North American vegetation dynamics observed with multi-resolution satellite data
Introduction
Land cover and land use change strongly influence terrestrial biogeophysical and biogeochemical process (Brovokin et al., 2004, DeFries et al., 1999, Houghton, 1999). Humans and changing climate, separately or in concert, have affected global vegetation, biogeochemical cycles, biophysical processes, and primary production. To infer North America vegetation changes we used a 1982 to 2005 record of normalized difference vegetation index (NDVI) data. This approach, using 8-km NDVI data from the (NOAA) National Ocean and Atmospheric Administration Advanced Very High Resolution Radiometer (AVHRR) instruments, has previously identified large-scale spatial and temporal patterns of vegetation response to climate (Lotsch et al., 2005, Myneni et al., 1997, Nemani et al., 2003, Zhou et al., 2003). Previous work by others, (Goetz et al., 2005, Gong and Shi, 2003, Ichii et al., 2002, Myneni et al., 1997, Nemani et al., 2003, Slayback et al., 2003, Tucker et al., 2001, Zhou et al., 2001), has addressed continental scale phenomena of changes in photosynthetic capacity since 1981–1982 from NDVI data. Investigations with coarse resolution data are limited in the ability to identify specific regional or local land use and land cover change mechanisms that could be responsible for NDVI anomalies.
A number of natural factors influence North American vegetation and primary production, and hence the NDVI: warming and possible reduced arctic snow cover (Chapin et al., 2000, Dye and Tucker, 2003); altered plant communities structure (Epstein et al., 2004, Sturm et al., 2005, Sturm et al., 2001, Tape et al., 2006); reduced permafrost extent and effects upon vegetative growth (Hobbie et al., 2002, Stokstad, 2004); insect and pathogen outbreaks (Ayres & Lombardero, 2000); severe drought (Angert et al., 2005, Barber et al., 2000, Ciais et al., 2005, Dai et al., 2004, Lotsch et al., 2005); and forest fire regimes (Flannigan et al., 2000). Anthropogenic influences on vegetation productivity include: more intensive agriculture practices (Malhi et al., 2001, Sainju et al., 2002); expansion of irrigated agriculture (Lemly et al., 2000, Tilman, 1999); decreasing productivity by removing biomass through urban expansion (Imhoff et al., 2000, Masek et al., 2000); and logging and subsequent regeneration (Howard et al., 2004). This analysis was undertaken to understand vegetation dynamics at a regional scale in North America, to explore possible mechanisms that can affect continental scale primary production. We were specifically interested in investigating NDVI anomalies and determining what caused them, through the combined use of AVHRR NDVI, Landsat, Ikonos, aerial photography, and ancillary data.
NDVI data from the NOAA series of AVHRR instruments over the past 24 years have shown variations in photosynthetic capacity across large areas of North America (Slayback et al., 2003). These observed NDVI trends have occurred in a variety of regions, implying possibly a variety of cause(s) (Goetz et al., 2005, Pickett and White, 1985, Tucker et al., 2001). Although a number of recent studies have found marked variations in NDVI throughout the Northern Hemisphere, they have not attributed these changes to regional factors that may include natural disturbances and/or human alterations to ecosystem functioning (Gong and Shi, 2003, Lotsch et al., 2005, Lucht et al., 2002, Myneni et al., 2001, Myneni et al., 1997, Nemani et al., 2003, Slayback et al., 2003, Tucker et al., 2001, Zhou et al., 2001). It is important to identify and quantify land cover type, because changes in land cover can alter ecosystem functioning and carbon storage (Baldocchi and Amthor, 2001, Olson, 1975).
