Pacific Forested Glacial Ablation Plain Biophysical Setting Southern Alaska

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1 Pacific Forested Glacial Ablation Plain Biophysical Setting Southern Alaska Conservation Status Rank: S4 (apparently secure) Introduction Forested glacial ablation plains are represented by mature trees and associated understory species growing in a periglacial environment on ice-cored deposits. Through various geomorphic processes, glaciers may accumulate rock, gravel and sand on their upper surfaces. Where this debris reaches a depth sufficient to insulate roots, plants may colonize and a vegetated glacial ablation plain may develop (Figure 1). In Southeast Alaska, seres occurring in this unique environment transition from pioneer Alnus viridis ssp. sinuata associations to mid-seral Picea sitchensis forests to mature Picea sitchensis-tsuga heterophylla forests (Figure 1 and Figure 2; Russell 1891, Molnia 2006, Stephens 1969, Post and Streveler 1976, Benn and Evans 1998). Additional study is required to evaluate whether these plant associations support unique vegetation, rare plants, and/or wildlife habitat. Many of these forests are 300 years and older, and many of the ice-cored ablation plains are estimated to last 600 years, ample time to allow forests to mature and even for secondary succession to occur (Post and Streveler 1976). However, in a rapidly warming climate, the melt processes that have produced these stable ablation plains become a liability to their existence (Tarr and Martin 1914). Figure 1. The Martin River glacier ablation plain showing the transition from barren debris to scattered spruce mixed with Alnus viridis ssp. sinuata to mature Picea sitchensis forest. Note the occurrence of craters and small lakes across the ablation plain. 1

2 Figure 2. Early seral communities on the supraglacial debris of the Martin River Glacier with pond in the foreground (photo by T. Boucher). Distribution Mature forests dominated or codominated by Picea sitchensis and/or Tsuga heterophylla on glacial ablation plains are rare and occur as isolated stands on the lower elevations of the Bering, Fairweather, Grand Plateau, Malaspina, Martin River, and Sherman Glaciers and on remnant ice on the north side of Lituya Bay (Russell 1891, Molnia 2006, Stephens 1969, Post and Streveler 1976, Benn and Evans 1998). Earlier seral-stages occur on additional ablation plains, and are more common than the mature forests. The distribution of forested glacial ablation plains (Figure 3) was developed from the intersection of glacial ice (GLIMS 2005) with Picea sitchensis-dominated landcover classes of the Alaska Vegetation Map (Boggs et al. 2015). 2

3 Figure 3. Distribution of the Pacific Forested Glacial Ablation Plain Biophysical Setting. Note that the areas of occupancy shown on the map are buffered for greater visibility. Climate Southern Alaska has a cool, wet maritime climate and is generally free of permafrost (Gallant et al. 1995, Nowacki et al. 2001). Mean annual precipitation ranges from 135 to 390 cm with 80 to 600 cm falling as snow. Average summer temperatures range from 7 to 18 C; average winter temperatures are between -3 and 3 C. Environmental Characteristics Supraglacial debris is largely derived from medial and lateral moraines or landslides to the glacier surface (Fickert et al. 2007); lesser sources may include alluvial or aeolian sediment transport and solifluction, as well as thrusting of bed-derived sediment from the bottom of a glacier to its surface (Alley et al. 1997). Depending on the energy of the depositional process, debris may include boulders 2 to 3 m in diameter and may reach thicknesses exceeding 0.5 m (Rampton 1970, Birks 1980). Of these varied sources, medial and lateral moraines are thought to be the dominant sources of supraglacial debris (Figure 4). Medial and lateral moraines form as narrow strips of debris, but increase in width and relief as they move downgradient past the equilibrium line to the ablation zone. In the ablation zone, where ice melt exceeds accumulation, debris is most commonly reworked by meltwater into outwash plains and ice may be degraded by above-freezing 3

