Influence of Competition on Longleaf Pine Seedling Recruitment in Selection Silviculture

Abstract

The longleaf pine (Pinus palustris Mill.) ecosystem’s decline has resulted in the loss of 97 percent of the 24-36 million hectares it covered prior to European settlement, but interest in longleaf pine restoration and management has increased in recent decades. This project sought to determine what levels of residual overstory in selection silviculture promote adequate longleaf pine seedling recruitment. Six hundred containerized longleaf pine seedlings were planted on two sites, one subxeric and one mesic, in the 2007-2008 dormant season. Half of the seedlings at each site were randomly selected for understory removal (with herbicide) in order to differentiate overstory influences from those of the understory. Canopy gap fraction above each seedling was determined using hemispherical photography, and average soil moisture was determined from four time domain reflectometer (TDR) measurements at each seedling during the 2008 and 2009 growing seasons. Seedling groundline diameter (GLD) was measured at time of planting and in August, 2008 and 2009. First year results indicated that mean moisture was not significantly different between herbicide and control treatments at either site. Regression analyses showed weakly positive relationships between moisture and seedling growth for both treatments at both sites, whereas a generally negative but non-significant relationship existed between gap fraction and seedling growth. Second year results showed a similar absence of a treatment effect on soil moisture at both sites. Total 2008-2009 mean seedling growth and soil moisture were significantly greater at the subxeric site than at the mesic site. Regression analyses showed few significant relationships, but generally positive trends existed between gap fraction and seedling GLD growth. In contrast, no general trend was present between mean soil moisture and GLD growth. Root excavations from a subsample of seedlings showed that root growth was not extensive, in that estimated mean taproot depth was shallower than 90 cm. Furthermore, no relationship existed between GLD and root depth. Due to confounding factors, hypothesis testing could not be conducted, and results represent circumstantial evidence only. However, results document the severity of longleaf pine’s growing environment on both mesic and subxeric sites and suggest that seedlings frequently experienced moisture stress during years slightly below average and above average for total rainfall. Although immature, data collected in this study support that of other research suggesting that early longleaf pine growth and survival is increased by moisture availability, but that with time light becomes the primary driver of longleaf pine seedling recruitment.