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ECOLOGICAL UNITS OF THE NORTHERN REGION: SUBSECTIONS

(This document is now available as a PDF file at Treesearch)

THE AUTHORS

ACKNOWLEDGEMENTS

Contents

• Introduction
• Development of the Map and Descriptions
• Uses of the Map and Descriptions
• Landscape and Climate Overview of the Northern Region
• Subsection Descriptions, Arranged by Section
• Appendix A: Subsection Acres
• Appendix B: Glossary of Selected Terms
• Appendix C: Selected References

INTRODUCTION

This subsection report is an effort to develop units that integrate information from various disciplines, single-purpose maps, and existing regionalizations. The development of ecological units and descriptive data is an iterative process that will continue as additional information becomes available. Peer review and comments by users of this information are important to this process. All ecological mapping and descriptions follow the Forest Service's National Hierarchical Framework of Ecological Units (ECOMAP 1993).

DEVELOPMENT OF THE MAP AND DESCRIPTIONS

Subsections are smaller areas of sections with similar surficial geology, lithology, geomorphic process, soil groups, subregional climate, and potential natural communities (ECOMAP 1993). Each subsection has landscape components that differentiate it from adjacent subsections. The differentia used to place lines on the map include geologic materials, geomorphic features, and climate. Other components are used to describe each subsection but are not used to delineate the units; these are called accessory characteristics and include soils and vegetation.

Geologic materials affect ecosystem processes. The physical and chemical properties of bedrock and surficial materials influence such processes as weathering, soil formation, vegetation distribution, and stream chemistry. Landscape features such as elevation, aspect, and slope influence the distribution of solar radiation and precipitation. This affects vegetation patterns, soil formation, hydrologic processes, and animal populations.

The U.S. Geological Survey 1:500,000 Albers Conic Equal-Area Maps for Montana, Idaho, and North Dakota were used for the base maps. Geologic materials were determined from 1:500,000 State geology maps with some minor refinements based on additional geologic maps and local knowledge. Geomorphic features were determined using 1:500,000 State topographic and shaded relief maps and local knowledge. Broad climatic zones were inferred from potential natural vegetation mapping using Kuchler (1964), regional vegetation information, and isohyetal maps.

The Forest Service's National Hierarchical Framework of Ecological Units (EOMAP 1993) numbering/lettering system was applied to all subsections. For example, in this publication, a capitalized letter following the number indicates a section; the lowercase letter that follows indicates a subsection.

USES OF THE MAP AND DESCRIPTIONS

Subsection maps and descriptions are used at the subregion scale for strategic, multi-forest, Statewide, and multiagency analysis and assessment (ECOMAP 1993). Use of this or any map should be consistent with the scale and methods used to compile it. The subsection map was designed for broad scale ecosystem analysis and should not be used for detailed work. The map was produced at 1:500,000 and is intended to be used at that or a smaller scale. Plotting and using the subsection map at scales larger than 1:500,000 (1:24,000 for example) constitutes a misuse of the map and should not be done.

Due to the great amount of variability within the subsections, interpretations for parameters such as erosion were not made. Interpretations at this level must necessarily be based on many assumptions, generalizations, and averages. Such ratings or interpretations generally lack any real value.

LANDSCAPE/CLIMATE OVERVIEW OF THE NORTHERN REGION

The following overview is intended to give the reader a general understanding of the landscapes and climate of the Northern Region. The overview is arranged by Section or groups of similar Sections.

331A Palouse Prairie — This Section has a maritime-influenced warm temperate climate with hot, dry summers and cool, moist winters. Columbia River Basalts formed much of the landscape of low relief plateaus and rolling hills dotted with steptoes. Major streams are deeply entrenched into the uplifted highlands forming large canyons. This Section covers 3,243,257 acres.