4 temperatures, stream erosion, or the exposure of ice by removal of sediment. Melt across steep ice faces can initiate small soil-vegetation slides, forming a chaotic accumulation of debris and vegetation (Figure 5; Russell 1891). Slides across slopes of craters may form bluffs 8 m high littered with standing, leaning and fallen dead trees. Under less rapid melt conditions debris may build over ice allowing vegetation to establish (Figure 6). Due to the insulation provided to the underlying ice by supraglacial debris, the thermodynamics of dirty glaciers differ from those of 'clean' glaciers. Supraglacial debris can reduce glacial ablation rates, allowing the glacier to extend further down valley than meteorology alone would suggest (Anderson 2000). Research on the vegetation communities on glacier ablation plains have shown that the lifespan of supraglacial trees is mainly controlled by glacier surface displacements and by the occurrence of backwasting and downwasting processes, whereas tree germination was associated with fine debris presence (Pelfini et al. 2012). Figure 4. The Martin River Glacier showing the formation and widening of its medial moraine from the confluence of the glacial arms at upper right. 4 Debris-covered glaciers have been estimated to last over 350 years in Alaska with another 300 years of ice remaining (Post and Streveler 1976). Trees more than 300 years in age have been documented on the supraglacial debris of the Martin River Glacier (Post and Streveler 1976) while trees more than 50 cm in diameter have been observed on the debris-covered termini of more than a dozen glaciers in southern Alaska, including the Bering, Malaspina, Fairweather, Grand Plateau, and Martin-River Glacier (Russell 1891, Molnia 2006, Stephens 1969, Post and Streveler 1976, Benn and Evans 1998). Vegetation and Soil Succession Newly stabilized supraglacial debris is invaded by a variety of pioneer plant associations. These include associations dominated by the shrub Alnus viridis ssp. sinuata (Sitka alder) such as Alnus viridis ssp. sinuata-oplopanax horridus/aruncus dioicus, Alnus viridis ssp. sinuata/rhytidiadelphus species or early seral Picea sitchensis/alnus viridis ssp. sinuata (Barrett and Christansen 2011, Stephens 1969, Russell 1891). Scattered Populus trichocarpa saplings are also common. Soil development is minimal, with multiple surface cracks exposing glacial ice and initiating soil and vegetation slumping. On mid-seral sites, Picea sitchensis gradually overtops the Alnus viridis ssp. sinuata, and within 100 years a dense forest dominated by Picea sitchensis or in combination with Tsuga heterophylla develops (Figure 6; Post and Streveler 1976, Stephens 1969). The forest floor is dominated by mosses in the Rhytidiadelphus and Hylocomium genera (Stephens 1969). Other species include shrubs in the Ribes genus, Oplopanax horridus, Pyrola secunda, and Dryopteris expansa. The height, diameter and age of Picea sitchensis ranges from 18 m tall, cm dbh, and 65 years old on the Kushtaka Glacier (Stephens 1969) to much larger and over 300 years old on the Martin River Glacier (Post and Streveler 1976, Russell 1891). Soil in these spruce forests are spodosols, with A, B and C horizons developed. The gravelly sandy loam comprising the C layer has 70% coarse fragments. Soil thickness ranges from 0.5 to 3 m (Post and Streveler 1976, Stephens

