2. Geologic and Edaphic Factors Influencing Susceptibility of Forest Soils to Environmental Change Scott W. Bailey There is great diversity in the structure and function of the northern forest across the 20-state portion of the United States considered in this book. The interplay of many factors accounts for the mosaic of ecological regimes across the region. In particular, climate, physiography, geology, and soils influence dominance and distribution of vegetation communities across the region. This chapter provides a review of the ecology of the northern forest, emphasizing the role of geology and soils. The chapter begins with descriptive material reviewing the physiog- raphy, bedrock geology, and soils of the various provinces that constitute the northern forest. The distribution of vegetation communities and the role of climate, while of prime importance in defining the ecology of the region, are given limited coverage here as these are discussed more thoroughly elsewhere in this volume (see Chapters 1 and 3). However, to the extent that climate and vegetation are important soil-forming factors (Jenny, 1941), their characteristics in each province are summarized here. As the historical vegetation, which developed prior to large-scale anthropogenic alterations of the landscape in the last 150 years, is more 28 S.W. Bailey germane to the distribution of soil types than current vegetation patterns, long-term climax vegetation or potential natural vegetation (Kuchler, 1964) is listed here. In the second portion of the chapter, particular attention is paid to the important resource of the soils, upon which our forests grow. Perhaps the single most important factor in determining the health and productivity of the forest, soils integrate many of the same influences that result in distribution of a variety of ecological types across the region. Distribution of major soil types is reviewed, highlighting the important differences in soil-forming factors and processes among soil taxonomic types. In particular, characteristics of each soil type that most affect forest nutrient cycling, and which might be most dynamic in a changing environment, are highlighted. Finally, efforts to predict distribution of soils susceptible to nutrient depletion are reviewed. Nutrient depletion is perhaps the aspect of environmental change that is most associated with soil processes and is the subject of much recent study. Opportunities for advancement of the methods used to evaluate this phenomenon are suggested. Ecological Regions The United States Department of Agriculture (USDA) Forest Service has adopted a hierarchical classification system for mapping ecological regions at multiple scales (McNab and Avers, 1994). At the highest level of classification, the domain, are broad climatic regions, such as Polar, Humid Temperate, Humid Tropical, and Dry. The entire northern forest region lies within the Humid Temperate Domain. At the second level of classification, four divisions occur in the northern forest. The Warm Continental, Hot Continental, Subtropical, and Prairie Divisions delineate broad differences in climate, primarily temperature and precipitation. Two divisions in this region are in mountainous terrain, characterized by altitudinal zonation of vegetation; the letter "M" designates these units (see Fig. 2.1 in the color insert). Broad distinctions within the northern forest region are best illustrated at the third level of the classification system, the province. The following is a breakdown of the study region at this level of detail, highlighting factors responsible for the distinction of ecological units at this scale. Laurentian Mixed Forest Province The Laurentian Mixed Forest occurs on flat to moderately hilly areas in the northern part of the region (see Fig. 2.1 in color insert). This province, which constitutes 22% of the region (Table 2.1), is characterized by 2. Geologic and Edaphic Factors 29 Table 2.1. Area of Ecological Provinces in the Northern Forest Region Svmbol Province Area (ha) Area (%) Laurentian Mixed Forest 37,656,021 New England-Adirondack 10,938,240 Mixed-Coniferous Forest-Alpine Meadow Eastern Broadleaf Forest (Oceanic) Central Appalachian Broadleaf-Coniferous Forest-Meadow Eastern Broadleaf Forest (Continental) Southeastern Mixed Forest Outer Coastal Plain Mixed Forest Lower Mississippi Riverine Forest Prairie Parkland (Temperate) Total northern hardwood forests (American beech [Fagus grandifolia Ehrh.], yellow birch [Betula alleglzaniensis Britton], sugar maple [Acer saccha- rum L.]). Lesser forest types include spruce (Picea spp.)-fir (Ahies spp.) in Maine and Minnesota, Appalachian oak (Quevcus spp.) forests in southern New York and adjacent Pennsylvania, and aspen (Populus spp.)