Can a satellite-derived estimate of the fraction of PAR absorbed by chlorophyll (FAPARchl) improve predictions of light-use efficiency and ecosystem photosynthesis for a boreal aspen forest?

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Abstract

We used daily MODerate resolution Imaging Spectroradiometer (MODIS) imagery obtained over a five-year period to analyze the seasonal and inter-annual variability of the fraction of absorbed photosynthetically active radiation (FAPAR) and photosynthetic light use efficiency (LUE) for the Southern Old Aspen (SOA) flux tower site located near the southern limit of the boreal forest in Saskatchewan, Canada. To obtain the spectral characteristics of a standardized land area to compare with tower measurements, we scaled up the nominal 500 m MODIS products to a 2.5 km × 2.5 km area (5 × 5 MODIS 500 m grid cells). We then used the scaled-up MODIS products in a coupled canopy-leaf radiative transfer model, PROSAIL-2, to estimate the fraction of absorbed photosynthetically active radiation (APAR) by the part of the canopy dominated by chlorophyll (FAPARchl) versus that by the whole canopy (FAPARcanopy). Using the additional information provided by flux tower-based measurements of gross ecosystem production (GEP) and incident PAR, we determined 90-minute averages for APAR and LUE (slope of GEP:APAR) for both the physiologically active foliage (APARchl, LUEchl) and for the entire canopy (APARcanopy, LUEcanopy).

The flux tower measurements of GEP were strongly related to the MODIS-derived estimates of APARchl (r2 = 0.78) but only weakly related to APARcanopy (r2 = 0.33). Gross LUE between 2001 and 2005 for LUEchl was 0.0241 µmol C µmol 1 PPFD whereas LUEcanopy was 36% lower. Time series of the 5-year normalized difference vegetation index (NDVI) were used to estimate the average length of the core growing season as days of year 152–259. Inter-annual variability in the core growing season LUEchl (µmol C µmol 1 PPFD) ranged from 0.0225 in 2003 to 0.0310 in 2004. The five-year time series of LUEchl corresponded well with both the seasonal phase and amplitude of LUE from the tower measurements but this was not the case for LUEcanopy. We conclude that LUEchl derived from MODIS observations could provide a more physiologically realistic parameter than the more commonly used LUEcanopy as an input to large-scale photosynthesis models.

Introduction

Realistic models of plant canopy photosynthesis are necessary for obtaining accurate estimates of the carbon cycle for use in land surface models (LSMs) and atmospheric general circulation models (GCMs) (Sellers et al., 1996a, Sellers et al., 1996b). In vegetative canopies, photosynthetically active radiation (PAR) is absorbed from sunlight by photosynthetic pigments, primarily chlorophyll a and its accessory pigments (chlorophyll b, carotenoids). When ecosystem photosynthesis is calculated with a process model, it is referred to as Gross Primary Production (GPP). When it is calculated from flux tower data, it is referred to as Gross Ecosystem Production, designated here as GEPtower.

Plant production efficiency models (PEMs) have been developed to estimate GPP at canopy, landscape, regional and global scales, utilizing optical remote sensing to provide the fraction of absorbed PAR (FAPAR). Examples include GLO-PEM (Prince et al., 1995, Prince et al., 2000, Prince and Goward, 1995, Prince and Goward, 1996), TURC (Ruimy et al., 1994, Ruimy et al., 1996a, Ruimy et al., 1996b), 3-PG (Landsberg and Waring, 1997, Law et al., 2000) and PSN (Running et al., 1994, Running et al., 1999a, Running et al., 1999b, Running et al., 2000, Running et al., 2004). This latter model is a satellite-based global photosynthesis product derived from the MODerate resolution Imaging Spectroradiometer (MODIS) on the Terra and Aqua platforms.

All of these models estimate GPP as the product of three terms: (1) the light use efficiency of the canopy (LUEcanopy), which is a measure of the PAR conversion efficiency into photosynthetically fixed CO2; (2) the FAPAR of the canopy (FAPARcanopy), which is estimated using radiative transfer models and remote sensing data or using empirical relationship between FAPARcanopy and the normalized difference vegetation index (NDVI, Tucker, 1979); and (3) the incident PAR where:GPP=LUEcanopyFAPARcanopyPAR.

Consequently, accurate estimates of FAPAR and LUE for ecosystems are essential for obtaining accurate GPP.

