Date of Award

Spring 1999

Document Type

Restricted Thesis

Terms of Use

© 1999 Georgine G. Yorgey. All rights reserved. Access to this work is restricted to users within the Swarthmore College network and may only be used for non-commercial, educational, and research purposes. Sharing with users outside of the Swarthmore College network is expressly prohibited. For all other uses, including reproduction and distribution, please contact the copyright holder.

Degree Name

Bachelor of Arts



First Advisor

Roger Latham


The species-rich Pocono heath barrens unexpectedly persist on a similar soil type and moisture regime as adjacent forests. Field and greenhouse experiments evaluated the hypothesis that barrens fragments have persisted despite forty years of fire suppression because the barrens plants alter nitrogen cycling, lowering mineral nitrogen levels and inhibiting invasion by most forest species. A greenhouse test using Carex pensylvanica suggests that there may be less available nitrogen in barrens soil than in forest soil. The difference in available nitrogen seems to be large enough to cause a physiological response in Carex pensylvanica growth, indicating that the difference is biologically significant. However, a bioassay of nitrogen uptake by fine roots of Acer rubrum and Amelanchier spp. suggests that i the few trees growing in the barrens are no more nitrogen stressed than those in the forest. It seems likely that these trees are able to invade the barrens because they are able to tolerate low nitrogen conditions. Because of this trait, Acer rubrum may be a key species in transitions from barrens to forest.