-birch (Betula spp.) and mixed pine (Pinus spp.) forests in the upper Midwest. Mean annual precipitation ranges from 530 mm in northern Minnesota to 1270 mm in coastal Maine and portions of the Allegheny Plateau of Pennsylvania and New York. The frost-free growing season is relatively short, ranging from 100 to 160 days. The age, structure, and composition of bedrock, as well as its influence on forests, is quite varied within the Laurentian Mixed Forest. In Maine, bedrock ranges from deformed but unmetamorphosed sedimentary rocks in the northeast to high-grade metasedimentary rocks in the southwest. Variable contributions of plutonic rocks, mostly granitic, as well as volcanic rocks also occur throughout Maine. The northern New York and Vermont portion is underlain by an assortment of unmetamorphosed sedimentary rocks, including carbonates, shale, and sandstone, their metamorphosed equivalents of marble, schist, and quartzite, as well as gneiss and amphibolite. The influence of Appalachian deformation, metamorphism and igneous activity is less prevalent in the Allegheny Plateau of southern New York and adjacent Pennsylvania. Here, bedrock is slightly deformed sandstone, siltstone, and shale, with lesser amounts of limestone, conglomerate, and coal. Slightly deformed sandstone, limestone, and dolomite characterize the Michigan portion of the forest. In western sections of the province, highly deformed and metamorphosed rocks, associated with the Canadian Shield, dominate, including felsic to mafic plutonic and volcanic rocks, their metamorphic equivalents gneiss and amphibolite, as well as quartzites, and banded iron formation. 30 S.W. Bailey Direct influence of bedrock on forest processes such as nutrient- and water-cycling, is minimal in portions of the Laurentian Mixed Forest where thick surficial deposits blanket the bedrock surface. However, in areas of shallow surficial deposits, bedrock influence may be great, depending on topography, bedrock composition, and degree of water flow through bedrock pores or fractures. In the only unglaciated portion of this province, in southwestern New York and adjacent Pennsylvania, bedrock influences the texture and mineralogy of surficial deposits ultimately derived from saprolite. Indirect influence of the bedrock is important in all glaciated areas, as primary mineralogy, and to a certain extent texture, of surficial deposits is dependent on the bedrock origin of the sediments. Surficial deposits in this province are nearly all of glacial origin, deposited during the final retreat of the continental glacier at the close of the Wisconsinan Stage, approximately 10,000 to 15,000 years ago. Minor areas of glacial deposits from earlier stages, up to 550,000 years old, are found in southern portions of the province adjacent to the limit of the Wisconsinan advance. Glacial deposits result from a variety of deposi- tional modes, resulting in a variety of configurations and textures. Unsorted, unstratified ground moraine, or till, is the most common glacial deposit in this region. The till tends to be relatively thin (meters to tens of meters thick) in eastern portions, while it is up to hundreds of meters thick in some portions of the upper Midwest. Stratified drift, including outwash plains, kames, and eskers of lesser areal extent are common in most regions, while large sandy outwash deposits dominate some parts of Michigan. Lacustrine deposits, ranging from clays to stratified silts and sands, are common in some low-lying portions, for example, east of Lake Champlain in Vermont and in northern Minnesota. Marine clays are found in east central Maine and the Saint Lawrence Valley. These were deposited when sea level was higher, due to land subsidence under the weight of the continental glaciers. Recent surficial deposits include alluvium in river valleys and organic accumulations common in wetland basins throughout the region. These range greatly in size and frequency, with large peat deposits common in extreme eastern Maine and especially in northern Minnesota. The only unglaciated portion of the Laurentian Mixed Forest occurs in southwestern New York and adjacent portions of Pennsylvania. This area is underlain by residuum in the most stable landscape positions, primarily gently sloping plateau tops. Colluvium dominates steeper slopes, whereas alluvium is found in lower slope positions. The Laurentian Mixed Forest and its mountain analog, the New England-Adirondack Province, are the only forest provinces in the study region with a predominance of Spodosols (Fig. 2.2 in color insert; Table 2.2). In this province, these are found in a large variety of parent materials and landscape positions. Several other soil orders dominate certain parent materials or more limited portions of the landscape.