Supplementary Information

Transcription

1 Supplementary Information The 46 sites that were used in this study represent gradients in climate, tree cover, and disturbance frequency (Figure 1; Table S3). The sites fall within a region of Canada that contains a substantial number of decreasing AVHRR NDVI trends. Tree cover, according to the MODIS VCF product, decreases from south to north and fire disturbance frequency, according to the Canadian Large Fire Database, decreases from west to east. Several different thresholds were used to define forest cover after aggregating the Canadian EOSD Landsat-based land cover map to a 500-m spatial resolution (Figure S12). Positive NDVI trends dominated for open forest and non-forest vegetation, while undisturbed forests, which made up most of the undisturbed pixels, have the smallest magnitude trends. Similar to previous studies using both ground and remote sensing data (Stow et al., 2004; Neigh et al., 2008; Fraser et al., 2011; McManus et al., 2012), tundra areas above the treeline showed strongly positive trends. It is important to note that EOSD data set and land cover definitions affect interpretation of the results. Specifically, forests were defined at the Landsat-scale as having more than 20% tree cover. Therefore, if 100% of one panel is forested the total tree cover could actually be as low as 20%. Mean annual temperatures across Canada show a strong latitudinal gradient, with southern sites having higher temperatures and more northern sites having lower mean annual temperatures (Figure S10; Table S1). In contrast, the gradient in precipitation is longitudinal, with a decline from east to west and then a sharp increase due to elevation in the Canadian Rockies. Most of the Landsat sidelap regions included in our study experienced warming over 31

2 the study period, with the highest warming observed in northern Quebec and Labrador. Much of this same region experienced increasing precipitation over the study period, with the exception of Central Quebec, where there is a local decrease in precipitation that is anomalously large, and which we suspect arises from low quality data in this data-sparse region. In the western study sites included in our analysis (i.e., west of Hudson Bay), precipitation mostly decreased during the study period. Landsat NDVI trends for undisturbed forests (Figure S11) appear to follow joint patterns of changes in precipitation and temperature that are mirrored by patterns in the amount of disturbed forest: eastern sites, which are more humid and have lower relative proportions of disturbance, show the most positive NDVI trends. Conversely, western sites are drier, have more disturbance, and show decreasing NDVI trends. Forest cover is partly controlled by disturbance history, but shows decreases from south to north in response to large temperature gradients (Table S3 and Figure S10). Modest greening trends at sites in Alberta (Figures S11 and S10) probably reflect a combination of gaps or errors of omission in disturbance data sets used in this study (Table S2), and expansion of woody vegetation in understory or canopy gaps. Our algorithm that detects non-fire disturbance only provides data from the start of the Landsat 5 mission, and hence does not include information on insects and harvest disturbance that occurred prior to Younger forests that are regrowing from logging or recovering from such disturbances would show positive trends that are similar to those that are recovering from fire disturbance (Figures 2 and S13). Landsat NDVI time series are shown aggregated to the regional scale 32

3 (Figure S1) or sampled from individual sites (Figures S2-S7). The regionalscale time series are for undisturbed forests only and correspond to the data presented in Figure 3. In Figure 3 there is very little variation in NDVI within each region, however, each panel shows a slight dip in NDVI around 1994 that might correspond to changes in the Landsat 5 orbit during this time. Another potential source of variation in the NDVI time series is a change or trend in the composite DOY. Our mean composite DOY was around DOY 212 and previous work has shown that the variation in compositing dates within the compositing window of DOY does not have much impact on the output NDVI time series (Sulla-Menashe et al., 2016). At the sitescale, pixels were sampled from the largest disturbance events to represent different periods of regrowth. For comparison, samples were also taken from undisturbed land cover types, which show on average smaller variation in NDVI than the disturbed time series. For disturbances that happen around the beginning of the time series, the trend is torqued in the positive direction by regrowth (e.g., the first panel in Figure S7). Disturbances that happen in the middle or end of the time series torque the trend in the negative direction (e.g., the second panel in Figure S7). 33

