Vegetation composition and structure in two hemlock stands threatened by the hemlock woolly adelgid.

We quantified the vegetation composition and structure of two hemlock (Tsuga canadensis) ravines in the Delaware Water Gap National Recreation Area threatened by the hemlock woolly adelgid (Adelges tsugae). Hemlock accounted for more than 50% of the canopy basal area (ravine mean = 52.3 m(2) ha(-1)) and more than 75% of the understory trees and saplings (ravine mean = 599 stems ha(-1)). Other common trees were black birch (Betula lenta) and red maple (Acer rubrum); white pine (Pinus strobus) was abundant at one site. The forest understories were shady with limited cover of vascular plants (ravine mean = 5%). Average understory light levels ranged from 6% of the above-canopy light in the more open stand to only 3% in the more closed-canopy stand. Two ferns, Dryopteris intermedia and Dennstaedtia punctilobula, were the most abundant herbaceous plants with a patchy distribution. Bryophyte cover averaged 9%; Mnium hornum and Hypnum imponens were the most abundant mosses. Birch seedlings were the most frequent vascular plant (>70% of the plots) followed by red maple (> 50% of the plots). Average tree seedling density in 1994 was 3.2 seedlings m(-2); seedling density doubled in 1995. The density of woody species germinating from buried seed was < 2 germinants m(-2). The dominance of hemlock in both the canopy and subcanopy stratum, the lack of a non-hemlock sapling bank, the absence of a buried seed pool, and the prevalence of black birch and red maple seedlings and adults, all suggest that the initial response to any adelgid caused mortality will be a rapid increase in birch and maple importance in these stands.