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Ecosystems (2005) 8: 808-824 DOI: 10.1007/~10021-005-0041-6

Recent History of Large-Scale 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 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, , and tropical . 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 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 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 , tropi- Key words: ecosystem disturbance; remote sens- cal storms, and large-scale forest logging. Summed ing; ; ; 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 , droughts, lava flows, and 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 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 (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 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 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 . 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 . 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 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 over North turbance event. Latitude is shown in decimal degrees N, America as a means to characterize major ecosys- and longitude in decimal degrees W. tem disturbance events and regimes at a spatial resolution of 8-km, or pixel sizes of about 64 km2. The same AVHRR greenness data set was applied by using a 19-year record of monthly satellite-ob- Hicke and others (2002) to compute net primary served FPAR. productivity (NPP) in North America using the Monthly FPAR values for the land surface were Carnegie-Ames-Stanford approach (CASA) carbon derived from the AVHRR data sets covering the cycle model. Although it was found that annual period 1982-2000. This FPAR data set was gener- ated using canopy radiative transfer algorithms NPP averaged 6.2 Pg C (1 Pg = loi5 gj and that regional trends in the CASA model NPP record (Knyazikhin and others 1998), which are designed varied substantially across the continent, this study to generate improved vegetation products for input by Hicke and others (2002) dealt only with one flux to terrestrial carbon flux calculations. These radia- in the continental (the terrestrial NPP tive transfer algorithms, developed for the moder- sink) and did not address ecosystem disturbances as ate resolution imaging spectroradiometer (MODIS) a potential source of carbon return to the atmo- aboard the NASA Terra satellite platform, account spheric pool of C02. In a more comprehensive for attenuation of direct and diffuse incident radi- study by Potter and others (2003b) using the ation by solving a three-dimensional formulation NASA-CASA , annual NPP in of the radiative transfer process in vegetation can- North America was reported to vary between 6 and opies. Monthly composite data from channels 1 7.5 Pg C per year over the period 1982-1999, and (visible) and 2 (near-infrared) of the AVHRR have the terrestrial sink for atmospheric C02 was fairly been processed according to the MODIS radiative consistent at between +0.2 and +0.3 Pg C per year. transfer algorithms and aggregated over the global land surface to 8-km spatial resolution. This aggregation level generates single grid cell (pixel) areas of approximately 6.4 x 10' ha (1 ha = 10" m2). The observed FPAR time series at each pixel was The AVHRR multi-year time series of vegetation first detrended (see example in Figure la) using a dynamics makes it possible to move beyond single linear adjustment, which is necessary to minimize disturbance events to conduct studies of "distur- the possibility that, in cases where there is a grad- bance regimes". A disturbance regime is defined ual but marked increase in monthly FPAR over the according to the spatial, temporal, and qualitative 18-years time series, any potential disturbance of disturbance events occurring within any events occurring relatively near the end of the given ecosystem type (Heinselman 1973). A natu- series are not overlooked. To remove the dominant ral disturbance regime (such as a forest fire cycle) seasonal oscilIations in vegetation phenology ob- can be described in terms of spatial extent (hect- served throughout the globe, our detrended FPAR ares) and distribution (patchiness), as well as the time series was subsequently 'deseasonalized' by frequency and seasonality of its occurrence over computing the 12-month running average time time, and its severity or intensity (that is, the en- series for every pixel location. ergy released per unit area and time). We describe An algorithm was next developed to identify any below an approach to better understand major significant and sustained declines in FPAR during ecosystem disturbance regimes on a global level the time series. Large-Scale Ecosystem Disturbances 81 1 a FPAR at la%tu& 20.55, longitude = -B6.?5

12-mom moving average (couM=l5 rnorrths)

12-motrtfi moving average (count= 18 months)

at-Id?LTr .-< $934 2385 $987 $988 $989 1QgZF); $!$2$ I:$%$?@ fmz 1M$995 1945 T8W $is98 fm cW0 "rj 812 C. Potter and others