The LUE concept was initially developed for agricultural crops at harvest to determine the conversion efficiency of available light into biomass (g C dry mass) over a full growing season and is typically expressed in units such as g C MJ 1 PAR (Monteith, 1972, Monteith, 1977). This seasonal crop-level LUE represents a direct measure of the average conversion efficiency of all above ground plant material (Gower et al., 1999), which is dominated by foliage for agricultural crops. Eddy covariance flux towers have the capability to provide near-continuous measurements of GEP — denoted as GEPtower, and absorbed PAR — denoted as APARtower (see Section 3.2.3 for more details, also see Krishnan et al., 2006), for an entire ecosystem for time periods as short as 30 min. Consequently, these instrumented flux towers also provide near-continuous measurements of LUE, denoted as LUEtower, over these same time periods as:LUEtower=GEPtowerFAPARtowerPARtower=GEPtowerAPARtower.

The LUEtower is typically expressed as µmol CO2 µmol 1 PAR or µmol C µmol 1 PPFD, where PPFD is the photosynthetic photon flux density. For these tower-based calculations, incident PAR is measured directly by radiometers attached to the flux tower and the FAPAR estimate is based both on detailed canopy structural measurements and on the radiometer measurements (Barr et al., 2007). An underlying assumption supporting the LUE retrieval through the MODIS modeling approach is that the LUEcanopy used in the models is a good approximation of LUEtower, at least when the measurement footprint of the tower is roughly equivalent to the area of the remote sensing pixel. Apparent ecosystem LUE obtained from flux tower measurements (i.e., LUEtower) directly describes the integrated response of the whole ecosystem to the prevailing environmental conditions, as do remotely acquired spectral snapshots although these latter are limited to specific acquisition times and viewing configurations.

On a canopy or ecosystem scale, GEP and APAR are typically linearly related (e.g., Waring et al., 1995), so that LUE can be determined from the slope of this relationship. This apparent linearity results from multiple scattering within the canopy, which involves 3-D contributions from foliage of multiple species and illumination conditions, as well as non-photosynthetic material (e.g., limbs, trunks, cones, litter). On the other hand, comparable light response curves for individual leaves of selected species yield non-linear responses for which the initial slope of the linear portion of the curve describes the quantum efficiency (Mohr et al., 1995). The quantum efficiency of individual leaves can also serve as an input to carbon cycle models but a means of scaling it to the canopy level is still required.

A common modeling approach is to set a maximum LUE for optimal environmental conditions (i.e., unstressed vegetation) and to simulate ecosystem responses when unfavorable environmental conditions occur (e.g., limitations of temperature, humidity, soil moisture, etc.) through down-regulation of the maximum LUE to achieve an apparent LUE (Medlyn, 1998). This is the approach used for the MODIS GPP product, an output of the PSN model. This GPP product has been compared with measurements made at flux towers by several research groups. For instance, Turner et al., 2003, Turner et al., 2004, Turner et al., 2006 found that the annual MODIS GPP totals calculated using MODIS standard photosynthesis products for a deciduous forest in Massachusetts, USA, matched well with the annual GEP totals from the flux tower. However, the seasonal time course of MODIS GPP dynamics differed significantly from the GEP measured by the flux tower (GEPtower) suggesting that a more physiologically realistic method of estimating GPP could be useful.

Even though maximum leaf LUE can be strongly influenced by leaf chlorophyll concentration (e.g., Waring et al., 1995), it is less clear how canopy chlorophyll concentration might influence apparent LUE at the ecosystem scale. Laboratory studies (Yoder and Waring, 1994) have shown that variation in canopy total chlorophyll content of miniature Douglas fir canopies was significantly correlated with their photosynthesis, although the correlation was higher for canopies exposed to full sun. Several other studies have shown a relationship between leaf or canopy nitrogen concentration and light use efficiency at the ecosystem scale (Kergoat et al., 2008, Ollinger et al., 2008), we believe that remote sensing techniques that evaluate chlorophyll rather than nitrogen could have even greater potential for estimating ecosystem light use efficiency and GPP.

