Our I.iviiifi Rrsiiiiitf\ — Ti'iifstriiil Ecos\src'ii:s 



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The Nature 



Conservancy/Natural Heritage 

 Network Classification System 



The basic goal of the comiiuinity classifica- 

 tion elloit is to piovide a complete listing oi' all 

 conimunities that represent the variation in eco- 

 logical systems. The classification hierarchy for 

 terrestrial communities is based on the biologi- 

 cal characteristics of existing vegetation types. 

 These types range from early successional 

 through climax associations and include serai 

 stages that are maintained by natural and 

 human-induced management and disturbance 

 regimes. 



The classification hierarchy is partitioned 

 into teiTestrial, aquatic, and subterranean "sys- 

 tems." The upper levels of the terrestrial system 

 have been derived through the modification of 

 United Nations Educational, Scientific and 

 Cultural Organization ( 1973) and Driscoll et al. 

 (1984) and refer to the physiognomic attributes 

 (structural) of the vegetation. The two finest 

 levels of the classification hierarchy are based 

 on tloristic analysis and are determined through 

 the identification of diagnostic species 

 (Westhoff and van der Maarel 1973). 



Ranking System 



The Nature Conservancy and Natural 

 Heritage Network rank all elements of natural 

 biological diversity according to their relative 

 rarity and vulnerability to aid in ranking critical 

 areas for conservation. The community ranks 

 are consistent with the overall conservation 

 ranking approach applied to all elements of nat- 

 ural diversity within The Nature Conservancy/ 

 Natural Heritage Network methodology 

 (Master 1991). The communities described in 

 this report have been ranked G 1 and G2 accord- 

 ing to The Nature Conservancy/Natural 

 Heritage Network ranking system {see Table 1 ). 



Listing Globally Rare 

 Community Types 



The development of the list of rare commu- 

 nities proceeded from the identification of rare 

 communities at the state level, to the production 

 of regional classifications of the rare state types, 

 and finally to the generation of a consistent list 

 of rare communities at the national level. Most 

 state heritage programs have developed a classi- 

 fication system at the state level; these systems 

 are based on available data and literature, input 

 from experts, and field verification. State con- 

 servation ranks have been assigned to most of 

 these communities based on the analysis of 

 existing information. 



Tal>li' I. The Naliire Conscivaiicy/Natural Heritage Network eonservatioii ranks for rare coiniiuinltles. 



Rank 



Definition 



G1 Critically imperiled globally because of extreme rarity Generally five or fewer occurrences or less than about 

 800 ha (or 2,000 acres) or because of some factor making the community particularly vulnerable to extinction 

 G2 Imperiled globally because of extreme rarity. Generally 6-20 occurrences or 800-4,000 ha (2,000-10,000 

 acres) or because of some factor making the community very vulnerable to extinction throughout range. 



A preliminary list of GI and G2 communi- 

 ties was compiled by each of the Nature 

 Conservancy's science regions (Table 2) 

 through a detailed evaluation of all rare state 

 types reported by the state heritage programs in 

 each region. Each rare state type was reclassi- 

 fied to conform to the classification and nomen- 

 clature standards of the national framework. 

 Rare communities that cross regional bound- 

 aries were identified and re-classified as neces- 

 sary to produce the national list of GI and G2 

 communities. 



'Not included in this report 



Patterns of Community Rarity 



Within the lower 48 United States, 371 glob- 

 ally rare terrestrial vegetated communities have 

 been documented (Grossman et al. 1994). 

 Preliminary evaluation of the proportion of GI 

 and G2 types indicates that these will account 

 for about 10%- 15% of all terrestrial communi- 

 ties. It is premature to attempt detailed national 

 analysis and synthesis of existing data because 

 of the preliminary nature of the overall classifi- 

 cation and the unevenness in available commu- 

 nity information among regions. We can. how- 

 ever, provide a preliminary examination of the 

 relative proportion of rare communities in each 

 physiognomic class within each region. 



Eastern Region 



Fourteen percent of the nationally rare com- 

 munities occur in the eastern region (Anderson 

 et al. 1994). Many new community types are 

 still being identified. Most of the rare commu- 

 nities reported from the eastern region were for- 

 est, followed by sparse woodland and herba- 

 ceous types (Figure). The rarity of these com- 

 munities is either related to the suitability of 



Table 2. Tlie Nature Conservancy 



science regions. 



