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Hybridization generates similarities among gene pools. This structure can be visualized and analyzed at the systems level using networks. Here we construct a network of the 315 woody plant species native or naturalized in the United States using data compiled by the Forest Service of the US Department of Agriculture (USFS). Each species is represented by a node in the network whose size is proportional to a recent census for live stems in the continental United States. Each of the 416 links between node pairs represents evidence for hybridization compiled from the USFS manual Silvics of North America. The total network resolved into 100 separate connected components or clusters (mean size, 3.15 species), with 44% of species linked to at least one other. Betula had the largest component (18 species) following by the separate Quercus clusters (17 red oaks and 16 white oaks); Q. velutina was the most genetically connected woody plant in the continental US. The number of species held together per component (i.e. size) scaled as a power-law albeit a slightly truncated one. The truncation suggests there are fewer than expected hybridizing species within the large woody genera of plants in the US. [ABSTRACT FROM AUTHOR]

Abstract: A dendroecological fire history study was conducted for The Nature Conservancy's Narrows Preserve on Peters Mountain, Virginia, where the predominant vegetation is oak (Quercus L.)-dominated forest containing some other hardwoods and pines (Pinus L.). The site encompasses all the known habitat of the endangered and endemic Peters Mountain mallow (lliamna corel Sherff.), a perennial herb that requires fire for seed germination and habitat maintenance. Fire scars from 73 pines indicate frequent burning in the past (Weibull median composite fire interval = 2.2 years), primarily during the dormant season. Fire frequency exhibited little temporal variability from the beginning of the fire chronology in 1794 until the 1940s, despite changing land uses. However, the incidence of fire declined subsequently with the advent of effective fire protection measures. Ageing trees near the mallow population indicates that the fire-tolerant chestnut oak (Quercus montana WilId.) recruited relatively continuously under frequent fire, but that other species were established primarily during the fire protection era. The decline in burning appears to have permitted an increase in tree density that likely inhibits the growth and recruitment of mallow plants. Our results suggest that reintroducing frequent fire would be an appropriate technique for managing the mallows and the greater Peters Mountain landscape. [ABSTRACT FROM AUTHOR]