Jane Liu

Daily canopy photosynthesis model through temporal and spatial scaling for remote sensing applications

Because Farquhar’s photosynthesis model is only directly applicable to individual leaves instantaneously, considerable skill is needed to use this model for regional plant growth and carbon budget estimations. In many published models, Farquhar’s equations were applied directly to plant canopies by assuming a plant canopy to function like a big-leaf. This big-leaf approximation is found to be acceptable for estimating seasonal trends of canopy photosynthesis but inadequate for simulating its day-to-day variations, when compared with eddy-covariance and gas-exchange chamber measurements from two boreal forests. The daily variation is greatly dampened in big-leaf simulations because the original leaf-level model is partially modified through replacing stomatal conductance with canopy conductance. Alternative approaches such as separating the canopy into sunlit and shaded leaf groups or stratifying the canopy into multiple layers can avoid the problem. Because of non-linear response of leaf photosynthesis to meteorological variables (radiation, temperature and humidity), considerable errors exist in photosynthesis calculation at daily steps without considering the diurnal variability of the variables. To avoid these non-linear effects, we have developed an analytical solution to a simplified daily integral of Farquhar’s model by considering the general diurnal patterns of meteorological variables. This daily model not only captures the main effects of diurnal variations on photosynthesis but is also computationally efficient for large area applications. Its application is then not restricted by availability of sub-daily meteorological data. This scheme has been tested using measured CO2 data from the Boreal Ecosystem–Atmosphere Study (BOREAS), which took place in Manitoba and Saskatchewan in 1994 and 1996

A process-based boreal ecosystem productivity simulator using remote sensing inputs

This paper describes a boreal ecosystems productivity simulator (BEPS) recently developed at the Canada Centre for Remote Sensing to assist in natural resources management and to estimate the carbon budget over Canadian landmass (106–107 km2). BEPS uses principles of FOREST biogeochemical cycles (FOREST-BGC) (Running and Coughlan, 1988) for quantifying the biophysical processes governing ecosystems productivity, but the original model is moth fled to better represent canopy radiation processes. A numerical scheme is developed to integrate different data types: remote sensing data at 1-km resolution in lambert conformal conic projection, daily meteorological data in Gaussian or longitude-latitude grided systems, and soil data grouped in polygons. The processed remote sensing data required in the model are leaf area index (LAI) and land-cover type. The daily meteorological data include air temperature, incoming shortwave radiation, precipitation, and humidity. The soil-data input is the available water-holding capacity. The major outputs of BEPS include spatial fields of net primary productivity (NPP) and evapotranspiration. The NPP calculated by BEPS has been tested against biomass data obtained in Quebec, Canada. A time series o f LAI over the growing season of 1993 in Quebec was derived by using 10-day composite normalized difference vegetation index images acquired by the advanced very high resolution radiometer at 1-km resolution (resampled). Soil polygon data were mosaicked, georeferenced, and rasterized in a geographic information system (ARC/INFO). With the use of the process-based model incorporating all major environmental variables affecting plant growth and development, detailed spatial distributions of NPP (annual and four seasons) in Quebec are shown in this paper. The accuracy of NPP calculation is estimated to be 60% for single pixels and 75% for 3×3 pixel areas (9 km9). The modeled NPP ranges from 0.6 kg C/m2/year at the southern border to 0.01 kg C/m2/year at the northern limit of the province. The total annual NPP in Quebec is estimated to be 0.24 Gt C in 1993, which is about 0.3–0.4% of the global NPP.

Multi-angular optical remote sensing for assessing vegetation structure and carbon absorption