occur during the dormant season, such as ice LSED = IF (FPARsd > 1.7) for >= damage or an extremely low season. Dor- (1) 12 MOc THEN 1 ELSE 0 mant season events may also affect the quality of the following leaf-out and green-up periods in the where FPARsd is the number of standard deviations growing season. Therefore, we cannot justify below the 18-years average monthly FPAR, and excluding the dormant season from our time series MO, is the number of consecutive months in the analysis. Furthermore, periodic errors in leafless 18-yr time series. Using a global 0.5" latitudellon- canopy (versus full canopy) estimates of FPAR- are gitude FPAR data set, the algorithm was tested with not likely to have a major ifnpact on the long-term a newly compiled data set of majored documented (19-year) 12-month running mean FPAR, against wildfires and cropland production failures world- which we have detected any and all of the FPAR- wide from 1982 to 1998 and was found to have a LO events. sensitivity of successful event detection at distur- In the use of a one-sided (LO) statistical t-test, bance area thresholds of at least 0.1 Mha in the rejection of the null hypothesis means that there is polar zones to 0.3 Mha at the equator (Potter and no difference between the 19-years average for the others 2003a). monthly FPAR level and the consistent FPAR-LO We hypothesized that significant declines in level identified in a string of 12 or more consecutive average annual FPAR levels can be defined to be time steps. An absolute value SD of 1.7 or more greater than 1.7 standard deviations (SD) below represents the 95% LO confidence level, SD of 2.0 or (LO) the 18-years average FPAR computed for any more represents the 97% LO confidence level, and specific pixel location. A "sustained" disturbance SD of 2.6 or more represents the 99% LO confidence event would be defined as any decline in average level (Stockburger 1998). Because we have first annual FPAR levels (at an assigned significance detrended the FPAR time series by linear regression, level) that lasts for a temporal threshold value of at the resulting data series should more closely least 12 consecutive monthly observations at any approximate a normal distribution. The resulting specific pixel location. The logic used here is that an data series have 18 degrees of freedom for tests of actual disturbance involves a sustained decline in significance (19 years - 1 for a one-tailed test). FPAR because the structure of the vegetation cover has been severely altered or destroyed during the FROM THE disturbance event, to a magnitude that lowers CONTINENTALRESULTS FPAR FPAR significantly for at least one seasonal growing TIMESERIES cycle, after which time regrowth and recovery of We applied the methods described above to the the former vegetation structure may permit FPAR 1982-2000 FPAR time series from AVHRR obser- to increase again. vations at 8-km resolution to identify potentiaI It is assumed that fairly common effects of LSED events in terrestrial ecosystems of North atmospheric interference with the AVHRR channel America, which we assume to include the nations of signals, such as heavy cover or smoke-de- the region commonly referred to as Central Amer- rived aerosols, would not persist (for example, as a ica. At a level of SD of 1.7 or more (95% confidence) false disturbance event) in the multi-year time for definition of disturbance intensity, we can detect series and thereby generate a FPAR-LO pattern 11,972 pixel locations at the threshold of 12 con- longer than about 6 months. By design, our dis- secutive monthly time steps for FPAR-LO events turbance algorithm should be insensitive to heavy (Figure 2a). For all vegetated land areas of North or smoke effects that occur practically America, the fraction of total land area that had at every year during the same season, or only epi- least one FRAR-LO event was 3.9%. Summed over sodically for 1 or 2 months at a time. If an inter- 19 years, these pixels together cover a total area of ference effect occurs every year at about the same just over 766,000 km2, which is slightly larger than time, it will be eliminated automatically as part of the state of Texas. We hypothesize from these re- the deseasonalization algorithm. One possible sults that each of these pixels identified in Figure 2a exception to this principle could be persistent has been affected to some degree by at least one atmospheric interference effects generated by ma- LSED over the past two decades. jor volcanic eruptions, such as the Pinatubo event of late 1991. Although it may be the case that FPAR can vary Geographic Distribution Patterns from year to year during dormant (non-growing) The distribution with latitude of all pixel areas de- , it is also possible for actual disturbances to tected at a SD of 1.7 or more level lasting more than 1

Large- Scale Ecosystem Disturbances 813

Figure 2. a Distribution throughout North America of 8-krn pixels as the first month and year for FPAR-LO lasting more than 12 consecutive months within the time series 1982-2000. b Continental sub- regions are listed as column headings in Table 1. Codes for sub-regi6ns are GLK, Great Lakes; PNW, Pacific Northwest; WUS, Western United States; SEG, Southeast and Gulf of Mexico; NES, Northeast; CCA, Central Canada; ALK, Alaska.