4 SI Figures 34

5 Figure S1: Average NDVI trends for each ecozone for undisturbed forests only. The panels are sorted by longitude with the most eastern ecozones at the top to the most western ecozones at the bottom. Each point in each plot represents an average across 500 sampled panels. The grey area presents ± 1 standard deviation from the sample mean and the solid black line is the linear trend of mean NDVI over time. 35

6 Figure S2: Average NDVI trends within disturbed and undisturbed areas for the Schefferville site within the East Taiga Shield ecozone. Each point represents an average across 500 sampled panels. The grey area presents ± 1 standard deviation from the sample mean and the solid black line is the linear trend of mean NDVI over time. 36

7 Figure S3: Average NDVI trends within disturbed and undisturbed areas for the Mistassini site within the Boreal Shield ecozone. Each point represents an average across 500 sampled panels. The grey area presents ± 1 standard deviation from the sample mean and the solid black line is the linear trend of mean NDVI over time. 37

8 Figure S4: Average NDVI trends within disturbed and undisturbed areas for the Missisa site in the Hudson Plain ecozone. Each point represents an average across 500 sampled panels. The grey area presents ± 1 standard deviation from the sample mean and the solid black line is the linear trend of mean NDVI over time. 38

9 Figure S5: Average NDVI trends within disturbed and undisturbed areas for the Amisk site within the West Taiga Shield ecozone. Each point represents an average across 500 sampled panels. The grey area presents ± 1 standard deviation from the sample mean and the solid black line is the linear trend of mean NDVI over time. 39

10 Figure S6: Average NDVI trends within disturbed and undisturbed areas for the Fort McMurray site within the Boreal Plain ecozone. Each point represents an average across 500 sampled panels. The grey area presents ± 1 standard deviation from the sample mean and the solid black line is the linear trend of mean NDVI over time. 40

11 Figure S7: Average NDVI trends within disturbed and undisturbed areas for the Zama site within the Taiga Plain ecozone. Each point represents an average across 500 sampled panels. The grey area presents ± 1 standard deviation from the sample mean and the solid black line is the linear trend of mean NDVI over time. 41

12 Figure S8: Boxplots for AVHRR NDVI trends from the GIMMS3g dataset grouped by time since disturbance and for undisturbed forests. The numbers to the right of the plot show the total number of 8-km pixels in each category, there are a total of 11,000 AVHRR pixels available for the analysis. Due to the data sparcity, these plots include all trends disregarding stastical significance. The historical disturbance information is from the Canadian LFDB (Stocks et al., 2002) and augmented by additional disturbances identified using the CCDC algorithm (Zhu et al., 2012). The forest cover information is from the EOSD land cover map of Canada (Wulder et al., 2008). At the AVHRR scale a pixel was considered undisturbed forest with a forest proportion of at least 0.6 and a disturbed proportion less than 0.1. For the two disturbance categories, the disturbed proportion during that era had to be higher than

13 Figure S9: Boxplots of AVHRR NDVI trends from the GIMMS3g dataset for undisturbed forest areas grouped by ecozone (from ca/english/ecozones/index.htm), with eastern ecozones plotted on the top and western ecozones on the bottom. The numbers to the right of the plot show the total number of 8-km pixels in each category, there are a total of 11,000 AVHRR pixels available for the analysis. Due to the data sparcity, these plots include all trends disregarding stastical significance. At the AVHRR scale a pixel was considered undisturbed forest with a forest proportion of at least 0.6 and a disturbed proportion less than

14 Figure S10: Median Landsat NDVI trends for undisturbed forest areas within each site overlain on maps of regional climate and climate change including (A) mean annual temperature for , (B) mean annual precipitation for , (C) mean annual temperature trends between 1984 and 2011, and (D) mean annual precipitation trends between 1984 and The climate data are derived from the Daily 10-km Gridded Climate Dataset for Canada, which covers all of Canada from 1950 to 2013 (National Land and Water Information Service, Agriculture and Agri-Food, Canada, 2015). The climate trends in panels C and D are Sen s slope estimates, calculated using the zyp.zhang package in R. 44

