Nicanor Z. Saliendra

Calibration of remotely sensed, coarse resolution NDVI to CO2 fluxes in a sagebrush-steppe ecosystem

The net ecosystem exchange (NEE) of carbon flux can be partitioned into gross primary productivity (GPP) and respiration (R). The contribution of remote sensing and modeling holds the potential to predict these components and map them spatially and temporally. This has obvious utility to quantify carbon sink and source relationships and to identify improved land management strategies for optimizing carbon sequestration. The objective of our study was to evaluate prediction of 14-day average daytime CO2 fluxes (Fday) and nighttime CO2 fluxes (Rn) using remote sensing and other data. Fday and Rn were measured with a Bowen ratio–energy balance (BREB) technique in a sagebrush (Artemisia spp.)–steppe ecosystem in northeast Idaho, USA, during 1996–1999. Micrometeorological variables aggregated across 14-day periods and time-integrated Advanced Very High Resolution Radiometer (AVHRR) Normalized Difference Vegetation Index (iNDVI) were determined during four growing seasons (1996–1999) and used to predict Fday and Rn. We found that iNDVI was a strong predictor of Fday (R 2 =0.79, n=66, P<0.0001). Inclusion of evapotranspiration in the predictive equation led to improved predictions of Fday (R 2 =0.82, n=66, P<0.0001). Crossvalidation indicated that regression tree predictions of Fday were prone to overfitting and that linear regression models were more robust. Multiple regression and regression tree models predicted Rn quite well (R 2 =0.75–0.77, n=66) with the regression tree model being slightly more robust in crossvalidation. Temporal mapping of Fday and Rn is possible with these techniques and would allow the assessment of NEE in sagebrush–steppe ecosystems. Simulations of periodic Fday measurements, as might be provided by a mobile flux tower, indicated that such measurements could be used in combination with iNDVI to accurately predict Fday. These periodic measurements could maximize the utility of expensive flux towers for evaluating various carbon management strategies, carbon certification, and validation and calibration of carbon flux models. D 2003 Elsevier Science Inc. All rights reserved.

Carbon fluxes on North American rangelands

Abstract Rangelands account for almost half of the earths land surface and may play an important role in the global carbon (C) cycle. We studied net ecosystem exchange (NEE) of C on eight North American rangeland sites over a 6-yr period. Management practices and disturbance regimes can influence NEE; for consistency, we compared ungrazed and undisturbed rangelands including four Great Plains sites from Texas to North Dakota, two Southwestern hot desert sites in New Mexico and Arizona, and two Northwestern sagebrush steppe sites in Idaho and Oregon. We used the Bowen ratio-energy balance system for continuous measurements of energy, water vapor, and carbon dioxide (CO2) fluxes at each study site during the measurement period (1996 to 2001 for most sites). Data were processed and screened using standardized procedures, which facilitated across-location comparisons. Although almost any site could be either a sink or source for C depending on yearly weather patterns, five of the eight native rangelands typically were sinks for atmospheric CO2 during the study period. Both sagebrush steppe sites were sinks and three of four Great Plains grasslands were sinks, but the two Southwest hot desert sites were sources of C on an annual basis. Most rangelands were characterized by short periods of high C uptake (2 mo to 3 mo) and long periods of C balance or small respiratory losses of C. Weather patterns during the measurement period strongly influenced conclusions about NEE on any given rangeland site. Droughts tended to limit periods of high C uptake and thus cause even the most productive sites to become sources of C on an annual basis. Our results show that native rangelands are a potentially important terrestrial sink for atmospheric CO2, and maintaining the period of active C uptake will be critical if we are to manage rangelands for C sequestration.

Inverse estimation of Vcmax, leaf area index, and the Ball-Berry parameter from carbon and energy fluxes

