Full ecoregional plan - Conservation Gateway

to obtain the detail we needed to make a determination of viability. The criteria for eachof the three factors are discussed below.SizeThe size of a contiguous forest example is particularly important with respect to theviability of matrix-forming ecosystems. To establish how large examples should be, twokey factors were considered: the size and frequency of natural disturbances and the sizeof the habitat needed by selected interior forest species within the ecoregion in order tobreed.Natural disturbances and minimum dynamic area: Examples of matrix forestecosystems should be large enough to withstand the full range of natural disturbances thatinfluence the system. To estimate the critical area needed to ensure that an ecosystemcould absorb, buffer, and recover from disturbance, we first listed the expectedcatastrophic disturbances typical of the ecoregion. In the Northeastern U.S., thesedisturbances include hurricanes, tornadoes, fires, ice storms, downbursts andinsect/pathogen outbreaks. Sizes of these disturbances were established from historicalrecords, vegetation studies, air photo analysis and expert opinion.Numerically, most disturbances are small and frequent; however large, infrequent,catastrophic events have had the greatest impact on most of the present landscapes. 3Thus, although Shugart and West (1981) suggested that minimum dynamic areas bescaled to the mean disturbance patch size, Baker (1992) emphasized that it should bescaled to the maximum disturbance size to account for the disproportional influence ofcatastrophic disturbances. Likewise, Peters et al. (1997) suggested scaling the minimumdynamic area to the largest disturbance event expected over a 500-1,000 year period.Damage from catastrophic natural disturbances is typically dispersed across a landscapein a uneven way such that severe damage patches are embedded in a larger area ofmoderate or light damage. We focused on this pattern and determined the maximum sizeand extent of severe damage patches expected over a one century interval for eachdisturbance type (see examples in Table MAT1 and Figure MAT1).Table MAT1. Comparison of characteristics among infrequent catastrophicdisturbances in the Northern Appalachian Ecoregion (adapted from Foster et al.1998)DisturbancecharacteristicTornado Hurricane DownburstsLargeFiresDuration Minutes Hours Minutes Weeks/monthsReturn 100-300 60-200 ? 400-interval in6000yearsMaximumsize of severedamagepatches(acres)InsectoutbreakIceStormFloodMonths Days Week/months10 2 50-1005000 803 3400 57-150 ?