12 consecutive months of FPAR-LO shows poten- (Figure 2a) at SD of 1.7 or more indicates that 52% tial LSED events detected from northern arctic ex- of continental LSED areas were located in forested tremes of 78"N, throughout the middle latitudes, ecosystems (mainly in evergreen and mixed and down to the continental southern extreme at coniferous-deciduous forests), whereas 28% were 6"N. The distribution among major global vegeta- located in savanna and shrubland ecosystems (Ta- tion classes [delineated for this study according the ble 1). Only about 20% of continental LSED areas 2001 MODIS land cover product from Fried1 and were located in grassland and cropland ecosystems others (2002)l of pixel areas from FPAR-LO events combined. 814 C. Potter and others

Table 1. Percentages of FPAR-LO Events (8-km pixel resolution) in Vegetation Cover Classes for North America as a Whole and for Selected Sub-regions over the Period 1982-2000.

Percentage FPAR-LO pixels

-- Vegetation class Fraction of class NAM GLK PNW WUS SEG NES CCA ALK Mixed wetland Evergreen needleleaf forest Evergreen broadleaf forest Deciduous needleleaf forest Deciduous broadleaf forest Mixed forest Closed shrubland Open shrubland Woody grasslandlsavanna Savannas Grasslands Croplands

"Fraction of Class" percentages were computed as the number of FPAR-LO events compared to the total number of 8-km pixels in North America for vegetation classes delineated according the 2001 MODlS land cover product from Fried1 and others (2002). Other percentages are computed as number of FPAR-LO events within each region, broken down by vegetation cover classes. Column percentages for sub-regions total to 100. Barren (desert) and urban lands make up less than 1 % of the FPAR-LO pixel total in all regions. Column heading codes: NAM, North America; and sub-regions GLK, Great Lakes; PNW, Paafic Northwest; WUS, Western United States; SEG, Southeast and Gulf of Mexico; NES, Northeast; CCA, Central Canada; ALK, Alaska. See Figure 2b for sub- region area boundaries.

As discussed briefly by Potter and others (2003a), The western U. S. ecoregion showed the highest our LSED detection method based on FPAR-LO proportion (16%) of FPAR-LO pixels detected in events is best suited to ecosystems where there is a grassland areas. predominance of perennial woody vegetation cover in the region. Forests and shrublands recover rel- Temporal Variations atively slowly from a sudden loss of green leaf cover, at least in contrast to grasslands and culti- When viewed in terms of the consecutive monthly vated ecosystems, where a notable fraction of the time steps for FPAR-LO more than 12 months, the green leaf biomass (and hence the FPAR) that is distribution of total pixel area at SD of 1.7 or more lost during a disturbance can be recovered fairly shows that 95% of the potential LSED coverage rapidly through herbaceous sprouting and plant had a duration of between 12 and 20 consecutive regrowth during the same year as the disturbance months for FPAR-LO events (Figure 3). Beginning event. Nonetheless, the results shown in Table 1 from the maximum concentration of pixel areas at indicate that our LSED detection frequency is just 13 consecutive months (34% of all FPAR-LO pix- as high within grassland and cropland classes as it is els), the decline in area coverage with an increase within certain forested vegetation classes. Hence, in the number of consecutive monthly time steps there is no overriding bias to forested vegetation in was nearly exponential (R~= 0.97), out to the the FPAR-LO detection method. maximum value of 28 consecutive months of On a sub-continental basis, a11 ecoregions within FPAR-LO. We could detect no significant trends in North America (Figure 2b) showed forests and the relationship between latitude zone and number tundra shrublands as the primary vegetation types of consecutive monthly time steps for FPAR-LO where the FPAR-LO algorithm detected potential events at SD of 1.7 or more. However, all of the LSED events, lead by the Pacific Northwest, Alaska, longest FPAR-LO events of between 26 and 28 and Central Canada with fewer than 10% FPAR- consecutive months for FPAR-LO were detected at LO events detected in areas dominated by grass- latitude zones south of 52%. lands and croplands (Table 1). The Great Lakes The distribution according to the start month for region showed the highest proportion (39%) of pixel areas detected at the SD of 1.7 or more level of FPAR-LO pixels detected in cropland areas, fol- FPAR-LO Iasting more than 1'2 consecutive months lowed by the eastern U. S. ecoregions with over shows that the periods of highest detection fre- 20% coverage of FPAR-LO pixels in cropland areas. quency were 1987-1989, 1995-1997, and 1999 Figure 4. Distribution according to start month of land area detected with at least one FPAR-LO event in the time series 1982-2000.