331D Northwestern Glaciated Plains and
331E Northern Glaciated Plains — These Sections have a cold continental climate with warm, dry summers and cold, dry winters. Large expanses of prairie with scattered highlands characterize these Sections. Glacial drift up to 100 feet thick covers the landscape and glacial knob and kettle topography is typical. Ice damming of major rivers also created areas of lacustrine and delta sediments. These Sections cover 40,078,875 acres.

331F Northwestern Great Plains,
331G Powder River Basin, and
342A Bighorn Basin — These Sections all have cold continental climates with warm to hot, dry summers and cold, dry winters. These non-glaciated plains are typically gently rolling low relief grasslands. Bedrock is Cretaceous marine shale, Tertiary fluvial sediments, and some large sandstone units. Clinker beds or "scoria" from burnt coal seams form colorful resistant beds in Tertiary outcrops. These Sections cover 41,524,948 acres.

332A Northeastern Glaciated Plains and
332B Western Glaciated Plains — These Sections both have a cold continental climate with warm to hot, humid summers and cold, moist winters. The topography is a rolling, glaciated drift prairie with numerous potholes and moraines. Thick glacial drift covers most of the landscape though lacustrine sediments also occur as a result of glacial ice damming. These Sections cover 16,592,171 acres.

M331A Yellowstone Highlands,
M331B Bighorn Mountains,
M332C Rocky Mountain Front,
M332D Belt Mountains, and
M332E Beaverhead Mountains — These Sections all have cold continental climates with warm, dry summers and cold, dry winters. The topography is dominantly uplifted and faulted mountains and high plateaus with a variety of bedrock. Higher elevations are typically shaped by mountain glaciation and exhibit typical cirqueland features. Steep dipslopes, flatirons, and hogbacks are typical of the Rocky Mountain Front and parts of Bighorn Mountains. Large gravel filled valleys are typical in the Beaverhead Mountains Section of southwest Montana. In central Montana, the mountains occur as widely separated uplifts surrounded and isolated by prairie. These Sections cover 23,835,539 acres.

M332A Idaho Batholith — This Section has a maritime influenced cool temperate climate with warm, dry summers and cool, moist winters. The batholith is a large, contiguous uplifted area of granitics characterized by large, rounded mountain masses and basin areas. Soils formed in gruss, loess, and volcanic ash. The landscape is deeply cut by the Idaho canyonlands. Mountain glaciation occurred at higher elevations. This Section covers 3,883,095 acres.

M332B Bitterroot Valley and
M333D Bitterroot Mountains — These Sections have maritime influenced cool temperate climates with warm, dry summers and cool, moist winters. Belt rocks (quartzite and argillite) are resistant to weathering and form typical landscapes of steep slopes and incised V-shaped valleys. Talus and rock outcrops are common. The Bitterroot Valley is a large intermontane valley formed in valley fill materials. The higher elevations are typically large U-shaped valleys with abundant rock outcrop, cirques, horns, and other glacial features. Till, outwash, and moraines fill the valley floors. These Sections cover 9,136,505 acres.

M332G Blue Mountains — This Section has a maritime influenced cool temperate climate with warm, dry summers and cool, moist winters. The landscape is dominated by deeply entrenched major streams. Steep, high relief breaks and narrow valleys are characteristic. Columbia River Basalts and related plateau landscapes also occur in this Section. This Section covers 1,345,904 acres.

M333A Okanogan Highlands,
M333B Flathead Valley, and
M333C Northern Rockies — These Sections have maritime influenced cool temperate climates with warm, dry summers and cool, moist winters. The landscape is dominated by rounded landforms that resulted from continental glaciation. Till deposits, often 50 to 100 feet thick, cover the mountainsides and valley floors. A mantle of loess and volcanic ash occurs over most of the glacial deposits. Some metasedimentary Belt rock landscapes remain. The Northern Rockies Section is dominantly metasedimentary mountains that have been characteristically shaped by alpine glaciation. These Sections cover 11,908,199 acres.

