productivity (r(2) = 0.86, relative bias: 5%) and evapotranspiration (r(2) = 0.86, relative bias: 15%) in data from 33 flux towers that cover seven plant functional types across arctic to tropical climatic zones. A sensitivity analysis revealed that the gross primary productivity and evapotranspiration computed in BESS were most sensitive to leaf area index and solar irradiance, respectively. We quantified the mean global terrestrial estimates of gross primary productivity and evapotranpiration between 2001 and 2003 as 118 +/- 26 PgC yr(-1) and 500 +/- 104 mm yr(-1) (equivalent to 63,000 +/- 13,100 km(3) yr(-1)), respectively. BESS-derived gross primary productivity and evapotranspiration estimates were consistent with the estimates from independent machine-learning, data-driven products, but the process-oriented structure has the advantage of diagnosing sensitivity of mechanisms. The process-based BESS is able to offer gridded biophysical variables everywhere from local to the total global land scales with an 8-day interval over multiple years.Keywords: Energy fluxes,
Water vapor exchange,
Foliage clumping index,
Carbon dioxide,
Stomal conductance,
Net ecosystem exchange,
Sitchensis bong carr,
Radiative transfer,
Oak hickory forest,
Leaf area inde
Productivity and evapotranspiration of two contrasting semiarid ecosystems following the 2011 global carbon land sink anomaly
© 2016 Elsevier B.V. Global carbon balances are increasingly affected by large fluctuations in productivity occurring throughout semiarid regions. Recent analyses found a large C uptake anomaly in 2011 in arid and semiarid regions of the southern hemisphere. Consequently, we compared C and water fluxes of two distinct woody ecosystems (a Mulga (Acacia) woodland and a Corymbia savanna) between August 2012 and August 2014 in semiarid central Australia, demonstrating that the 2011 anomaly was short-lived in both ecosystems. The Mulga woodland was approximately C neutral but with periods of significant uptake within both years. The extreme drought tolerance of Acacia is presumed to have contributed to this. By contrast, the Corymbia savanna was a very large net C source (130 and 200gCm-2yr-1 in average and below average rainfall years, respectively), which is likely to have been a consequence of the degradation of standing, senescent biomass that was a legacy of high productivity during the 2011 anomaly. The magnitude and temporal patterns in ecosystem water-use efficiencies (WUE), derived from eddy covariance data, differed across the two sites, which may reflect differences in the relative contributions of respiration to net C fluxes across the two ecosystems. In contrast, differences in leaf-scale measures of WUE, derived from 13C stable isotope analyses, were apparent at small spatial scales and may reflect the different rooting strategies of Corymbia and Acacia trees within the Corymbia savanna. Restrictions on root growth and infiltration by a siliceous hardpan located below Acacia, whether in the Mulga woodland or in small Mulga patches of the Corymbia savanna, impedes drainage of water to depth, thereby producing a reservoir for soil moisture storage under Acacia while acting as a barrier to access of groundwater by Corymbia trees in Mulga patches, but not in the open Corymbia savanna
Evaluating the performance of land surface model ORCHIDEE-CAN v1.0 on water and energy flux estimation with a single- and multi-layer energy budget scheme
Canopy structure is one of the most important vegetation characteristics for land-atmosphere interactions, as it determines the energy and scalar exchanges between the land surface and the overlying air mass. In this study we evaluated the performance of a newly developed multilayer energy budget in the ORCHIDEE-CAN v1.0 land surface model (Organising Carbon and Hydrology In Dynamic Ecosystems - CANopy), which simulates canopy structure and can be coupled to an atmospheric model using an implicit coupling procedure. We aim to provide a set of accept-able parameter values for a range of forest types. Top-canopy and sub-canopy flux observations from eight sites were collected in order to conduct this evaluation. The sites crossed climate zones from temperate to boreal and the vegetation types included deciduous, evergreen broad-leaved and evergreen needle-leaved forest with a maximum leaf area index (LAI; all-sided) ranging from 3.5 to 7.0. The parametrization approach proposed in this study was based on three selected physical processes - namely the diffusion, advection, and turbulent mixing within the canopy. Short-term sub-canopy observations and long-term surface fluxes were used to calibrate the parameters in the sub-canopy radiation, turbulence, and resistance modules with an automatic tuning process. The multi-layer model was found to capture the dynamics of sub-canopy turbulence, temperature, and energy fluxes. The performance of the new multi-layer model was further compared against the existing single-layer model. Although the multi-layer model simulation results showed few or no improvements to both the nighttime energy balance and energy partitioning during winter compared with a single-layer model simulation, the increased model complexity does provide a more detailed description of the canopy micrometeorology of various forest types. The multi-layer model links to potential future environmental and ecological studies such as the assessment of in-canopy species vulnerability to climate change, the climate effects of disturbance intensities and frequencies, and the consequences of biogenic volatile organic compound (BVOC) emissions from the terrestrial ecosystem.Peer reviewe