NDVI (Tucker, 1979) is calculated from channel 1 (0.58–0.68 μm) and channel 2 (0.72–1.10 μm) from the NOAA AVHRR series of polar orbiting satellites as:
NDVI has been found to have a strong linear relationship to the fraction of photosynthetically active radiation (FPAR), the radiation that drives photosynthesis (0.4–0.7 μm) (Myneni et al., 1995, Sellers, 1985). FPAR is the main determinant of net primary productivity (NPP) of the ecosystem (Monteith, 1981). NDVI changes across North America are thus important because they represent variability in vegetation photosynthetic capacity. Few investigations have explored regional vegetation changes in North America responsible for the observed increases in Northern Hemisphere NDVI; (Jia et al., 2003, Stow et al., 2003, Walker et al., 2003) our investigation seeks to understand regional vegetation dynamics across the North American Continent observed by increases in the AVHRRs' NDVI.
Section snippets
AVHRR NDVI
We used the Global Inventory Modeling and Mapping Studies (GIMMS) version “g”, 1982 to 2005 bi-monthly AVHRR NDVI record (Tucker et al., 2005) because a consistent inter-calibrated data set is critical for long-term vegetation studies. These data, at 8-km (64 km2) resolution and bi-monthly intervals, have been processed to account for orbital drift, minimize cloud cover, compensate for sensor degradation, and effects of stratospheric volcanic aerosols (Brown et al., 2004, Tucker et al., 2005).
Results
NDVI anomalies in the AVHRR data revealed six areas for investigation with high-resolution data to evaluate land cover land use change. We selected six contiguous regions based on two assumptions: (I) A contiguous region greater than 2000 km2 has an NDVI trend greater than 0.1 from selected observational periods and; (II) High-resolution remote sensing data and corresponding validation data were available for intensive analysis for the entire AVHRR record. Six regions met these criteria: (1)
Synthesis
Six different areas displaying marked increases in NDVI values for the period from 1982 to 2005 have been examined using multi-temporal Landsat data and other ancillary data sources to provide attribution for the changes. These results indicated a complex interaction between anthropogenic changes and direct biophysical impacts. In some areas, especially in northern latitudes areas of the Mackenzie Delta and Newfoundland, the changes appear to be the result solely of biophysical response to a
Conclusion
North American vegetation dynamics driven by a number of different biophysical phenomena were revealed in this research. Discrete events over the past two decades induced change in ecosystem functioning that were identified with multi-resolution satellite imagery. We found that: (1) Multi-resolution data provides information critical to the state of knowledge of vegetation dynamics in North America; (2) land cover land use change driven by humans had a marked impact to North American
Acknowledgements
This research was funded by the National Aeronautics and Space Administration, North American Carbon Program grant NRA-04-OES-01. We would like to thank Dan Slayback, for the assistance in data preparation and are grateful to the anonymous referees for their comments which increased the clarity of the manuscript.
References (113)
- et al.
Patterns of seasonal dynamics of remotely sensed chlorophyll and physical environment in the Newfoundland region
Remote Sensing of Environment
(2001) - et al.
Assessing the consequences of global change for forest disturbance from herbivores and pathogens
The science of the total environment
(2000) - et al.
On the use of NDVI profiles as a tool for agriculture statistics: The case study of wheat yield estimate and forecast in Emilia Romagna
Remote Sensing of Environment
(1993) - et al.
Climate change and forest fires
The Science of the Total Environment
(2000) - et al.
Comparison of time series tasseled cap wetness and the normalized difference moisture index in detecting forest disturbances
Remote Sensing of Environment
(2005) - et al.
Impacts of vegetation dynamics on the identification of land-cover change in a biologically complex community in North Carolina, USA
Remote Sensing of Environment
(2002) - et al.
Spatial and temporal patterns in Arctic river ice breakup observed with MODIS and AVHRR time series
Remote Sensing of Environment
(2004) - et al.
Sources of error in accuracy assessment of thematic land-cover maps in the Brazilian Amazon
Remote Sensing of Environment
(2004) Thematic mapping from multitemporal image data using principal components transformation
Remote Sensing of Environment
(1984)- et al.
Structural stage in Pacific Northwest forest estimated using simple mixing models of multispectral images
Remote Sensing of Environment
(2002)