The vast landscapes of interior Alaska are changing: large glaciers are melting and rapidly receding up valleys, ancient permafrost is degrading and turning frozen soils into soupy gelatin, woody vegetation is spreading dramatically into open areas, and boreal ponds and wetlands are shrinking. Climate data for interior Alaska show a pronounced warming trend over the past several decades. A growing scientific consensus suggests that a tide of relative warmth is stimulating many of the changes in Alaska's ecosystems. Yet the ultimate trajectory and outcome are unknown. What is almost certain, however, is that these changes will have profound consequences for all life in the Far North.
In 2005 the Central Alaska Network received a serendipitous gift of several hundred 35 mm slides, photographed from the backseat of a two-seater airplane in 1976. The donor, Dr. Fred Dean (professor of wildlife biology at the University of Alaska, Fairbanks), and his graduate student, Debbie Heebner, used these photographs to help produce the first land-cover map of Denali National Park. Central Alaska Network staff scanned the slides at high resolution, entered the locations of the photos into a Geographic Information System, and printed hard copies of the slides along with location maps. Then, from a helicopter, the original photographs were repeated as closely as possible. Now examined and analyzed, these photo pairs are a treasure trove of information about visible vegetation changes over the last 30 years.
From 1976 (top) to 2005 (bottom), white spruce trees (Picea glauca) colonized a subalpine terrace along the upper Savage River in Denali National Park and Preserve. This photo pair highlights an example of the ongoing replacement, presumably permanent, of one vegetation community by another. Over the past 30 years many such examples appear to have impacted park landscapes.
Credit top photo: Fred Dean (1976). Credit bottom photo: NPS (2005)
The magnitude of the observed changes in many of these photo pairs was surprising. The primary types of changes were (1) expansion of spruce into formerly treeless areas, (2) invasion of open wetland areas by woody vegetation, and (3) widespread colonization of formerly open floodplains and terraces by vegetation. In many cases these changes appear directional; that is, they represent a qualitative shift in the landscape mosaic, not simply a shift in vegetation due to succession.
The rapid invasion of trees and shrubs in this area of open sedge meadow had begun by 1976 (top) but was nearly complete by 2005 (bottom). This wetland, near Corner Lake in the northern lowlands of Denali National Park, had likely supported only open sedge meadow for centuries before the recent invasion by woody plants.
Credit top photo: Fred Dean (1976). Credit bottom photo: NPS (2005)
The repeat photo pairs provide dramatic visual evidence of recent vegetation changes. Understanding and responding to these changes requires more rigorous and detailed information. To gather the necessary data, the Central Alaska Network is implementing intensive, landscape-scale monitoring of vegetation across the three parks in the network. Monitoring according to this design began in Denali National Park and Preserve in 2001, in Yukon-Charley Rivers National Preserve in 2006, and in Wrangell–St. Elias National Park and Preserve in 2007. These units comprise 21.7 million acres (8.8 million ha), or 25% of the land area of the U.S. National Park System. The goals of this program are to detect and quantify vegetation changes like those captured anecdotally by repeat photography, and to document the dimensions and ecological consequences of these changes using reproducible, statistically rigorous protocols.
The vegetation monitoring program for the Central Alaska Network uses a two-stage systematic grid design wherein mini-grids (red squares), consisting of 25 (200 sq m [2,153 sq ft]) sample plots arranged in a grid pattern of five rows of five plots spaced 500 m (1,640 ft) apart, are themselves located on a macro-grid of points spaced 10 km (6.2 mi) or 20 km (12.4 mi) across the park landscape. Investigators record a full suite of physical and biological characteristics at each sample plot.
Credit: NPS
The Central Alaska Network has established a sampling design based upon a multistage systematic grid for detecting changes at individual sample plots and across park landscapes. At each plot, network ecologists measured and recorded the types and abundances of vascular plants, mosses, and lichens; dimensions and locations of all trees; and physical attributes, including those from soil samples. Network staff also collected cores from trees at the perimeter of the permanent plots and marked the center of each plot with a monument and mapping-grade Global Positioning System point. Subsequent sampling, to be conducted every seven years, will allow detection of trends in the vegetation cover at multiple nested spatial scales.
With nearly 500 permanent vegetation plots installed to date in Denali National Park and Preserve, network ecologists are already learning a great deal about vegetation-landscape relationships from these data. This work has revealed new information regarding the distribution and diversity of vascular plants across the landscape. For instance, across all spatial scales, the average species richness of plant communities increased dramatically with increasing elevation into the high alpine zone of the park.
Alpine areas also supported the greatest diversity of rare and endemic plants. The data offer an early warning of potential threats to plant conservation: with continued warming, woody vegetation will increasingly invade alpine tundra, thereby displacing these highly diverse plant communities. These data are a single strand in a multifaceted monitoring program that should allow detection, understanding, and management of dramatically changing landscapes in interior Alaska.
Carl Roland
Plant Ecologist, Denali National Park and Preserve, Alaska
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