Overstory Structure and Regeneration Processes in Longleaf Pine Forests

Research Team: Michael A. Battaglia (Graduate Student), Glen Stevens (Graduate Student), Dr. Robert J. Mitchell (Scientist), Dr. Brian Palik (USDA Forest Service), Dr. Robert Jones (VPI), Pu Mou (UNC - Greensboro), and Stephen D. Pecot (Research Technician IV)

Recent interest in restoring greater age structure to even-aged longleaf pine stands has led to the exploration of overstory retention approaches. This silvicultural approach is thought to more closely resemble the complex structure after natural disturbance and provide some benefit to unique ecosystem structures. However, the benefits of overstory retention are tempered by increased competition between seedlings and the overstory, causing reduced seedling survival and growth. Furthermore, the competitive environments and mechanisms that influence regeneration in longleaf pine systems differ substantially from that of closed-canopy ecosystems and those under even-aged management that have been much more extensively studied. We initiated a study in 1998 to address the effects of residual overstory structure and competing herbaceous vegetation on the survival and growth of planted longleaf pine seedlings. Stands were harvested to a similar residual basal area using three removal techniques: single-tree selection, small group (~0.20 ac) selection, and large group (~0.45 ac) selection. We also used an uncut control stand. Each treatment and control was randomly assigned to three replications. Subplots encompassed the range of overstory density in each stand. The understory in each subplot received either an herbicide solution to remove herbaceous vegetation or was left intact.

We found below-ground gaps were short-lived with neighboring trees or understory plants quickly colonizing below-ground space vacated by tree removal. Above-ground gaps, however, were more persistent. Annual estimates of light transmittance reaching the understory varied significantly both spatially (intact canopy to gap center) and temporally (time of year and sky condition). Hemispherical photograph estimates of light reaching the understory were more precise and less biased than ten-minute average measures using photodiodes during overcast or cloudless days. Seedling growth was closely related to overstory density, with the greatest growth in areas where 70% of the sky was unobstructed by overstory crowns. The spatial arrangement of overstory retention affected mean seedling growth response at a stand level. By manipulating the spatial distribution of remaining overstory trees the benefits of overstory retention can be retained while allowing for robust regeneration of longleaf pine.

Additional Information:

McGuire JP, Mitchell RJ, Moser EB, Pecot SD, Gjerstad DH, Hedman CW. Gaps in a gappy forest: plant resources, longleaf regeneration, and understory response to overstory removal. Can J For Rsch 2001; 31:765-78.

Jones RH, Mitchell RJ, Stevens GL, Pecot SD. Influence of canopy gap size and distribution on local and ecosystem wide fine root production in a pine woodland. Oeco.

Palik BJ, Mitchell RJ, Houseal G, Pederson N. Effects of canopy structure on resource availability and seedling response in a longleaf pine ecosystem. Can J For Res 1997;27:1458-64.

Funded by: USDA NRI Ecosystems