SUBSECTION DESCRIPTIONS, ARRANGED BY SECTION

• Section 331A - Palouse Prairie
• Section 331D - Northwestern Glaciated Plains
• Section 331E - Northern Glaciated Plains
• Section 331F - Northwestern Great Plains
• Section 331G - Powder River Basin
• Section 332A - Northeastern Glaciated Plains
• Section 332B - Western Glaciated Plains
• Section M331A - Yellowstone Highlands
• Section M331B - Bighorn Mountains
• Section M332A - Idaho Batholith
• Section M332B - Bitterroot Valley
• Section M332C - Rocky Mountain Front
• Section M332D - Belt Mountains
• Section M332E - Beaverhead Mountains
• Section M332G - Blue Mountains
• Section M333A - Okanogan Highlands
• Section M333B - Flathead Valley
• Section M333C - Northern Rockies
• Section M333D - Bitterroot Mountains
• Section 342A - Bighorn Basin

APPENDIX A: SUBSECTION ACRES

Subsection	Acres	Number of Polygons	Subsection	Acres	Number of Polygons
331Aa	728,027	2	M332Bk	1,069,316	1
331Ab	1,793,288	2	M332Bp	225,199	1
331Ac	721,942	1	M332Ca	475,328	1
331Da	2,040,563	1	M332Cb	508,740	1
331Db	1,512,751	4	M332Cc	449,506	1
331Dc	488,863	1	M332Da	1,370,953	1
331De	841,265	1	M332Db	1,978,690	2
331Df	1,655,619	1	M332Dc	332,144	1
331Dh	17,925,132	2	M332De	433,262	2
331Ea	15,614,682	1	M332Dg	516,304	1
331Fb	417,135	1	M332Dh	378,384	1
331Fc	2,400,691	1	M332Dj	883,612	1
331Fd	12,210,346	2	M332Dk	2,646,330	1
331Fe	2,136,832	4	M332Dm	170,881	1
331Fg	1,623,611	1	M332Dn	224,838	1
331Ga	3,498,342	1	M332Dp	313,076	1
331Gb	2,411,965	1	M332Ea	672,022	1
331Gc	2,102,060	1	M332Eb	390,737	1
331Gd	581,179	1	M332Ec	176,147	1
331Ge	13,846,942	1	M332Ee	430,626	1
332Aa	12,697,638	1	M332Eg	225,887	1
332Ab	773,695	1	M332Eh	354,714	1
332Ac	1,539,631	1	M332Ej	2,097,837	4
332Ad	799,566	1	M332Ek	412,257	1
332Ae	290,446	1	M332En	809,777	1
332Ba	491,195	1	M332Ep	312,070	1
M331Aa	1,217,591	1	M332Er	410,301	1
M331Af	1,124,175	1	M332Gd	1,345,904	1
M331Ag	1,041,921	1	M333Aa	992,148	1
M331Ah	693,979	1	M333Ab	1,131,763	1
M331Ai	420,704	1	M333Ac	972,811	3
M331Ak	132,481	1	M333Ba	875,779	1
M331Al	178,628	1	M333Bb	2,443,060	1
M331Am	392,925	1	M333Bc	1,679,192	1
M331Ap	473,663	1	M333Be	468,665	1
M331Ar	524,484	1	M333Ca	610,341	1
M331Ba	660,565	1	M333Cb	1,523,827	1
M332Aa	1,297,211	2	M333Cc	690,014	1
M332Ab	830,336	5	M333Ce	520,599	1
M332Ac	1,755,548	4	M333Da	1,321,043	1
M332Ba	450,712	1	M333Db	1,363,689	1
M332Bb	407,651	1	M333Dc	1,444,859	1
M332Bc	729,657	1	M333Dd	1,807,880	1
M332Bd	670,441	1	M333De	940,573	1
M332Be	247,421	1	342Ad	295,845	1
M332Bg	1,185,779	1

GLOSSARY OF SELECTED TERMS

The definitions listed are taken from the Glossary of Landforms in the USDA Natural Resources Conservaton Service National Soil Survey Handbook and from the American Geological Institute Glossary of Geology.