0 1982 84 86 88 90 92 94 96 98 2000 Start Month

(Figure 4). It is plausible that these time periods of Tropical Storms of the high detection frequency for FPAR-LO events can be verified as coincident with major climate events A series of major tropical storms in Category 3 or 4 during the time series. This kind of verification (sustained winds in excess of 178 km per hr) made approach follows in a region-by-region examina- landfall across the southeastern coast of the United tion of the patterns of potential LSED areas shown States and along the Gulf Coasts of the United in Figures 2-4. States and Mexico during the 1980s (Table 2; Landsea 1993). By locating 8-km pixels in the Reconstruction of Regional Ecosystem vicinity of documented landfall points of storms Disturbance Types (Powell and Aberson 2001), five of the strongest hurricanes making North American landfall in the Verification of actual changes in ecosystems due 1980s were readily detected as FPAR-LO events in to LSED events must be based on demonstrated the 19-years AVHRR time series. These five hurri- relationships between the results shown in Fig- canes (in order of occurrence) were called Alicia, ures 2, 3 and 4 and independently confirmed Elena, Gloria, Gilbert, and Hugo. historical events, together with any major climate The impacts of Hurricane Gilbert in 1988 on the anomalies in the regions where potential LSED Yucatan Peninsula of Mexico and Hurricane Hugo events are detected. In the topic sections that in 1989 on the North Carolina coast (Figure 1) are follow, we summarize regional disturbance types representative of disturbance to forest ecosystems chronologically, with focus on weI1-documented detected as FPAR-LO events throughout south- tropical storms, wildfires, droughts, heat waves, eastern North America and Caribbean Islands dur- cold waves, and during the 1980s and ing this period of severe tropical damage. The 1990s. hurricane season in North America extends from 816 C. Potter and others

Table 2. Hurricanes (Category 3 and higher) of the 1980s Detected as FPAR-LO Events (after Powell and Aberson 2001 )

Year Hurricane Category Landfail location Landfall LatiLon

1983 Micia 3 SE Texas, USA 28.9% 95.0w 1985 Elena 3 Mississippi, USA 30.2% 88.8w 1985 Gloria 3 East Coast, USA 35.5% 75.5OW 1988 Gilbert 3 East Coast, Mexico 20.4% 86.5?N, 23.9% 97.0°W 1989 Hugo 4 North Carolina, USA 33.5-%J80.3w

June to November, with storms most common in 1998). Versions of these 30-m resolution maps September, when ocean temperatures are warmest. were created following methods from Cohen and The FPAR time series in Figure 1 show this ex- others (2002) and aggregated to 8-km spatial grids pected timing of FPAR-LO events detected in late for OR and WA using extended geographic areas or early of 1988 and 1989. and time periods. Comparisons between our 8-km Unlike the examples shown in Table 2, Hurri- FPAR-LO pixels and the PNW Landsat-derived cane Andrew, a Category 4 storm that struck disturbance locations showed that the maximum southern Florida in 1992, could not be detected as a disturbance area coverage in any 8-km grid cell for FPAR-LO event in the 19-years AVHRR time series. OR and WA was about 33%. Compared to 1984- The probable explanation is that the landfall areas 1988 and 1988-1992 Landsat-derived maps for in southern Florida for Hurricane Andrew were not WA, we find that the FPAR-LO disturbance detec- dominated by forest vegetation cover, but instead tion algorithm can reliably differentiate FPAR sig- by grassland and wetland areas. Hence, the. dis- in relatively disturbed (FPAR-LO at SD 2 turbance detection method with thresholds of SD 2.0) versus non-disturbed forest areas for the re- of 1.7 or more FPAR-LO lasting more than 12 gion (Figure 5). consecutive months will not reveal areas of pre- More specifically, Landsat products generated dominantly annual herbaceaous cover that would from the method of Cohen and others (2002) show only be disrupted for several weeks or a month due disturbed forest cover in 95% of the 8-km pixels to a severe wind event. where our AVHRR algorithm detects an FPAR-LO event during the matching time period, provided

