Peatland Distribution and Characterization in the Susitna River Basin, Alaska

Peatlands are widely distributed across the northern latitudes and store large amounts of carbon. Ongoing and future changes in climate may lead to changes in peatland surface area and carbon storage. Therefore, baseline data describing present-day peatland distribution and possible recent changes in peatland coverage are critically needed, as they provide a better understanding of the regional controls on peatland distribution. The Susitna River Basin, Alaska, drains ~ 52,000 km2 and includes part of the Alaska Range, the Talkeetna Mountains as well as extensive, formerly glaciated lowlands. In this study, we used Landsat Enhanced Thematic Mapper (ETM+) images (2001-2002) to classify landcover focusing on peatlands. This map was refined through the use of QuickBird and other spatial data to verify peatland classification and to distinguish among cover classes with varying amounts of standing water, peat, and forest. Categories included (1) wet, (2) wooded, and (3) open peatlands. Aerial photographs from the 1950s were compared to 2005 QuickBird and Google Earth images to determine whether peatland areas were expanding or contracting in response to climatically-induced extensive glacier shrinkage and increased moisture in the region over the last 50 years. Spatial data analysis, extracted from the National Elevation Dataset (2-arc-second), was performed on ~ 66% of the total basin, from which peatlands covered 8% (2,675 km2). Elevation and slope analysis for areas < 750 m in altitude revealed that peatlands are primarily distributed on gentle slope (<1° for 55% peatlands, with an overall average slope of 0.9°) and low elevation (51% peatlands below 200 m, overall average of 222 m above sea level), with a key transition at approximately 400 m (85% of peatlands are found below 400 m). Average slopes for peatlands found above 400 m range from 1 to 3°, whereas overall landscape slopes are much higher, ranging from 0° to 68° (average of 5.5°). Most peatlands within the basin do not show detectable changes in size over the last 50 years; however, changes were observed in some low-elevation (<60 m) valleys, possibly due to warming air temperature. Peatland surface area is positively correlated with stream density in a non-linear manner, with rapid rates of changes at low stream density and reduced response >0.2 km/km2. We are conducting this landcover change study in conjunction with other field investigations to assess how peatlands in this key northern watershed are responding to the moisture changes caused by changing climate and glacier melt.