Spatiotemporal remote sensing of ecosystem change and causation across Alaska

Abstract

Contemporary climate change in Alaska has resulted in amplified rates of press and pulse disturbances that drive ecosystem change with significant consequences for socio-environmental systems. Despite the vulnerability of Arctic and boreal landscapes to change, little has been done to characterize landscape change and associated drivers across northern high-latitude ecosystems. Here we characterize the historical sensitivity of Alaska's ecosystems to environmental change and anthropogenic disturbances using expert knowledge, remote sensing data, and spatiotemporal analyses and modeling. Time-series analysis of moderate-and high-resolution imagery was used to characterize land- and water-surface dynamics across Alaska. Some 430,000 interpretations of ecological and geomorphological change were made using historical air photos and satellite imagery, and corroborate land-surface greening, browning, and wetness/moisture trend parameters derived from peak-growing season Landsat imagery acquired from 1984 to 2015. The time series of change metrics, together with climatic data and maps of landscape characteristics, were incorporated into a modeling framework for mapping and understanding of drivers of change throughout Alaska. According to our analysis, approximately 13% (~174,000 ± 8700 km ) of Alaska has experienced directional change in the last 32 years (±95% confidence intervals). At the ecoregions level, substantial increases in remotely sensed vegetation productivity were most pronounced in western and northern foothills of Alaska, which is explained by vegetation growth associated with increasing air temperatures. Significant browning trends were largely the result of recent wildfires in interior Alaska, but browning trends are also driven by increases in evaporative demand and surface-water gains that have predominately occurred over warming permafrost landscapes. Increased rates of photosynthetic activity are associated with stabilization and recovery processes following wildfire, timber harvesting, insect damage, thermokarst, glacial retreat, and lake infilling and drainage events. Our results fill a critical gap in the understanding of historical and potential future trajectories of change in northern high-latitude regions.

Landscape Change Detected over a Half Century in the Arctic National Wildlife Refuge Using High-Resolution Aerial Imagery

“…This is similar to the amount of change detected in the Arctic Network of National Parks, located to the west of the Refuge, where a similar methodology found 24% of 206 systematically distributed plots showed change between 1975-1985 and 2008-2010 [27]. Our finding also compare to an Alaska-wide analysis, in which Pastick et al [37] found 14% of the landscape had undergone change from 1984 to 2015, based on an analysis of spectral trends in Landsat imagery.…”

“…The dataset developed by this study has been used for projecting future changes across a broader region of northern Alaska using state-transition modeling [62] and for landscape change analysis using satellite remote sensing and decision-tree modeling [37]. We found that environmental variables were very useful in explaining variations in change types across the region and can be incorporated into other studies.…”

Variability in the expansion of trees and shrubs in boreal Alaska

“…) and cover (Pastick et al. ) in areas already classified as Shrub (i.e., that did not undergo a coarse land‐cover transition). Indeed, it seems likely that increases in the stature and biomass of existing individuals and clones would be a common initial response to ameliorating conditions in systems such as ours, which are dominated by long‐lived perennial species (e.g., Walker et al.…”

“…, Swanson , Pastick et al. ), the variation in shrub and tree expansion is not well understood and is spatially variable, representing a complex interplay between climate and soil conditions (Brenning , Tape et al. , Roland et al.…”

“…Among the most dramatic climate-driven ecological change occurring at high latitudes is the rapid advance of shrubs and trees into low-statured tundra (Myers-Smith et al 2011, Lin et al 2012, Pastick et al 2018), particularly at low elevations (Rinas et al 2017). Over the latter half of the 20th century, Alaska has experienced a 43% increase in tall shrub cover (Chapin et al 2005) as well as upward and northward expansions of tree lines (Harsch et al 2009).…”