Pacific Northwest Logging-- - of the 1980s that there is greater than 50% forest cover overall in the 8-krn pixel area. Wherever the MODIS 1-km The extent to which mature forests (>80 years old) land cover map indicates that there is less than have been disturbed in the Pacific Northwest 50% forest cover in a 8-kin resolution pixel, our (PNW) has been estimated at over 75% by the result is less than a 20% match (8-km pixel-by- 1990s (NRC 2000), the majority of which is pixel for disturbed area cover) between the two attributable to logging. The PNW states of Oregon disturbance mapping methods. Based on these re- (OR) and Washington (WA) are notable as having sults, we surmise that more than 50% forest cover had relatively large forest logging and im- is a reasonable lower cutoff for the differentiation pacts during the 1980s. Additionally, repeated of forest disturbance versus non-disturbance FPAR stand-replacing disturbances have been rare signatures in the AVHRR time series. (0.03% of the forest area in OR) over the time frame of the past 30 years (Cohen and others Droughts Heat 2002). These unique attributes of the PNW region and Waves facilitate comparisons of the FPAR-LO event pat- Dry hot events were common throughout terns shown in Figure 2a with many large-scale the 1982-1998 time period in the south-central forest disturbance events throughout OR and WA. Cohen and others (2002) have developed a new Figure 5. FPAR monthly time series for single 8 km set of Landsat-based maps for historical forest dis- pixels in NW Washington state with a 4.3% disturbed turbance in OR and WA. These maps of stand- forest area detected during the period 19841988, and b replacing forest disturbances in the ~orthwest 0% disturbed forest area detected duling the period Forest Plan area between 1972 and 2002 were 1984-1 988 [determined from Landsat imagery according generated using a change detection method that is to the rnethbds of Cohen and others (2002)]. Both approximately 90% accurate (Cohen and others are in areas with more than 50% forest cover. Large-Scale Ecosystem Disturbances 81 7

a FPAR zd igtitude 48.13, longitude s -123.93

rm2 1983 rm 1985 1m 1987 $9~!989 tw 1993 1- ts5 19s -1m $999 2000 290:

f, FPAR at lathxie P 47-37, longitude = -123.77

12mrtth moving average 818 C. Potter and others

United States. The first regional summer drought drought of 1983 seems to have had an impact early and heat wave to occur in the 1980s affected large in the FPAR time series, and the return of dry hot parts of the southeastern United States in 1986. summer conditions could be detected a second time Extreme July temperatures in Georgia, North Car- as the downturn of FPAR in 1993 through 1994, olina, and South Carolina were the warmest re- and a third time in 1998. corded in the 20th century (Bergman and others, In some cases, slight increases in detection sen- 1986; Karl and Young 1987). The unusually dry sitivity revealed further evidence of drought-in- hot weather resulted in severe losses to agriculture. duced crop failures. For instance, we found that by In 1988, a severe summer drought affected the reducing our SD of 1.7 or more threshold to SD of central and eastern United States. Dry weather 1.5 or more, sensitivity of the disturbance detection began in April and persisted through June. Record algorithm was increased notably in croplands of the high temperatures occurred throughout the sum- Midwestern states. The detection with a threshold mer in the midwest and northeastern regions, with of SD of 1.5 or more identified many more FPAR- many locations setting all-time records for June. LO pixels in Nebraska and South Dakota where the Record numbers of forest fires broke out across the 1988 droughts appear to be the cause of extensive western United States, including the Yellowstone reduction in plant greenness. National Park fire (Renkin and Despain 1992). Again in 1989, severe summer drought over much Large-Scale Forest Fires of the northern plains resulted in significant losses A critical set of historical disturbance events avail- to agriculture. Drought severely impacted the able for verification of FPAR-LO events as LSEDs northwestern and southwestern parts of . are well-documented wildfires that burned areas At this same time, California was experiencing the reported to cover tens of thousands of hectares in a third year of an historic 7-year drought (1986- single year or growing season. A list of such events 1993). was compiled (Table 3) using publications and re- During the June-July period of 1993, much of ports from the North American fire literature. The the southeastern United States received less than list in Table 3 is not intended to represent an 50% of normal rainfall along with temperatures exhaustive set of North American fire events over several degrees above normal. The southeast as a the 19-years period of the FPAR record, but instead whole recorded the second driest July on record is a list of the largest fire events that could be since 1895 (the driest was 1983). The states of confirmed for their timing of initiation (to within Alabama, Georgia, North Carolina, South Carolina, about 3 months) and geographic location (to Tennessee, and Virginia had their hottest July on within approximately 1 latitude and longitude). record since 1895. Selected wildfire areas have been confirmed for In 1998, drought and extreme heat affected a timing and location using Landsat and other rela- large area of the south-central United States from tively high resolution satellite images (Arino and Texas and Oklahoma eastward to the Carolinas, Plurnrner 1999). Georgia, and Florida. Dry weather began in mid- We find that within each geographic area of the March 1998. Many locations set daily maximum confirmed wildfire events listed in Table 3, an temperature records during the summer of 1998. FPAR-LO event was detected during the reported Locations along the east coast of Florida set a time period of actual wildfire activity. As an number of high minimum records probably as a example, the FPAR time series for the Yellowstone result of the insulating affect of the aerosols gen- ' National Park fire (Figure 7) shows a significant erated by wildfires and the abnormally warm wa- FPAR-LO event (SD 2 2.0) beginning during the ters in the Gulf of Mexico and Atlantic Ocean. summer of 1988. This pixel location coincides with Impacts of each of these severe drought years can the North Fork Fire that spread on the edge of the be detected in the start times and general locations Park toward West Yellowstone, Montana. The of FPAR-LO events throughout the southeastern, recovery back to long-term average FPAR required central, and northern plains regions of the United nearly 2 years. States (Figure 2a). A time series example from North Carolina (Figure 6) illustrates the impact of 1995-1997 repeated summer droughts and heat waves on the and Blizzards of regional greenness profile. The start of the largest According to the U. S. , FPAR-LO event was detected early in 1986 and the of 1995-1 996 featured abnormally cold lasted for slightly longer than 12 months, followed and snowy conditions in Canada and United States. by a gradual recovery during the next 2 years. The For the winter season as whole, temperatures Large-Scale Ecosystem Disturbances 819