The utility of multi-angle optical remote sensing for terrestrial carbon cycle estimation is demonstrated through theoretical development, POLDER data analysis, and a case study of carbon cycle in a boreal forest. Progress in canopy-level photosynthesis modeling suggests that simpler big-leaf photosynthesis models are giving ways to more complex sunlit/shaded leaf separation models. This advancement in ecological modeling has increased the demand for advanced description of canopy architecture. Such demand may be mostly met through the use of multi-angle remote sensing techniques. In addition to leaf area index (LAI), another canopy parameter, the foliage clumping index, can be derived from multi-angle remote sensing. These two parameters are the basis for separating sunlit and shaded leaves. As leaf photosynthesis is nonlinearly related to incident radiation, such separation avoids the problems of big-leaf models that only make use of the total radiation absorption by the canopy without considering the distribution of radiation among leaves. A practical conclusion is that the traditional way of mapping the net primary productivity (NPP) through its correlation with the remotely sensed fraction of photosynthetically active radiation (FPAR) absorbed by plant canopies is only a very crude approximation and could be replaced with mapping LAI and clumping index and modeling NPP with advanced photosynthesis models. This is a step forward in remote sensing applications because single-angle remote sensing can only acquire information on the effective LAI related to the canopy gap fraction in the viewing direction and the amount of shaded leaf area is unknown.

Annual carbon balance of Canada's forests during 1895–1996

This paper reports annual carbon (C) balance of Canadas forests during 1895–1996 estimated using the Integrated Terrestrial Ecosystem C-budget model (InTEC) [Chen et al. this issue]. During 1895–1910, Canadas forests were small sources of 30±15 Tg C yr−1 due to large disturbances (forest fire, insect-induced mortality, and harvest) in late nineteenth century. The forests became large sinks of 170±85 Tg C yr−1 during 1930–1970, owing to forest regrowth in previously disturbed areas and growth stimulation by nondisturbance factors such as climate, atmospheric CO2 concentration, and N deposition. In recent decades (1980–1996), Canadas forests have been moderate sinks of 50±25 Tg C yr−1, as a result of a tradeoff between the negative effects of increased disturbances and positive effects of nondisturbance factors. The nondisturbance factors, in order of importance, are (1) atmospheric N deposition (measured by a national monitoring network), (2) net N mineralization and fixation (estimated from temperature and precipitation records), (3) growing season length increase (estimated from spring air temperature records), and (4) CO2 fertilization (estimated from CO2 records using a leaf-level photosynthesis model). The magnitudes of modeled nondisturbance effects are consistent with simulation results by the Carnegie-Ames-Stanford Approach (CASA) and are also in broad agreement with flux measurements above mature forest stands at several locations in Canada. Results for the disturbance effects agree with a previous study [Kurz and Apps, 1996]. The overall C balance from InTEC generally agrees with that derived from tree ring data [Auclair and Bedford, 1997] and from forest inventories. The combination of our result and that of Houghton et al. [1999] for the United States suggests that North America (> 15°N) was probably a C sink of 0.2-0.5 Pg C yr±−1 during 1980s, much less than that of 1.7 Pg C yr±−1 estimated by Fan et al. [1998] using an atmospheric inversion method.

Net primary productivity distribution in the BOREAS region from a process model using satellite and surface data

The purpose of this paper is to upscale tower measurements of net primary productivity (NPP) to the Boreal Ecosystem-Atmosphere Study (BOREAS) study region by means of remote sensing and modeling. The Boreal Ecosystem Productivity Simulator (BEPS) with a new daily canopy photosynthesis model was first tested in one coniferous and one deciduous site. The simultaneous CO2 flux measurements above and below the tree canopy made it possible to isolate daily net primary productivity of the tree canopy for model validation. Soil water holding capacity and gridded daily meteorological data for the region were used as inputs to BEPS, in addition to 1 km resolution land cover and leaf area index (LAI) maps derived from the advanced very high resolution radiometer (AVHRR) data. NPP statistics for the various cover types in the BOREAS region and in the southern study area (SSA) and the northern study area (NSA) are presented. Strong dependence of NPP on LAI was found for the three major cover types: coniferous forest, deciduous forest and cropland. Since BEPS can compute total photosynthetically active radiation absorbed by the canopy in each pixel, light use efficiencies for NPP and gross primary productivity could also be analyzed. From the model results, the following area-averaged statistics were obtained for 1994: (1) mean NPP for the BOREAS region of 217 g C m−2 yr−1; (2) mean NPP of forests (excluding burnt areas in the region) equal to 234 g C m−2 yr−1; (3) mean NPP for the SSA and the NSA of 297 and 238 g C m−2 yr−1, respectively; and (4) mean light use efficiency for NPP equal to 0.40, 0.20, and 0.33 g C (MJ APAR)−1 for deciduous forest, coniferous forest, and crops, respectively.