15 Figure S11: Site-specific information on (A) total forest proportion, (B) disturbed fraction, and (C) NDVI trends for undisturbed forests in that site. The first two barplots (A and B) show the fraction of each site that is (A) considered forest and (B) considered disturbed. Note that these two categories overlap but are not mutually exclusive, nonforest areas can also be disturbed. In the third panel, the distribution of NDVI trends for undisturbed forested pixels in each site is presented as a boxplot. The colors represent the sites ecozones and the sites are sorted by longitude from west (left) to east (right). The ecozone information is from ecozones/index.htm. The forest cover was calculated by aggregating the EOSD land 45 cover map of Canada to the 500-m scale (Wulder et al., 2008). The historical disturbance information is from the Canadian LFDB (Stocks et al., 2002) and augmented by additional disturbances identified using the CCDC algorithm (Zhu et al., 2012).

16 Figure S12: Landsat NDVI trends grouped by percentage forest cover for undisturbed pixels across all sites. The numbers to the right of the plot show the total area covered by Landsat pixels in each category. Forest cover was estimated from the EOSD land cover map of Canada (Wulder et al., 2008). 46

17 Figure S13: Maps showing land cover and disturbance history (left) and Landsat NDVI trends (right) for the Fort McMurray site just east of Fort McMurray, Alberta. The maps are shown in the same Alber s Equal Area projection as Figures 1 and S10, and have the same spatial resolution as the panel analysis (500-m). The first panel shows the spatial distribution of land cover and disturbance for this site. The second panel shows the spatial distribution of NDVI trends. Trends that have an absolute magnitude smaller than NDVI units per year are shown in grey. The historical disturbance information is from the Canadian LFDB (Stocks et al., 2002) and augmented by additional disturbances identified using the CCDC algorithm (Zhu et al., 2012). The land cover information is from the EOSD land cover map of Canada aggregated to the 500-m scale (Wulder et al., 2008). 47

18 SI Tables Table S1: Summary of climate change and NDVI trends of undisturbed forests for each site. Sites are ordered by longitude from east to west. The climate data are derived from the Daily 10-km Gridded Climate Dataset for Canada, which covers all of Canada from 1950 to 2013 (National Land and Water Information Service, Agriculture and Agri- Food, Canada, 2015). The forest cover was calculated by aggregating the EOSD land cover map of Canada to the 500-m scale (Wulder et al., 2008). The historical disturbance information is from the Canadian LFDB (Stocks et al., 2002) and augmented by additional disturbances identified using the CCDC algorithm (Zhu et al., 2012). Site Latitude Longitude Start temperature ( C) End temperature ( C) Start precipitation (cm) End precipitation (cm) Median NDVI trend Proportion undisturbed forest Marystown Stephenville Blanc Sablon Mingan Michikamau Schefferville Torngat Manicouagan Kuujuaq Saguenay Pletipi Gouin Mistassini Ottawa Radisson Algonquin Sudbury Kesagami Pukaskwa Missisa Winisk Nipigon Pipestone Sachigo Kabetogama Gillam Winnipeg Amisk NOBS Yorkton La Ronge Cree Saskatoon Athabasca Bonnyville Fort McMurray Slave Yellowknife Wabasca Edmonton Whitecourt Chinchaga Kakwa Zama Williston Nahanni

