Search

Earlier snowmelt, warmer temperatures and herbivory are among the factors that influence high-latitude tundra productivity near the town of Utqiaġvik in northern Alaska. However, our understanding of the potential interactions between these factors is limited. MODIS observations provide cover fractions of vegetation, snow, standing water, and soil, and fractional absorption of photosynthetically active radiation by canopy chlorophyll (fAPAR _chl ) per pixel. Here, we evaluated a recent time-period (2001–2014) that the tundra experienced large interannual variability in vegetation productivity metrics (i.e. fAPAR _chl and APAR _chl ), which was explainable by both abiotic and biotic factors. We found earlier snowmelt to increase soil and vegetation cover, and productivity in June, while warmer temperatures significantly increased monthly productivity. However, abiotic factors failed to explain stark decreases in productivity during August of 2008, which coincided with a severe lemming outbreak. MODIS observations found this tundra ecosystem to completely recover two years later, resulting in elevated productivity. This study highlights the potential roles of both climate and herbivory in modulating the interannual variability of remotely retrieved plant productivity metrics in Arctic coastal tundra ecosystems.

Highlights: [•] We utilized different approaches to derive LUE and fPAR and examined their capabilities for carbon modeling. [•] Both crops, maize and soybean, and deciduous forest were selected to study. [•] fAPARchl showed closer seasonal dynamics with GPP than MODIS MOD15A2 fPAR. [•] Estimated GPP using fAPARchl was closer to flux tower based GPP. [•] Using a site-specific LUEmax value derived from an analytic approach can improve GPP estimates. [Copyright &y& Elsevier]

In the last three decades, substantial advancements have been made in understanding the global carbon cycle. Some of these advancements involve using the fraction of absorbed photosynthetically active radiation (fAPAR) by an entire canopy (fAPARcanopy) and/or the Normalized Difference Vegetation Index (NDVI) in modeling studies. In spite of these advancements, large uncertainties still remain. Zhang (Remote Sens. Environ., 2005) [1] tried to mitigate some of these uncertainties with the concept of using fAPAR that is restricted to the foliage chlorophyll (fAPARchl) instead of the entire canopy. In this current study, we calculated fAPARcanopy, fAPARchl, and foliage non-chlorophyll fAPAR (fAPARnon\mathchar "702D chl) for the Harvard Forest using a radiative transfer model and multi-temporal Earth Observing One (EO-1) Hyperion satellite images. The canopy-level proportions of foliar chlorophyll and non-chlorophyll absorption were determined at different seasons (spring, summer, autumn) in an effort to demonstrate temporal variations of three plant functional types: deciduous forest, coniferous forest, and grass. Comparisons were made for NDVI versus fAPARcanopy and for the Enhanced Vegetation Index (EVI) versus fAPARchl. In addition, EO-1 Hyperion images were utilized to simulate these new fAPARcanopy, fAPARchl, and fAPARnon\mathchar "702D chl products at 60 m as prototypes for the proposed NASA HyspIRI satellite spectrometer. These products should prove useful for future terrestrial carbon cycle and ecosystem studies. [ABSTRACT FROM PUBLISHER]

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 (r 2 =0.78) but only weakly related to APARcanopy (r 2 =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. [Copyright &y& Elsevier]