Spatial distribution of carbon sources and sinks in Canada's forests

Annual spatial distributions of carbon sources and sinks in Canada’s forests at 1 km resolution are computed for the period from 1901 to 1998 using ecosystem models that integrate remote sensing images, gridded climate, soils and forest inventory data. GIS-based fire scar maps for most regions of Canada are used to develop a remote sensing algorithm for mapping and dating forest burned areas in the 25 yr prior to 1998. These mapped and dated burned areas are used in combination with inventory data to produce a complete image of forest stand age in 1998. Empirical NPP age relationships were used to simulate the annual variations of forest growth and carbon balance in 1 km pixels, each treated as a homogeneous forest stand. Annual CO2 flux data from four sites were used for model validation. Averaged over the period 1990–1998, the carbon source and sink map for Canada’s forests show the following features: (i) large spatial variations corresponding to the patchiness of recent fire scars and productive forests and (ii) a general south-to-north gradient of decreasing carbon sink strength and increasing source strength. This gradient results mostly from differential effects of temperature increase on growing season length, nutrient mineralization and heterotrophic respiration at different latitudes as well as from uneven nitrogen deposition. The results from the present study are compared with those of two previous studies. The comparison suggests that the overall positive effects of non-disturbance factors (climate, CO2 and nitrogen) outweighed the effects of increased disturbances in the last two decades, making Canada’s forests a carbon sink in the 1980s and 1990s. Comparisons of the modeled results with tower-based eddy covariance measurements of net ecosystem exchange at four forest stands indicate that the sink values from the present study may be underestimated.

Approaches for reducing uncertainties in regional forest carbon balance

Accurate estimation of regional terrestrial ecosystem carbon (C) balance is critical in formulating national and global adaptation and mitigation strategies in response to global changes. Since the regional C balance cannot be measured directly, it has been estimated using various models. In such studies, errors often exceeded the magnitude of the estimated C balance due to two types of uncertainties: noninclusion of some important factors in the C cycle and the fact that the C balance is a small difference between several large fluxes that can each be determined with only a limited accuracy. In this study, we propose new approaches to reduce these uncertainties and implement them in an Integrated Terrestrial Ecosystem C-budget model (InTEC). To minimize the first type of uncertainties, InTEC considers all the major factors presently known to affect C balance (including climate, atmospheric CO 2 concentration, N deposition, and disturbances). To reduce the second type of uncertainties, InTEC estimates the C balance from historical changes in these factors, relative to the preindustrial period. InTEC is built on the basis of widely tested Century C cycling model, Farquhars leaf photosynthesis model, and age-NPP relationships, and is constrained by N cycling. As a general regional-scale terrestrial ecosystem C budget model, InTEC has so far been applied to Canadas forests [Chen et al., this issue]. The sensitivity analysis showed that these two new approaches reduce the uncertainty in the C balance of Canadas forests substantially.

Measurement of low‐altitude CO over the Indian subcontinent by MOPITT

We show that the dayside MOPITT retrievals in the lower troposphere can provide useful information on surface sources of atmospheric CO over the Indian subcontinent. We find that MOPITT retrievals at 850 hPa show localized enhancements over the Indian subcontinent, which correlate with similar enhancements seen in the tropospheric NO2 columns from the SCIAMACHY instrument. In particular, high concentrations of CO over the Indo-Gangetic basin and some prominent cities are captured in the lower-tropospheric retrievals in spring. MOPITT averaging kernels (normalized to take into account the absorber amounts in the layers) indicate that the retrievals are sensitive to CO in the lower troposphere. In winter, MOPITT retrievals at 850 hPa can detect the strongest source areas over the eastern states of Bihar and West Bengal, thus confirming the so-called “Bihar pollution pool,” which was detected earlier in the aerosol measurements by the multiangle imaging spectroradiometer (MISR) aboard Terra. The pollution features are consistent with the spatial distribution of CO emissions in India, as reflected in the GEOS-Chem simulation of CO. Furthermore, these lower-tropospheric features in the simulation are still present after smoothing the modeled fields using the MOPITT averaging kernels and a priori profile, demonstrating that the retrievals do have sensitivity in the lower troposphere. This work indicates that although MOPITT retrievals are often most sensitive to CO in the middle and upper troposphere, they do provide information on lower-tropospheric CO in selected continental regions with strong thermal contrast and could be useful for pollution studies.

