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Recent History of Large-Scale Ecosystem Disturbances in North America Derived from the AVHRR Satellite Record

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Recent History of Large-Scale Ecosystem Disturbances in North America Derived from the AVHRR Satellite Record Ecosystems (2005) 8: 808-824 DOI: 10.1007/~10021-005-0041-6 Recent History of Large-Scale Ecosystem Disturbances in North America Derived from the AVHRR Satellite Record Christopher potter,'" Pang-Ning d an,' Vipin ~umar,'Chris ~ucharik,~ Steven ~looster; Vanessa ~enovese,~Warren ohe en,^ and Sean ~eale~~ 'NASA Ames Research Center, Moffett Field, California 94035, USA;2~niversity of Minnesota, Minneapolis, Minnesota 55415, USA; 3~niversityof Wisconsin, Madison, Wisconsin 53706, USA; *~aliforniaState University Monterey Bay, Seaside 93955, California, USA; USDA Forest Service, Corvallis, Oregon 97331, USA Ecosystem structure and function are strongly af- over i9 years, areas potentially influenced by ma- fected by disturbance events, many of which in jor ecosystem disturbances (one FPAR-LO event North America are associated with seasonal tem- over the period 1982-2000) total to more than perature extremes, wildfires, and tropical storms. 766,000 km2.The periods of highest detection fre- This study was conducted to evaluate patterns in a quency were 1987-1989, 1995-1 997, and 1999. 19-year record of global satellite observations of Sub-continental regions of the Pacific Northwest, vegetation phenology from the advanced very high Alaska, and Central Canada had the highest pro- resolution radiometer (AVHRR) as a means to portion (>90%) of FPAR-LO pixels detected in characterize major ecosystem disturbance events forests, tundra shrublands, and wetland areas. The and regimes. The fraction absorbed of photosyn- Great Lakes region showed the highest proportion thetically active radiation (FPAR) by vegetation (39%) of FPAR-LO pixels detected in cropland canopies worldwide has been computed at a areas, whereas the western United States showed monthly time interval from 1982 to 2000 and the highest proportion f 16% ) of FPAR-LO pixels gridded at a spatial resolution of 8-krn globally. detected in grassland areas. Based on this analysis, Potential disturbance events were identified in the an historical picture is emerging of periodic FPAR time series by locating anomalously low droughts and heat waves, possibly coupled with values (FPAR-LO) that lasted longer than 12 con- herbivorous insect outbreaks, as among the most secutive months at any 8-km pixel. We can find important causes of ecosystem disturbance in North verifiable evidence of numerous disturbance types America. across North America, including major regional patterns of cold and heat waves, forest fires, tropi- Key words: ecosystem disturbance; remote sens- cal storms, and large-scale forest logging. Summed ing; fire; drought; forests. Ecosystem structure and function are strongly im- pacted by major disturbance events (Pickett and Received 29 March 2004; accepted 2 September 2004; published online 21 October 2005. White 1985; Walker and Willig 1999), many of *Corresponding author; e-mail. [email protected] which in North America are associated with sea- Large-Scale Ecosystem Disturbances 809 sonal temperature extremes, droughts, wildfires, the 18-year record of global satellite observations of and tropical storms. Potter and others (2003a,b) vegetation phenology from the advanced very high characterized a large scale ecological disturbance as resolution radiometer (AVHRR) as a time series to an event that results in a sustained disruption of characterize major ecosystem disturbance events ecosystem structure and function generally with and regimes. The fraction absorbed of photosyn- effects that last for time periods longer than a single thetically active radiation (FPAR) by vegetation seasonal growth cycle for native vegetation. Phys- canopies worldwide was computed at a monthly ical disturbance categories include fires, hurricanes, time interval from 1982 to 1999 and gridded at a floods, droughts, lava flows, and ice storms. Bio- spatial resolution of 0.5O I+titude/longitude. Po- genic disturbance categories include the impacts of tential disturbance events of large extent (greater herbivorous insects, mammals, and pathogens. than a single 8-km pixel area of 6400 ha) were Anthropogenic disturbance categories include log- identified in the FPAR time series by locating ging, deforestation, drainage of wetlands, clearing anomalously low values (FPAR-LO) that lasted for cultivation, chemical pollution, and alien spe- longer than 12 consecutive months at any pixel. cies introductions. Many of these events alter eco- Potter' and others (2003a) reported that nearly 400 system productivity and resource availability (light Mha of the global land surface could be identified and nutrients) for organisms on large spatial and with at