Bryophyte response to eastern hemlock decline caused by hemlock woolly adelgid infestation

The understory conditions of eastern hemlock stands in the Delaware Water Gap National Recreation Area have been altered by canopy decline caused by an exotic insect pest, the hemlock woolly adelgid. We consider the response of bryophytes to these changes, particularly increased availability of understory light and coarse woody debris (CWD). Pre-adelgid surveys and environmental measurements were taken at a network of 72 permanent plots in 1994 and 1995. Re-measurement of understory variables was conducted in 2003 and 2006. Bryophytes have responded to adelgid impacts through a sustained increase in plot-level richness, mainly achieved through a greater frequency of species occurring on CWD and to a lesser extent on bare soil in the plots. Greater gains in species richness took place in plots closer to the streams. Bryophyte responses to understory light availability and shrub density suggest that light availability may be more important for species maintenance than for species colonization. Orthodicranum montanum (Hedw.) Loeske had the greatest increase in frequency, occurring in two-thirds of all plots on one or more substrata by 2006. Several nitrophilous species had appeared by 2003 and 2006 including Brachythecium rutabulum (Hedw.) Schimp., which has now colonized almost one-third of the plots since the hemlock woolly adelgid invasion. Bryophytes are a significant component of the understory vegetation in eastern hemlock stands and this study is the first to document the dynamic nature of their response to hemlock woolly adelgid impacts.