Impact of multiple fires on stand structure and tree regeneration in central Appalachian oak forests

The National Guard Training Center at Fort Indiantown Gap (NGTC-FIG) near Harrisburg, PA, has experienced frequent fires since the 1950s on the ridges and 1980s in the valleys as a result of military training exercises. This represented a unique opportunity to investigate the role of recent and repeated fire in oak (Quercus) forests in the eastern USA. We investigated four frequently burned and two unburned sites replicated in ridge and valley ecosystems. Burned sites generally had lower tree density and a higher proportion of overstory oak species (64-92% relative importance value) than unburned stands (47-49% importance). Oak saplings averaged 875 ha−1 in burned forests and 31 ha−1 in unburned forests. Red maple (Acer rubrum, L.), the most aggressive oak replacement species in the eastern USA, had overstory importance of 7% and 24% in burned and unburned stands, respectively. Oak saplings ranged from 824 to 1545 ha−1 in three of the four burned stands and 0-62 ha−1 in the unburned stands. Oak sapling density was only 62 ha−1 one recently (2002) burned stand where fire had not resulted in reduced tree density; this stand had the highest tree density of all sampled stands. There were no red maple saplings in three of the four burned stands. Oak saplings were most abundant when overstory density was less than 400 trees/ha and understory tree density was less than 200 trees/ha. When overstory or understory tree density exceeded 400 and 200 trees/ha, respectively, oak regeneration was virtually absent. The results of this study suggest that periodic fire often reduces overstory and understory stand density and promotes successful regeneration of relatively shade intolerant oak species in the eastern USA. However, high tree density in forests will retard the development of oak understories and subsequent recruitment, even if periodic burning occurs.