19 Table S2: Summary of ancillary disturbance information used to mask out disturbance from results. The two maps that are being compared are the Canadian Large Fire Database (LFDB) and a disturbance map that was produced using the Continuous Change Detection and Classification algorithm (CCDC). Sites are ordered by longitude from east to west. Site Disturbed area (km 2 ) LFDB pre-1985 Disturbed area (km 2 ) LFDB Disturbed area (km 2 ) CCDC Marystown Stephenville Blanc Sablon Mingan Michikamau Schefferville Torngat Manicouagan ,482 Kuujuaq Saguenay ,183 Pletipi 9 2,316 2,331 Gouin 243 1,000 2,701 Mistassini 603 3,284 3,432 Ottawa Radisson 515 5,539 2,081 Algonquin Sudbury ,662 Kesagami Pukaskwa ,333 Missisa Winisk 532 1,138 1,186 Nipigon ,818 Pipestone 517 1,596 1,127 Sachigo 260 1,534 1,979 Kabetogama ,913 Gillam 1,409 2,935 2,164 Winnipeg 934 5,249 2,623 Amisk 210 5,439 3,805 NOBS 2,512 3,092 2,858 Yorkton 1, ,102 La Ronge 1,993 2,578 2,256 Cree 2,784 7,061 5,773 Saskatoon ,553 Athabasca 5,960 6,163 4,598 Bonnyville 1,294 1,082 3,496 Fort McMurray 3,685 4,760 4,432 Slave 4,944 2,907 4,128 Yellowknife 609 1, Wabasca 1,468 2,012 2,583 Edmonton 1, ,499 Whitecourt 2,114 1,677 3,616 Chinchaga 4, ,179 Kakwa ,306 Zama 1, ,028 Williston 2, ,283 Nahanni

20 Table S3: Summary of site attributes including their locations and the proportion of significant AVHRR NDVI trends. Sites are ordered by longitude from east to west and correspond to Figure S11. The biome information is taken from the ecozones of canadianbiodiversity.mcgill.ca/english/ecozones/index.htm and the forest cover was calculated by aggregating the EOSD land cover map of Canada to the 500-m scale (Wulder et al., 2008). The historical disturbance information is from the Canadian LFDB (Stocks et al., 2002) and augmented by additional disturbances identified using the CCDC algorithm (Zhu et al., 2012). Note that the forested and disturbance areas overlap but are not mutually exclusive, non-forest areas can also be disturbed. The AVHRR NDVI trends were computed using a similar method to Beck et al. (2011). To derive the proportion of pixels with either greening or browning trends, only significant trends were counted and divided by the total number of AVHRR pixels for that site. Site Biome Latitude Longitude Area (km 2 ) Proportion forest Proportion disturbed Proportion AVHRR browning Proportion AVHRR greening Marystown Boreal Shield , Stephenville Boreal Shield , Blanc Sablon Boreal Shield , Mingan Boreal Shield , Michikamau East Taiga Shield , Schefferville East Taiga Shield , Torngat East Taiga Shield , Manicouagan Boreal Shield , Kuujuaq East Taiga Shield , Saguenay Boreal Shield , Pletipi Boreal Shield , Gouin Boreal Shield , Mistassini Boreal Shield , Ottawa Boreal Shield , Radisson East Taiga Shield , Algonquin Boreal Shield , Sudbury Boreal Shield , Kesagami Hudson Plain , Pukaskwa Boreal Shield , Missisa Hudson Plain , Winisk Boreal Shield , Nipigon Boreal Shield , Pipestone Boreal Shield , Sachigo Boreal Shield , Kabetogama Boreal Shield , Gillam Boreal Shield , Winnipeg Boreal Plain , Amisk West Taiga Shield , NOBS Boreal Shield , Yorkton Boreal Plain , La Ronge Boreal Plain , Cree Boreal Shield , Saskatoon Boreal Plain , Athabasca West Taiga Shield , Bonnyville Boreal Plain , Fort McMurray Boreal Plain , Slave Boreal Plain , Yellowknife West Taiga Shield , Wabasca Boreal Plain , Edmonton Boreal Plain , Whitecourt Boreal Plain , Chinchaga Boreal Plain , Kakwa Boreal Plain , Zama Taiga Plain , Williston Boreal Plain , Nahanni Taiga Plain ,