[1] Canopy fluxes of CO2 and energy can be modeled with high fidelity using a small number of environmental variables and ecosystem parameters. Although these ecosystem parameters are critically important for modeling canopy fluxes, they typically are not measured with the same intensity as ecosystem fluxes. We developed an algorithm to estimate leaf area index (LAI), maximum carboxylation velocity (Vcmax), the Ball-Berry parameter (m), and substrate-dependent ecosystem respiration rate (bA) by inverting a commonly used modeling paradigm of canopy CO2 and energy fluxes. To test this algorithm, fluxes of sensible heat (H), latent heat (LE), and CO2 (Fc) were measured with eddy covariance techniques in a pristine grassland-forb steppe site in northern Kazakhstan. We applied the algorithm to these data and identified ecosystem characteristics consistent with data across a time series of meteorological drivers from the Kazakhstan data. LAI was calculated by fitting the model to measured H + LE, Vcmax and bA were solved simultaneously by fitting the model to measured CO2 fluxes, and m was calculated by varying the partitioning of available energy between H and LE. Seasonal changes in LAI ranged from 2.0 to 2.4, Vcmax declined from 20 to 5 mmol CO2 m � 2 s � 1 , respiration as a percentage of assimilation ranged from 0.5 to 0.75, and m varied from 17 to 24. Our results with the Kazakhstan data showed that LAI, Vcmax, ecosystem respiration, and m can be solved to accurately predict (R 2 = 80 to 95%) carbon and energy fluxes with nonsignificant bias at 20-min and daily timescales. The ecosystem characteristics calculated in our study were consistent with independent measurements of the seasonal dynamics of a shortgrass steppe in Kazakhstan and with values published in the literature. These characteristics were closely linked to mean daily fluxes of CO2 but were not dependent on the environmental drivers for the periods they were measured. We conclude that process model inversion has potential for comparing CO2 and energy fluxes among different ecosystems and years and for providing important ecosystem parameters for evaluating climatic influences on CO2 and energy fluxes.

Growing season CO2 fluxes in a sagebrush-steppe ecosystem in Idaho: bowen ratio/energy balance measurements and modeling

Abstract The sagebrush-steppe ecosystem covers more than 36 million ha and could play an important role in the global carbon cycle; however, quantitative estimates of CO 2 fluxes on these extensive ecosystems are not available. The Bowen ratio/energy balance technique (BREB) was used to continuously monitor CO 2 fluxes during the 1996 to 1999 growing seasons at a sagebrush-steppe site near Dubois, Idaho, USA. The daytime and night-time CO 2 fluxes were modeled to provide estimates of occasionally missing or aberrant data points so that daily (24-h) integrals across the entire growing season could be quantified. Depending on the particular time of the season, daytime fluxes were best described by a rectangular hyberbolic, nonrectangular hyperbolic, or hysteresis-type functions that included radiation, relative humidity, and soil temperature. Night-time CO 2 fluxes exhibited greater variability than daytime fluxes and were not closely correlated with any single meteorological characteristic. Night-time fluxes were predicted using a nonlinear parameter identification technique that estimated values of daytime respiration, which were significantly correlated with night-time fluxes. For the four growing seasons of our study, the integrated seasonal fluxes ranged from 284 to 1,103 g CO 2 m −2 with an overall average of 635 g CO 2 m −2 . Respiratory losses during the non-growing season were estimated to be about 1.5 g CO 2 m −2 day −1 or a total of 270 g CO 2 m −2 . This gives an annual net positive flux (carbon sequestration) estimate of 365 g CO 2 m −2 (or 1.0 t C ha −1 ). These results suggest that the combination of BREB measurements and modeling techniques can be used to provide estimates of CO 2 fluxes on important rangeland ecosystems. Das Wustenbeifus-Steppenokosystem erstreckt sich uber mehr als 36 Millionen Hektar und konnte eine wichtige Rolle im globalen Kohlenstoffkreislauf spielen. Quantitative Schatzungen der CO 2 -Flusse dieser ausgedehnten Okosysteme sind jedoch nicht verfugbar. Die “Bowen-Verhaltnis”-Energiebilanz-Technik (BREB) wurde benutzt, um die CO 2 -Flusse wahrend der Wachstumssaisonen 1996 bis 1999 in einer Wustenbeifus-Steppe bei Dubois, Idaho, USA, kontinuierlich zu erfassen. Die Tages- und Nacht-CO 2 -Flusse wurden modelliert um Schatzwerte fur gelegentlich fehlende oder abweichende Datenpunkte zu bekommen, so dass Tagesintegrale (24h) uber die gesamte Wachstumssaison quantifiziert werden konnten. Abhangig vom jeweiligen Zeitpunkt der Saison wurden die Tagesflusse am Besten durch rechtwinklig hyperbolische, nicht-rechtwinklig hyperbolische oder hystereseahnliche Funktionen beschrieben, die Strahlung, relative Luftfeuchtigkeit und Bodentemperatur enthielten. Die Nacht-CO 2 -Flusse zeigten eine grosere Variabilitat als Tagesflusse und waren mit keiner einzelnen meteorologischen Kenngrose eng korreliert. Die Nachtflusse wurden vorhergesagt, indem eine nichtlineare Parameter-Identifizierungstechnik verwendet wurde, mit der die Werte der Tagesrespiration geschatzt wurden, die wiederum mit den Nachtflussen signifikant korreliert waren. Fur die vier Wachstumssaisonen unserer Studie bewegten sich die integrierten saisonalen Flusse zwischen 284 und 1103 g CO 2 m −2 mit einem Gesamtmittelwert von 635 g CO 2 m −2 . Die Respirationsverluste auserhalb der Wachstumssaison wurden auf ungefahr 1.5 g CO 2 m −2 Tag −1 bzw. insgesamt auf 270 g CO 2 m −2 geschatzt. Dies ergibt einen geschatzten positiven jahrlichen Nettofluss (Kohlenstofffixierung) von 365 g CO 2 m −2 (oder 1 t C ha −1 ). Diese Ergebnisse lassen erkennen, dass eine Kombination von BREB-Messungen und Modellierungstechniken verwendet werden kann, um Schatzungen des CO 2 -Flusses in wichtigen Weideland-Okosystemen zur Verfugung zu stellen.

