Woody undestory response to changes in overstory density: Thinning in Allegheny hardwoods

Understanding the effects of silvicultural treatments on understory vegetation is important in predicting the consequences of such treatments, not only on regeneration but also on wildlife habitat, visual qualities, and recreation. We sought to develop an empirical model of understory response that could be generalized to other forest types. We analyzed understory populations of tree species for 15 years following thinning to different residual relative densities in 50- and 55-year-old Allegheny hardwoods. The average number of stems 1 ft (0.3 m) tall to 1 in (2.5 cm) dbh increased for 3 to 5 years after thinning and then leveled-off or decreased after 10 or 15 yers. The greatest density of understory stems developed at low residual density. In stems 1 to 3 ft (0.3 to 0.9 m) tall, the densities of shade-tolerant species were unresponsive to thinning while the shade-intolerant were most responsive. The shade-intolerant and -intermediate species increased in importance over time in the more heavily thinned treatments. In the 3 ft (0.9 m) tall to 1 in (2.5 cm) dbh size class, shade-intolerant and -intermediate species were more responsive to thinning than tolerant species, but shade-tolerant species remained more important numerically throughout the study. Ingrowth to > 1 in diameter classes was greatest by shade-tolerant stems, increased over time, and was enhanced by thinning. We used repeated measures analysis of variance to model the number of stems in these three size classes and three shade-tolerance classes as a function of residual relative density at thinning and time since treatment. These models explained 0.08 to 0.80 of the variation in stem numbers, depending on the size and tolerance class. These descriptions might be improved by reference to prior conditions of the regeneration or interfering herbaceous competition, but a model that required this information would not be capable of predicting responses to future treatments.