From a biochemical perspective, only the PAR absorbed by photosynthetic pigments (designated as APARchl) enables photosynthetic processes, whereas the PAR absorbed by non-photosynthetic components such as boles, branches, stems, and litter is not used for CO2 fixation. We designate chlorophyll-based FAPAR here as FAPARchl. By definition, APARcanopy (the product of FAPARcanopy and PAR) is greater than APARchl (the product of FAPARchl and PAR). For linking to remote sensing applications, estimates of APARchl should provide more realistic GEPtower and LUEtower values than similar estimates using APARcanopy. We define LUE based on APARchl versus APARcanopy as follows:LUEchl=GEPtowerFAPARchlPAR=GEPtowerAPARchl.LUEcanopy=GEPtowerFAPARcanopyPAR=GEPtowerAPARcanopy.

In earlier studies (Zhang et al., 2005, Zhang et al., 2006), an approach to estimate FAPARchl was proposed using daily MODIS data. Since then, we have refined our algorithm to retrieve FAPARchl from MODIS imagery using the modified PROSPECT-SAIL2 model, PROSAIL-2 (Zhang et al., 2005, Zhang et al., 2006). The new version of this algorithm provides a statistical distribution of likely FAPARchl values for each cloud-free MODIS observation.

In this article, we combine five years of flux, meteorological, and remote sensing data from a boreal aspen flux site to attain the following four objectives: (1) to present a method for estimating FAPARchl and FAPARcanopy using single-date, scaled-up MODIS observations; (2) to apply the FAPARchl and FAPARcanopy algorithms to MODIS data acquired for 2001–2005 over this aspen flux site in Saskatchewan; (3) to link our estimates of MODIS FAPARchl and FAPARcanopy to the tower-based observations of PAR and GEP so as to derive LUE on both a unit chlorophyll area basis (LUEchl, Eq. (3) above) and for the whole canopy (LUEcanopy, Eq. (4) above); and (4) to compare our LUEchl, LUEcanopy and tower-based LUE estimates (i.e., LUEtower) to see if the LUEchl could provide a more physiologically realistic input to land surface process models. For this latter objective, we test the hypotheses that: (i) LUEcanopy = LUEchl; (ii) LUEchl = LUEtower; and (iii) LUEcanopy = LUEtower.

Section snippets

Southern Old Aspen

The Southern Old Aspen forest (SOA) was established in 1919 after a forest fire in Prince Albert National Park at the southern edge of the Canadian boreal forest (Barr et al., 2007). The eddy flux tower site (53.7°N, 106.2°W, 600 m elevation) is located ~ 50 km northwest of Prince Albert, Saskatchewan. SOA originated as part of the BOReal Ecosystem Atmosphere Study (BOREAS), and has continued operations under the Boreal Ecosystem Research and Monitoring Sites (BERMS) project and the Canadian

Spatial integration of MODIS data

A possible source of discrepancy between tower-based photosynthesis and the MODIS standard GPP is the way the MODIS reflectance products are used to calculate FAPARcanopy and the MODIS standard GPP (Justice et al., 1998, Wolfe et al., 1998). This occurs because the areal coverage of the MODIS products used in the GPP calculations are not constant over the growing season and may not match the footprint of the flux tower site. The MODIS observations made at multiple times over a target actually

PROSAIL-2 derived canopy variables including APARchl and APARcanopy

The posterior distributions of SOA canopy variables from the PROSAIL-2 model display seasonal variation, as shown for nine of the seventeen MODIS daily 5 × 5 observations in 2005 (Table 3). The other observations had similar distributions as those shown in Table 3 so they are not presented. Several canopy variables are shown: LAI, CF, total photosynthetic pigment content, water content, dry matter content, FAPARcanopy, and FAPARchl. Differences in total chlorophyll concentration were related to

Discussion

LUEchl captured more seasonal (Table 3) and inter-annual variation than LUEcanopy and provided an improved overall relationship to GEP (Fig. 3, Fig. 4). Krishnan et al. (2006) reported that the annual average LUEtower at the SOA tower for 2001–2005 was 0.0229–0.0302 µmol C µmol 1 PPFD. Their in situ average LUEtower estimate matched well with our average MODIS-derived LUEchl (0.0229–0.0302 versus 0.0225–0.0310 µmol C µmol 1 PPFD from Fig. 4) but not with the MODIS-derived LUEcanopy over five

Acknowledgments

This project was supported by the NASA Carbon Cycle Science Program (Dr. Diane Wickland, Program Manager) under the auspices of the North American Carbon Program. Support was provided to the FCRN and BERMS programs through several Canadian funding sources (Environment Canada, the Canadian Forest Service, NSERC, CFCAS, BIOCAP, Action Plan 2000 and PERD).

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