Long-Term Dynamics of Production, Respiration, and Net CO2 Exchange in Two Sagebrush-Steppe Ecosystems

Abstract We present a synthesis of long-term measurements of CO2 exchange in 2 US Intermountain West sagebrush-steppe ecosystems. The locations near Burns, Oregon (1995–2001), and Dubois, Idaho (1996–2001), are part of the AgriFlux Network of the Agricultural Research Service, United States Department of Agriculture. Measurements of net ecosystem CO2 exchange (Fc) during the growing season were continuously recorded at flux towers using the Bowen ratio-energy balance technique. Data were partitioned into gross primary productivity (Pg) and ecosystem respiration (Re) using the light-response function method. Wintertime fluxes were measured during 1999/2000 and 2000/2001 and used to model fluxes in other winters. Comparison of daytime respiration derived from light-response analysis with nighttime tower measurements showed close correlation, with daytime respiration being on the average higher than nighttime respiration. Maxima of Pg and Re at Burns were both 20 g CO2·m−2·d−1 in 1998. Maxima of Pg and Re at Dubois were 37 and 35 g CO2·m−2·d−1, respectively, in 1997. Mean annual gross primary production at Burns was 1 111 (range 475–1 715) g CO2·m−2·y−1 or about 30% lower than that at Dubois (1 602, range 963–2 162 g CO2·m−2·y−1). Across the years, both ecosystems were net sinks for atmospheric CO2 with a mean net ecosystem CO2 exchange of 82 g CO2·m−2·y−1 at Burns and 253 g CO2·m−2·y−1 at Dubois, but on a yearly basis either site could be a C sink or source, mostly depending on precipitation timing and amount. Total annual precipitation is not a good predictor of carbon sequestration across sites. Our results suggest that Fc should be partitioned into Pg and Re components to allow prediction of seasonal and yearly dynamics of CO2 fluxes.

Research ArticlesLong-Term Dynamics of Production, Respiration, and Net CO2 Exchange in Two Sagebrush-Steppe Ecosystems

Abstract We present a synthesis of long-term measurements of CO2 exchange in 2 US Intermountain West sagebrush-steppe ecosystems. The locations near Burns, Oregon (1995–2001), and Dubois, Idaho (1996–2001), are part of the AgriFlux Network of the Agricultural Research Service, United States Department of Agriculture. Measurements of net ecosystem CO2 exchange (Fc) during the growing season were continuously recorded at flux towers using the Bowen ratio-energy balance technique. Data were partitioned into gross primary productivity (Pg) and ecosystem respiration (Re) using the light-response function method. Wintertime fluxes were measured during 1999/2000 and 2000/2001 and used to model fluxes in other winters. Comparison of daytime respiration derived from light-response analysis with nighttime tower measurements showed close correlation, with daytime respiration being on the average higher than nighttime respiration. Maxima of Pg and Re at Burns were both 20 g CO2·m−2·d−1 in 1998. Maxima of Pg and Re at Dubois were 37 and 35 g CO2·m−2·d−1, respectively, in 1997. Mean annual gross primary production at Burns was 1 111 (range 475–1 715) g CO2·m−2·y−1 or about 30% lower than that at Dubois (1 602, range 963–2 162 g CO2·m−2·y−1). Across the years, both ecosystems were net sinks for atmospheric CO2 with a mean net ecosystem CO2 exchange of 82 g CO2·m−2·y−1 at Burns and 253 g CO2·m−2·y−1 at Dubois, but on a yearly basis either site could be a C sink or source, mostly depending on precipitation timing and amount. Total annual precipitation is not a good predictor of carbon sequestration across sites. Our results suggest that Fc should be partitioned into Pg and Re components to allow prediction of seasonal and yearly dynamics of CO2 fluxes.

