The Alaska Vegetation Classification - Alaska Geobotany Center ...

General Descriptionof theClassification SystemWe have Constructed a hierarchical classification containing units at five levels ofresolution (levels I through V). In the ideal hierarchical system, each unit is exclusiveof all others, and when one class at any level is known, all levels above it are automaticallyknown. The broadest, most generalized level (level I) consists of threeformations-forest, scrub, and herbaceous. At the finest level of resolution (level V)units are discrete plant communities, with levels 11, 111, and IV intermediate in re-solution. We have not attempted to name levels 11, 111, and IV, although level IV inforest is comparable to Daubenmire's (1952) series. The scope of the system isshown by the number of units: level II contains 11 units; level 111, 30 units; level IV,146 units: and level V, 888 units.To be considered as a vegetation type in this system, at least 2 percent of covermust be vegetation. Any area with less than 2 percent in cover is not included hereand is considered to be unvegetated or barren.The forest units are based on tree crown canopy coverage and tree species compositiondown through level IV. The level II classes for forest are needleleaf, broadleaf,and mixed. A needleleaf forest is one where over 75 percent of total tree coveris contributed by needleleaf (coniferous) species. Similarly, a broadleaf forest hasover 75 percent of the tree cover in broadleaf tree species. In a mixed forest, neitherneedleleaf nor broadleaf species have clear dominance: both contribute 25 to 75 percentof the total canopy cover. Classes in level 111 are based on amounts of total treecanopy cover and are those suggested by Fosberg (1967): closed, open, and woodland.Closed stands have from 60 to 100 percent crown canopy. Open stands havefrom 25 to 60 percent crown canopy cover. Woodland has only scattered trees anda canopy cover of 10 to 25 percent. Level IV units are defined by the dominant treespecies in the overstory. To be listed under a level IV unit, a tree species mustcomprise at least 25 percent of the total tree canopy.Scrub vegetation classes are based on shrub height, shrub canopy coverage, andspecies composition down through level IV. The level II scrub classes are dwarf treescrub, tall scrub, low scrub, and dwarf scrub. Dwarf tree scrub is defined as vegetationhaving 10 percent or more of cover in tree species that on the site will notachieve 3 meters (10 ft) in height at maturity. Tall scrub vegetation is 1.5 meters(5 ft) or more in height, with 25 percent or more of the cover in tall shrubs. Lowscrub vegetation is between 20 centimeters (8 in) and 1.5 meters (5 ft) in heightand has 25 percent or more cover in low shrubs. Dwarf scrub vegetation is less than20 centimeters (8 in) high, and has 25 percent or more cover in dwarf shrubs. Level111 classes in dwarf tree scrub are the same used for forest; that is, closed, open, andwoodland. Level 111 classes for tall and low scrub are closed and open. Closed talland low scrub units have over 75 percent shrub canopy cover, and open units aredefined as having less than 75 percent shrub cover. For dwarf scrub, the level 111units are based on dominant plant species groups. These classes are dryas dwarfscrub, ericaceous dwarf scrub, and willow dwarf scrub.3