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Impact of time on interpretations of forest fragmentation: Three-decades of fragmentation dynamics over Canada

Hermosilla, Txomin, Wulder, Michael A., White, Joanne C., Coops, Nicholas C., Pickell, Paul D., Bolton, Douglas K.
Remote sensing of environment 2019 v.222 pp. 65-77
Landsat, ecological function, forest damage, forest ecosystems, forests, habitat fragmentation, habitats, harvesting, insects, land cover, land use, landscapes, remote sensing, water stress, wildfires, Canada
Measures of forest fragmentation, and how fragmentation is changing through time, offer required information for understanding the status and dynamics of forest ecosystems, habitat conditions, and ecosystem functions. In this research, we investigate the multi-temporal characterization of forest fragmentation across the forested ecosystems of Canada (>650 million ha) and characterize the past three decades of forest fragmentation, providing useful context against which future analyses can be compared. Using 33 years of annual land cover maps produced from classified Landsat image best-available pixel composites (1984–2016), we describe and quantify the different forest patterns and dynamics that emerged in areas that were not disturbed in the analysis period, as well as following stand-replacing (i.e., wildfire, harvest) and non-stand-replacing (e.g., insects, water stress) disturbances. Baseline levels of fragmentation for each ecozone were determined by analyzing unchanged areas. Fragmentation dynamics by dominant forest disturbance showed that harvest activities generally lead to an increase in fragmentation related to the amount of forest cover (composition), while wildfires result in increasing fragmentation as a function of the spatial arrangement of the forest. The results presented herein also allow for characterization of the recovery of vegetation spatial patterns following various disturbance types, with areas dominated by fire presenting slower spatial recovery rates compared to harvest. By the end of the analysis period following disturbance events, forest fragmentation metrics in harvest-dominated landscapes were comparable to the pre-harvesting baseline, reaching 96% of mean pre-disturbance levels for mean forest patch size, and 83% for number of forest patches. In contrast, fire-dominated landscapes resulted in more event related fragmentation, with reduced forest cover (mean Thiel Sen slope = −0.13% year−1), mean forest patch size (−0.22 ha year−1), an increase in forest patches (0.11 year−1), and forest–non-forest join counts (0.83 year−1). By the end of the analysis period following disturbance events, mean forest patch size reached 52% of mean pre-disturbance levels and 68% the number of forest patches. Overall, non-stand replacing changes had no impact on the behaviour of the forest fragmentation metrics. The open access to Landsat's image archive combined with the analysis methods presented herein enable the systematic quantification and characterization of Canada-wide trends in forest fragmentation trends and post-disturbance spatial patterns over three decades. The results reported herein provide detailed information on the temporal evolution of spatial forest patterns, and illustrate that given an adequate time period, spatial patterns in areas where land use has not changed, recover to resemble pre-disturbance conditions.