FPAR at latitude - 35.87, ton$tude = -7 9.72

i982 39233 13% $985 1w 19~7I= 1s~1936 1991 im 19% 1394 1995 19% 1297 4998 3999 sow zoo: 12-month moving amp(cuuntil3 months)

Figure 6. FPAR-LO event in North Carolina 1986, showing drought and heat wave impacts on southeastern U.S. greenness profiles.

Table 3. List of Major Forest Wildfires on Record for North America in the 1980s and 1990s

Year Location Area Burned (ha) Lat/Lon Notes

1987 Stanislaus, 59,000 38ON 120w Stanislaus National California USA Forest 1987 Siskiyou,Oregon 5 1,000 42% 124OW Silver fire, Siskiyou USA National Forest 1988 Yellowstone, > 500,000 44.6ON 110.7°W Wyoming USA* 1989 Manitoba, Canada >400,000 51°N 97OW Lake Manitoba, spread northward for 800 km 1989 Quebec, Canada > 200,000 52ON 75w and 1991 1997 Alaska USA >200,000 63-64ON 159OW Inowak fire (100 miles SW of McGrath), Simels and Magitchlie Creek fires Galena District 1998 Mexico* > 500,000 17-22ON 94O-98OW Chiapas, Oaxaca

Sources: Weatherspoon and Skinner (1995); Jeffrey (1989); Kasischke and others (1999); Galindo and others. (2003); Canadian Interagency Fire Center (2001) Reports, Winnipeg, Manitoba *Fire location has been confirmed using Landsat and ATSR (Along Track Scanning Radiometer) satellite images (Arino and Plummer 1999)

averaged 3-5°C below normal across central Can- began in late January 1996 across the northern ada, and 1-2°C below normal across the northern plains and northern Rockies. The most notable cold tier of the United States. Arctic airmass movement wave occurred during early February 1996, when 820 C. Potter and others

FPAR at f ati#ude E 4-62, londtude = -111.72

72-mt)rrth moving average (~bljnfr14rnmfbs]