The Turbulent Lagrangian Time Scale in Forest Canopies Constrained by Fluxes, Concentrations and Source Distributions
Author Correction: The FLUXNET2015 dataset and the ONEFlux processing pipeline for eddy covariance data
The FLUXNET2015 dataset and the ONEFlux processing pipeline for eddy covariance data
The FLUXNET2015 dataset provides ecosystem-scale data on CO2, water, and energy exchange between the biosphere and the atmosphere, and other meteorological and biological measurements, from 212 sites around the globe (over 1500 site-years, up to and including year 2014). These sites, independently managed and operated, voluntarily contributed their data to create global datasets. Data were quality controlled and processed using uniform methods, to improve consistency and intercomparability across sites. The dataset is already being used in a number of applications, including ecophysiology studies, remote sensing studies, and development of ecosystem and Earth system models. FLUXNET2015 includes derived-data products, such as gap-filled time series, ecosystem respiration and photosynthetic uptake estimates, estimation of uncertainties, and metadata about the measurements, presented for the first time in this paper. In addition, 206 of these sites are for the first time distributed under a Creative Commons (CC-BY 4.0) license. This paper details this enhanced dataset and the processing methods, now made available as open-source codes, making the dataset more accessible, transparent, and reproducible.Peer reviewe
Aspects of flow characteristics and turbulence in complex terrain : results from the MAP-RIVIERA project
In this thesis turbulence measurements from
the MAP-RIVIERA project are analysed. During the
project a number of permanent turbulence stations
were installed on a cross section of the Riviera valley.
Furthermore measurements from e.g. radio sounding
systems and a passive microwave profiler were
carried out during intensive observation periods. As large fractions of the alps are forested, a
tower with amongst others six levels of ultrasonic
anemometer thermometers was installed in a mixed
forest at the slope (35°). This data set is compared to
studies carried out over forests in more ideal, flat
terrain. The analysis is carried out for 30 min mean
data, joint probability distributions, length scales and
spectral characteristics. Furthermore patterns of
coherent structures are determined and dominating
time scales as well as flux fractions are calculated. Thermally induced slope and valley winds
are interacting on different spatial and time scales
leading to complex patterns in momentum transport
which differ significantly from surface layer
characteristics. Directional shear causes lateral
momentum transports that are in the same order as the
longitudinal ones. In the canopy a sharp attenuation of
turbulence is observed. Skewed distributions of
velocity components indicate that intermittent
turbulent transports play an important role in the
energy distribution. In the absence of larger scale
waves energy is removed from the mean flow above
the canopy and injected into coherent eddies. Length
scales of these disturbances are comparable to those in
forests in flat terrain. In the canopy work is done
against pressure drag and against the viscous
component of canopy drag. Kinetic energy is
converted into fine scale wake turbulence and heat
leading to vanishing second moments. It is shown that wave like structures are a
very common feature of the stable boundary layer. In
the valley atmosphere temperature fluctuations are
observed which are in phase through the whole
measured layer indicating terrain generated waves.
These temperature oscillations are not confined to the
valley centre but they are also observed on stations at
the slope. Having a phase shift between valley and
slope oscillations it is proposed that terrain generated
waves back up or even enable the mechanism of
compressional warming on the valley slopes. It is
shown, that due to compressional warming a cycle is
initiated in which (relatively colder) air is advected
slope upwards above the canopy while in the canopy
(relatively warmer) air drains at low speed. When the
downslope winds occur above the canopy relatively
warmer air is mixed into the canopy. This effect is
strongest just before the onset of a new ´upslope
event´
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