Influence of Subfacet Heterogeneity and Material Properties on the Urban Surface Energy Budget

AbstractUrban facets—the walls, roofs, and ground in built-up terrain—are often conceptualized as homogeneous surfaces, despite the obvious variability in the composition and material properties of the urban fabric at the subfacet scale. This study focuses on understanding the influence of this subfacet heterogeneity, and the associated influence of different material properties, on the urban surface energy budget. The Princeton Urban Canopy Model, which was developed with the ability to capture subfacet variability, is evaluated at sites of various building densities and then applied to simulate the energy exchanges of each subfacet with the atmosphere over a densely built site. The analyses show that, although all impervious built surfaces convert most of the incoming energy into sensible heat rather than latent heat, sensible heat fluxes from asphalt pavements and dark rooftops are 2 times as high as those from concrete surfaces and light-colored roofs. Another important characteristic of urban areas—t...

Precipitation Regulates the Response of Net Ecosystem CO2 Exchange to Environmental Variation on United States Rangelands

Abstract Rangelands occupy 50% of Earths land surface and thus are important in the terrestrial carbon (C) cycle. For rangelands and other terrestrial ecosystems, the balance between photosynthetic uptake of carbon dioxide (CO2) and CO2 loss to respiration varies among years in response to interannual variation in the environment. Variability in CO2 exchange results from interannual differences in 1) environmental variables at a given point in the annual cycle (direct effects of the environment) and in 2) the response of fluxes to a given change in the environment because of interannual changes in biological factors that regulate photosynthesis and respiration (functional change). Functional change is calculated as the contribution of among-year differences in slopes of flux-environment relationships to the total variance in fluxes explained by the environment. Functional change complicates environmental-based predictions of CO2 exchange, yet its causes and contribution to flux variability remain poorly defined. We determine contributions of functional change and direct effects of the environment to interannual variation in net ecosystem exchange of CO2 (NEE) of eight rangeland ecosystems in the western United States (58 site-years of data). We predicted that 1) functional change is correlated with interannual change in precipitation on each rangeland and 2) the contribution of functional change to variance in NEE increases among rangelands as mean precipitation increases. Functional change explained 10–40% of the variance in NEE and accounted for more than twice the variance in fluxes of direct effects of environmental variability for six of the eight ecosystems. Functional change was associated with interannual variation in precipitation on most rangelands but, contrary to prediction, contributed proportionally more to variance in NEE on arid than more mesic ecosystems. Results indicate that we must account for the influence of precipitation on flux-environment relationships if we are to distinguish environmental from management effects on rangeland C balance.

Land Use Influences Carbon Fluxes in Northern Kazakhstan

Abstract A mobile, closed-chamber system (CC) was used to measure carbon and water fluxes on four land-use types common in the Kazakh steppe ecoregion. Land uses represented crop (wheat or barley, WB), abandoned land (AL), crested wheatgrass (CW), and virgin land (VL). Measurements were conducted during the growing season of 2002 in northern Kazakhstan at three locations (blocks) 15–20 km apart. The CC allowed the measurement of the carbon flux components of net ecosystem exchange (NEE), ecosystem respiration (RE) and soil respiration (RS), together with evapotranspiration (ET). Nonlinear regression analyses were used to model gross primary production (GPP) and ET as a function of photosynthetically active radiation (Q); RE and RS were modeled based on air (Tair) and soil (Ts) temperature, respectively. GPP, RE, RS, and ET were estimated for the entire year with the use of continuous 20-min means of Q, Tair, and Ts. Annual NEE indicated that AL gained 536 g CO2 · m−2, WB lost − 191 g CO2 · m−2, CW was near equilibrium (− 14 g CO2 · m−2), and VL exhibited considerable carbon accumulation (153 g CO2 · m−2). The lower GPP values of the land-use types dominated by native species (CW and VL) compared to WB and AL were compensated by positive NEE values that were maintained during a longer growing season. As expected, VL and CW allocated a larger proportion of their carbon assimilates belowground. Non–growing-season RE accounted for about 19% of annual RE in all land-use types. The results of this landscape-level study suggest that carbon lost by cultivation of VLs is partially being restored when fields are left uncultivated, and that VLs are net sinks of carbon. Estimations of carbon balances have important management implications, such as estimation of ecosystem productivity and carbon credit certification.

Prediction of senescent rangeland canopy structural attributes with airborne hyperspectral imagery

Canopy structural and chemical data are needed for senescent, mixed-grass prairie landscapes in autumn; yet data-driven models are lacking for rangelands dominated by non-photosynthetically active vegetation (NPV). We report how field data and aerial hyperspectral imagery were modeled to predict canopy attributes post growing-season using two approaches: (1) application of narrow spectral regions with Vegetation Indices (VIs) and (2) application of the full spectrum with Partial Least Squares Regression (PLSR). Analyses of the full spectrum using PLSR resulted in slightly lower root-mean-square error of prediction, as compared to VIs, which represent reflectance ratios for specific spectral bands.