The interactions between anthropogenic aerosols and the East Asian summer monsoon using RegCCMS

An online coupled regional climate-chemistry model called RegCCMS is used to investigate the interactions between anthropogenic aerosols and the East Asian summer monsoon (EASM) over East Asia. The simulation results show that the mean aerosol loading and optical depth over the region are 17.87 mg/m2 and 0.25, respectively. Sulfate and black carbon (BC) account for approximately 61.2% and 7.8% of the total aerosols, respectively. The regional mean radiative forcing (RF) is approximately −3.64, −0.55, and +0.88 W/m2 at the top of the atmosphere for the total aerosol effect, the total aerosol direct effect, and the BC direct effect, respectively. The surface direct RF of BC accounts for approximately 31% of the total RF of all aerosols. Because of the total aerosol effect, both the energy budgets and air temperature are considerably reduced in the region with high aerosol loadings, leading to decreases in the land-ocean air temperature gradient in summer. The total column-absorbed solar radiation and surface air temperature decrease by 8.4 W/m2 and 0.31 K, respectively. This cooling effect weakens horizontal and vertical atmospheric circulations over East Asia. The wind speed at 850 hPa decreases by 0.18 m/s, and the precipitation decreases by 0.29 mm/d. The small responses of solar radiation, air temperature, and atmospheric circulations to the BC warming effect are opposite to those of the total aerosol effect. The BC-induced enhancement of atmospheric circulation can increase local floods in south China, while droughts in north China may worsen in response to the BC semidirect effect. The total aerosol effect is much more significant than the BC direct effect. The East Asian summer monsoon becomes weaker due to the total aerosol effect. However, this weakness could be partially offset by the BC warming effect. Sensitivity analyses further indicate that the influence of aerosols on the EASM might be more substantial in years when the southerlies or southwesterlies at 850 hPa are weak compared with years when the winds are strong. Changes in the EASM can induce variations in the distribution and magnitude of aerosols. Aerosols in the lower troposphere over the region can increase by 3.07 and 1.04 µg/m3 due to the total aerosol effect and the BC warming effect, respectively.

Carbon Offset Potentials of Four Alternative Forest Management Strategies in Canada: A Simulation Study

Using an Integrated TerrestrialEcosystem C-budget model (InTEC), we simulated thecarbon (C) offset potentials of four alternativeforest management strategies in Canada: afforestation,reforestation, nitrogen (N) fertilization, andsubstitution of fossil fuel with wood, under differentclimatic and disturbance scenarios. C offset potentialis defined as additional C uptake by forest ecosystemsor reduced fossil C emissions when a strategy isimplemented to the theoretical maximum possibleextent. The simulations provided the followingestimated gains from management: (1) Afforesting allthe estimated ∼ 7.2 Mha of marginal agricultural landand urban areas in 1999 would create an average Coffset potential of ∼ 8 Tg C y-1 during 1999–2100,at a cost of 3.4 Tg fossil C emission in 1999. (2)Prompt reforestation of all forest lands disturbed inthe previous year during 1999–2100 would produce anaverage C offset potential of ∼ 57 Tg C y-1 forthis period, at a cost of 1.33 Tg C y-1. (3)Application of N fertilization (at the low rate of 5kg N ha-1 y-1) to the ∼ 125 Mha ofsemi-mature forest during 1999–2100 would create anaverage C offset of ∼58 Tg C y-1 for this period,at a cost of ∼0.24 Tg C y-1. (4) Increasingforest harvesting by 20% above current average ratesduring 1999–2100, and using the extra wood products tosubstitute for fossil energy would reduce averageemissions by ∼11 Tg C y-1, at a cost of 0.54 TgC y-1. If implemented to the maximum extent, thecombined C offset potential of all four strategieswould be 2–7 times the GHG emission reductionsprojected for the National Action Plan for ClimateChange (NAPCC) initiatives during 2000–2020, and anorder of magnitude larger than the projected increasein C uptake by Canadas agricultural soils due toimproved agricultural practices during 2000–2010.