PS 25-92
To plant or not to plant? Results from a century-long forest planting experiment
The Lehigh University Experimental Forest (LUEF) is a 5.5-ha forest in eastern Pennsylvania that was densely planted with 22 species of evergreen and deciduous tree seedlings in 1915. The seedlings were planted in 43 distinct plots, arranged either in monospecific or bispecific associations. This planted forest was left unmanaged for approximately a century, and therefore provides a unique opportunity to explore the potential long-term effects of planting on tree community structure, spatial distribution patterns and recruitment history. In this study, the community composition and recruitment history of the LUEF were compared to a nearby non-planted control site in order to contrast the effects of planting versus natural succession alone. Both forests were surveyed and all trees with a measurable diameter identified, tagged and spatially mapped. Recruitment dates were obtained for 460 black birches (Betula lenta L.) in both forests according to standard dendrochronological procedures.
Results/Conclusions
Even after a century of succession, the legacy of initial planting arrangement is apparent in the community composition, species richness and evenness, spatial distribution of species, and spatiotemporal recruitment patterns. Although both forests are dominated by black birch and oak (Quercus spp.), community composition was significantly different between the sites, and the planted forest had greater species richness and evenness. Trees in the planted forest tended to have a clumped distribution pattern whereas species in the unplanted forest tended to be more randomly distributed. This may highlight the vulnerability of monospecific plot plantings in the LUEF, as unsuccessful planting and species-specific mortality may have created large canopy gaps. Results from tree-ring analyses show significant differences in spatial and temporal patterns of recruitment in Betula lenta between the two forests. In the planted forest, most Betula lenta established almost immediately and subsequently infiltrated the forest in discrete episodes and locations; establishment in the non-planted forest was temporally more gradual and was more spatially random. In recent decades both forests show decreased rates of tree recruitment, and understory vegetation and camera traps identify herbivory from white-tailed deer (Odocoileus virginianus) as a likely cause. Results of this study indicate that while planting may increase total species diversity, species should be chosen carefully to best suit the particular environmental conditions of a site and monospecific plots likely influence the size and frequency of subsequent canopy gaps. Insights from natural experiments such as this one are particularly timely, given ongoing global afforestation efforts.