5 Figure 5. Melt across steep ice faces can initiate small landslides, which expose glacial ice; Matanuska Glacier ablation plain. 1969). Following the eventual melt of the underlying ice, it is expected that the organic matter, nitrogen and other soil nutrients accumulated will make significant contributions to young post-glacial ecosystems (Crocker and Dickson 1957, Stephens 1969). Conservation Status Rarity: Mature Picea sitchensis or Tsuga heterophylla-picea sitchensis forests rarely develop on glacial ablation plains and are only documented from seven periglacial environments in Southern Alaska (Russell 1891, Molnia 2006, Stephens 1969, Post and Streveler 1976, Benn and Evans 1998). Their estimated potential range is less than 1,000 km 2. Threats: Change in glacier movement threatens this system. In a rapidly warming climate, the melt processes that have produced these stable ablation plains become a liability to their further existence (Tarr and Martin 1914, Stephens 1969). In contrast, it is unclear as to whether advancing glaciers would support an ablation plain stable enough to allow the development of forests. Trend: Ice-cored ablation plains are estimated to last well beyond the time required for forests to mature and even for secondary forest succession to occur (600 years; Rampton 1970, Birks 1980). Thus in the absence of significant glacier recession or advance, change in the extent and condition of this system in not expected. It is not known how increased ablation rates due to a warming climate will affect the maintenance of this system. Species of Conservation Concern No animal or plants species of conservation concern are known or suspected to occur in this biophysical setting. Additional study is required to evaluate whether this biophysical setting supports animal and plants of conservation concern. Classification Concept Source The classification concept for this biophysical setting is based on Russell (1891). 5

6 Figure 6. Supraglacial debris on the Martin River Glacier supporting Picea sitchensis forest on left side of the image, a crater with a lake, and scattered spruce mixed with Alnus viridis ssp. sinuata on the right (source: Google Earth, accessed September 2, 2015). Literature Cited Alley, R. B., K. M. Cuffey, E. B. Evenson, J. C. Strasser, D. E. Lawson, and G. J. Larson How glaciers entrain and transport basal sediment: Physical constraints, Quat. Sci. Rev. 16: Anderson, R. S A model of ablation-dominated medial moraines and the generation of debrismantled glacier snouts: Journal of Glaciology Volume 46: Barrett, T.M. and G.A. Christensen, tech. eds Forests of southeast and south-central Alaska, : five-year forest inventory and analysis report. Gen. Tech. Rep. PNW-GTR-835. Portland, OR: U.S. Department of Agriculture, Forest Service, Pacific Northwest Research Station. 156 p. Benn, D. I., and D. J. A. Evans Glaciers and glaciation. New York: Wiley, 734 pp. Birks, H. J. B The present flora and vegetation of the moraines of the Klutlan Glacier, Yukon Territory, Canada: a study in plant succession. Quaternary Research 14: Boggs, K., T. V. Boucher, T. T. Kuo Vegetation map and classification: Southern Alaska and Aleutian Islands. Alaska Natural Heritage Program, University of Alaska Anchorage, Anchorage, Alaska. Crocker, R. L., and B. A. Dickson Soil development on the recessional moraines of the Herbert and Mendenhall Glaciers, south-eastern Alaska. Journal of Ecology 45: Fickert, T., D. Friend, F. Grüninger, B. Molina, and M. Richter Did Debris-Covered Glaciers Serve as Pleistocene Refugia for Plants? A New Hypothesis Derived from Observations of Recent Plant Growth on Glacier Surfaces. Arctic, Antarctic, and Alpine Research 39: GLIMS (Global Land Ice Measurements from Space) and National Snow and Ice Data Center. 2005, updated GLIMS Glacier Database, Version 1. [subset 40]. Boulder, Colorado USA. NSIDC: 6

7 National Snow and Ice Data Center. doi: [accessed February 2016]. Molnia, B. F., Satellite Image atlas of the glaciers of the world. Alaska. U.S. Geological Survey Professional Paper 1386-K, 750 pp. Post, A., and G. Streveler The tilted forest: glaciological-geological implications of vegetated neoglacial ice at Lituya Bay, Alaska. Quaternary Research 6: Rampton, V Neoglacial fluctuations of the Natazhat and Klutlan Glaciers, Yukon Territory, Canada. Canadian Journal of Earth Sciences 7: Russell, I. C An expedition to Mount St. Elias. National Geographic Magazine 3: Stephens, F. R., A forest ecosystem on a glacier in Alaska. Arctic 22: Tarr, R. S., and L. Martin Alaskan Glacier Studies. Washington, D.C.: National Geographic Society. 498 pp. 7