least one FPAR-LO event over the 18-year temporal scales (Pickett and White 1985; Tihan time series. The majority of these potential distur- 1985). bance events occurred in tropical savanna and Ecosystem disturbances can contribute to the shrublands or in boreal forest ecosystem classes. current rise of carbon dioxide (C02)levels in the Verification of potential disturbance events from atmosphere (Schimel and others 200 1). Because the FPAR-LO analysis was carried out using docu- major 'pulses' of C02 and other trace gases from mented records of the timing of large-scale wildfires terrestrial biomass loss can be emitted to the at locations throughout the world. Disturbance atmosphere during large disturbance events, the regimes were further characterized by association timing, location, and magnitude of vegetation dis- analysis with historical climate events worldwide. turbance is presently a major uncertainty in This FPAR-LO detection approach is based on the understanding global biogeochemical cycles concept that leafy vegetation cover is likely the (Canadell and others 2000). Numerous studies most fragile and therefore perhaps the single most have been conducted to quantify carbon emissions vulnerable biotic component of terrestrial ecosys- from single categories of disturbance, principally tems to detectable alteration during major distur- biomass burning events, and generally with na- bance events, Vegetation leaf cover burns relatively tional or continental levels of resolution. These easily or can be readily blown down, cut to the studies include Kurz and Apps (1999), Houghton ground, or defoliated by herbivores. Leaf litter then and others (1999), Murph and others (2000), decomposes rapidly to blend in with background Amiro and others (2001) for portions of North soil attributes, at least compared to the large woody America, Fearnside (1997), Nepstad and others biomass components of shrub, woodland, and for- (1999), Potter and others (2001) for portions of est ecosystems. South America, Scholes and others (1996), Barbosa Earth-observing satellites have monitored daily and others (1999) for portions of Africa, Houghton leafy vegetation cover on land (also called 'green- and Hackler (1999) for portions of Southeast Asia, ness' cover) for more than 20 years (Myneni and and Hurst and others (1994) for Australia. Several others 1998). Like the normalized difference veg- studies have dealt- with global level effects of etation index (NDVI), FPAR is a common measure deforestation of carbon emissions (Andreae 1991; of greenness cover (Knyazikhin and others 1998), Houghton 1999; Potter 1999). ranging from zero (on barren land) to 100°h (for Potter and others (2003a) reported an approach dense cover). In theory, the higher the FPAR level for detection of large-scale ecosystem disturbance observed over the course of a seasonal plant (LSEDs) events based on sustained declines in growing cycle, the denser the green leaf cover and vegetation greenness observed by daily satellite (presumably, on average) the less disturbed the observations. This approach was global in scope, vegetation cover, and/or the longer the time period covered more than a decade of analysis, and since the last major disturbance. It is plausible that encompassed all potential categories of major eco- any significant and sustained decline in vegetation system disturbance-physical, biogenic, and FPAR observed from satellites represents a distur- anthropogenic-using a consistent method of bance event, a hypothesis we evaluated here using detection and analysis. This method was based on independent records of such disturbance events 810 C. Potter and others throughout North America. It is also possible that extreme climate events, such as droughts or cold Figure 1. FPAR-LO events in a 1988 on the Yucatan waves, that occur during the same season of the Peninsula of Mexico and b 1989 on the coast of North year, but are separated by 9-12 months in two Carolina, corresponding to the landfall areas of Hurri- consecutive yearly cycles could result in a lower canes Gilbert and Hugo, respectively. Top panels are than average FPAR over the period of impact. original (raw) FPAR values, scaled 0-256, and bottom panels are the deseasonalized FPAR anomalies in units of Hence, it is possible to expand the definition of an standard deviation (SD) in the1 2-month moving average. ecological disturbance to include consecutive years Dashed vertical lines show the iongest consecutive period of unfavorable growing season conditions for a gi- of anomalously LO monthly values. Owing to the use of a ven plant cover. moving average, the apparent start time of the FPAR-LO This study was conducted to evaluate patterns in events shown in the bottom panels may be up to 12- a 19-year (1982-2000) record of satellite AVHRR months shifted from the documented timing of the dis- observations of vegetation phenology
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