Figure 7. FPAR-LO event in the summer of 1988, centered several kilometres south of West Yellowstone, Montana at the North Fork Fire locations spreading from Yellowstone National Park. temperatures averaged 11-1 7°C below normal southwestern Ontario, and over much of south- from the southern Canadian prairies southeastward central Canada (Figure 2a). A time series example to the western Great Lakes. The Midwestern Cli- from Wisconsin (Figure 8) demonstrates the impact mate Center reported that in four states (Illinois, of extended winter conditions into the sea- Iowa, Minnesota, Wisconsin) this was one of sons of 1996 and 1997 on the regional greenness coldest weather events of the 20th Century. profile. The seasonal pattern of monthly greenness In the first week of January, 1996, much of the values for these pixels suggests a significant delay in eastern seaboard received from 1 to 3 feet of snow the spring 'green-up' in both 1996 and 1997, during the " of '96". Snowfall totals by the compared to all the other spring seasons in the 19- end of February 1996 averaged more than 15O0/0 of years FPAR time series. Instead of a typical rapid normal across the -upper midwestern United States. greening from March to May, both 1996 and 1997 The Aviation Weather Center of Kansas City, MO showed a 1-2 month delay in spring greening and a and Intellicast reported extensive tree damage in shorter growing season overall. The 1996 spring- the north-central United States due to the heavy, summer growing season was the shortest of all, wet nature of the snow fall. The winter of 1996- whereas the end of the 1997 spring-summer 1997 brought a second consecutive season of growing season tends back toward an average year unusually heavy snowfall across the upper Mid- duration. western states, with 2-3 times the mean annual As shown in Table 1, about 60% of the 8-krn amounts. pixels in the Great Lakes region that show the ex- Impacts of these two consecutive cold waves in tended decline in mean greenness over 1996-1 997 1996 and 1997 could be detected in the start times were classified as predominantly coniferous and and general locations of FPAR-LO events mixed forest cover (including wetlands). The throughout lower Wisconsin, Michigan, and remaining 40% of the pixels were classified as Large-Scale Ecosystem Disturbances 82 1

FPAR at latitude = 43.89, fongitt.de -88.88

12-mar& moving average [count=20 mo&)

Figure 8. FPAR-LO event in Wisconsin 1996-1997, showing 2 consecutive years of winter-spring cold wave impacts on midwestem U.S. greenness profiles. predominantly cropland cover, although many could have been a delayed spring planting date of stands of trees will be mixed with crop cover at the 20-25 days compared to optimal planting dates in 8-krn pixel resolution. Because the 1996-1 997 these same regions (JA. Foley and others, submit- Great Lakes FPAR-LO anomaly was detected in ted). many different vegetation types, this suggests re- Although we can characterize the observed gional climate phenomena at work, rather than a FPAR-LO patterns throughout the Great Lakes re- species-specific production decline in just one type gion during 1996-1997 as a LSED, this type of of plant cover. anomaly in the satellite greenness record may not To corroborate these anomalies in FPAR-LO involve a conventional destruction of forest wood patterns throughout the Great Lakes region, we biomass that is typical of a wildfire or a hurricane. examined 2 1-year (-1980-2000) monthly and daily Rather, 2 consecutive years of lower than average climate records at 0.5" latitude x 0.5" longitude growing season length and decreased GDD accu- resolution (New and others 2000; IGstler and oth- mulations due to prolonged winter seasons would ers 200 1). Annual growing degree day (GDD; base represent a similar disruption in annual produc- 8°C) anomalies for 1996 and 1997 (relative to the tivity that is traceable to the slow phenologic re- 1980-2000 annual averages) show that plant sponses illustrated in Figure 8. growth was likely slowed. Lower than average GDD accumulations were observed during 1996 Western Canadian Drought and Heat and 1997 over most of Michigan, southwestern 1999-2000 Ontario, and in lower Wisconsin (Figure 9). Wave of Numerical simulations with an agroecosystem An extensive coverage of FPAR-LO events is model (Kucharik 2003) suggested that a key limi- notable (Figure 2a) starting in late 1998 and early tation to crop deveIopment during these two years 1999 over the Pacific coastal regions of Canada and 822 C. Potter and others

Figure 9. Anomalies in growing season (defined as the time period from the last spring freeze to the first fall freeze, inclusive, using 0°C as a temperature threshold) growing degree days (GDD, base 8°C) for 1996 (left) and 1997 (rQht).