Alluvium

—Unconsolidated clastic material deposited by running water, including gravel, sand, silt, clay, and various mixtures of these.

Cirque

—Semicircular, concave, bowl-like area with steep face primarily resulting from erosive activity of a mountain glacier.

Cirqueland

—Areas dominated by cirques and related glacial features, rock, and rubbleland.

Colluvium

—Unconsolidated, unsorted earth material being transported or deposited on sideslopes and/or at the base of slopes by mass movement (e.g. direct gravitational action) and by local, unconcentrated runoff.

Ecological Unit

—A mapped landscape unit designed to meet management objectives, comprised of one or more ecological types.

Floodplain

—The nearly level plain that borders a stream and is subject to inundation under flood-stage conditions unless protected artificially. It is usually a constructional landform built of sediment deposited during overflow and lateral migration of streams.

Fluvial

—Of or pertaining to rivers; produced by river action.

Intermontane Basin

—A generic term for wide structural depressions between mountain ranges that are partly filled with alluvium and called "valleys" in the vernacular. Intermontane basins may be drained internally (bolsons) or externally (semi-bolson).

Lacustrine deposit

—Clastic sediments and chemical precipitates deposited in lakes.

Landscape

—A collection or related, natural landforms; usually the land surface which the eye can comprehend in a single view.

Loess

—Fine-grained, wind-deposited material, dominantly of silt size.

Metasediment

—A sediment or sedimentary rock that shows evidence of having been subjected to metamorphism.

Moraine

—A mound, ridge, or other distinct accumulation of unsorted, unstratified glacial drift, predominantly till, deposited chiefly by direct action of glacier ice, in a variety of topographic landforms that are independent of control by the surface on which the drift lies.

Outwash

—Stratified detritus (chiefly sand and gravel) removed or "washed out" from a glacier by melt-water streams and deposited in front of or beyond the end moraine or the margin of an active glacier.

Potential Vegetation

—The biotic community that would be established if all successional sequences of its ecosystem were completed without additional human-caused disturbance under present environmental conditions.

Residuum

—Unconsolidated, weathered or partly weathered mineral material that accumulates by disintegration of bedrock in place.

Section

—An ecological unit in the subregion planning and analysis scale of the National Hierarchical Framework corresponding to subdivisions of a Province having broad areas of similar geomorphic process, stratigraphy, geologic origin, drainage networks, topography, and regional climate. Such areas are often inferred by relating geologic maps to potential natural vegetation groupings as mapped by Kuchler (1964).

Soil Great Group

—A category of soil classification where soils are placed together based on close similarities in kind, arrangement, and degree of expression of horizons; close similarities in soil moisture and temperature regimes; and similarities in base status.

Soil Moisture Regime

—Classes of soil moisture that are based on the assumption that the soil supports whatever vegetation it is capable of supporting. Moisture regimes are defined in terms of the ground-water level and in terms of the presence or absence of water held at a tension of <15 bars in the moisture control section by periods of the year.

Soil Temperature Regime

—The characteristic temperature regime of a soil that is described by the mean annual soil temperature, the average seasonal fluctuations from that mean, and the mean warm or cold seasonal soil temperature gradient within the main root zone, which is the zone from a depth of 5 to 100 centimeters.

Subsection

—An ecological unit in the subregion planning and analysis scale of the National Hierarchical Framework corresponding to subdivisions of a Section into areas with similar surficial geology, lithology, geomorphic process, soil great groups, subregional climate, and potential vegetation.

Till

—Dominantly unsorted and unstratified drift, deposited by a glacier, and consisting of a heterogeneous mixture of clay, silt, sand, gravel, stones, and boulders.

SELECTED REFERENCES