the boreal mountain zones of British Columbia and quency over time. To this end, we are able to draw the Yukon. The most probable explanation for a several conclusions on the basis of results from the LSED event in the region is extreme drought and analysis of the 19-years FPAR time series from heat waves. The Meteorological Service of Canada AVHRR observations. reported that 1998 was one of the driest and First, our method to detect LSED using satellite warmest in the past 55 years (-41 % greenness images is most effective in places where lower rainfall and + 1.3"C higher temperature, there is a predominance of perennial forest or compared to averages over 1948-2003) for the re- shrub vegetation cover. In grasslands and culti- gion of the northern British Columbia Mountains vated ecosystems, green leaf biomass is produced and the Yukon (Environment Canada 1999). A annually and any leaf cover that is altered during a second consecutive heat wave followed in the disturbance could be recovered in large part summer of 1999, particularly along the Pacific through regrowth during the same season or year Coast region of Canada, with temperatures 1.2"C as the disturbance event. Consequently, our above the same long-term annual mean. method of defining FPAR-LO events is designed to The 1998-1999 LSED event in British Columbia perfom best in the detection of major disturbances may also be linked to recent outbreaks of bark in forest and shrubland ecosystems, where biomass beetles (Logan and Powell 2001) in forests domi- storage in perennial woody tissues may be lost or nated by pine, spruce, or fir species. These wood- damaged in fires, hurricanes, or logging operations. boring insects favor mild (warm) and then This is especially applicable where green leaf cover typically spread in search of new tree food sources in the canopy cannot regenerate fully during the in July and August, when trees are most vulnerable same year as the disturbance event. In the case of to infestation due to water deficiency. Damage prolonged droughts, heat waves, or herbivorous done by the bark beetles can eventualIy kill trees, insect outbreaks, our FPAR-LO methods can detect turning green leaf canopies to brown over large major disturbances in grasslands and cultivated areas, and subsequently increasing the risk of ecosystems, as well as in forested areas. wildfire. Second, the widespread impacts of repeated winter cold waves and delayed springtime warming suggest that these events can contribute to regional disturbance regimes on a geographic scale similar to A main objective of this study was to better that of major summer droughts and heat waves. understand historical patterns of ecosystem distur- Although there may be a need for further valida- bance events throughout North America and to tion of the causes for observed FPAR-LO patterns characterize major disturbance regimes in terms of throughout the Great Lakes region during 1996- spatial extent, geographic distribution, and fre- 1997, the large spatial coverage of this potential Large-Scale Ecosystem Disturbances 823

LSED event has not been reported previously in Bergman KH, Ropelewski CF, HaIpert MS. 1986. The record any public documents known to us. southeast drought of 1986. Weathenvise 39:262-6. Third, several notable limitations exist in the Canadell JG, Mooney HA, Baldocchi DD, Berry JA, Ehleringer JR, Field CB, Gower ST, Hollinger DY, Hunt JE, Jackson RB, use of satellite image pixels as coarse as 8-km Running SW, Shaver GR, Steffen W, Trumbore SE, Valentini resolution. Small-scale logging and partial tree R, Bond BY. 2000. Carbon metabolism of the terrestrial bio- removal activities cannot be detected reliably at sphere: a multi-technique approach for improving under- this resolution. The same can be said for wildfires standing. Ecosystems 3: 115-30. smaller than 6,400 ha of area burned. Flooding Canadian Interagency Forest Fire Centre (CIFFC). 2001. Canada along major rivers is likely to be localized in its Report 2001, Winnipeg, Canada. p 6. disturbance impacts, and therefore not detected at Cohen WB, FiorelIa M, Gray J, Helmer E, Anderson K. 1998. An efficient and accurate method for mapping forest clearcuts in this resolution. Ice storms and localized wind the Pacific Northwest using Landsat imagery. Photogram Eng storms seem to fall into this same category of Remote Sens 64:293-300. being below the 8-krn detection level for effects Cohen W, Spies T, Alig R, Oetter D, Maiersperger T, Fiorella M. on satellite FPAR. 2002. Characterizing 23 years (1972-1995) of stand replace- In closing, we suggest that the historical AVHRR ment' disturbance in western Oregon forests with Landsat satellite greenness record holds numerous undis- imagery. Ecosystems 5: 122-1 37. covered patterns that may change scientific views DeFries R, Townshend J. 1994. NDVI-derived Iand cover classi- of continental and global alterations in the land fication at global scales. Intern1 J Remote Sens 15:3567-86. Environment Canada. 1999. Climate Trends and Variations 20 surface over the past years. In North America Bulletin for Canada, Atmospheric Environment Service alone, a picture is emerging of periodic droughts Downsview, Ontario, p 2. and heat waves, possibly coupled with herbivorous Fearnside PM. 1997. Greenhouse gases from deforestation in insect outbreaks, as among the most important Brazilian Amazonia: net committed emissions. Clim Change causes of ecosystem disturbance in recent times. If 35:321-60. temperatures continues to warm over interior for- Fried1 MA, McIver DK, Hodges JCF, Zhang XY, Muchoney D, ested areas of the continent, the results presented StrahIer AH, Woodcock CE, Gopal S, Schneider A, coopei A, Baccini A, Gao F, Schaaf C. 2002. 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