Long-term effects of group opening size and site preparation method on gap-cohort development in a temperate mixedwood forest

Management aimed at regenerating softwoods and hardwoods in the same stand (i.e., a mixedwood) has some appeal in the pine-hardwood forests of the southern United States. We revisited a group selection experiment installed in 1991 to evaluate the influence of gap size and site preparation treatments on a pine-hardwood stand in Louisiana, USA. Experimental treatments included a factorial combination of three group opening sizes (0.1, 0.25, 0.4 ha) and three methods of site preparation (chemical, mechanical, untreated) in the harvest gaps. Specifically, we tested two hypotheses: 1) regeneration of shade-intolerant tree species increased with gap size and the addition of site preparation and 2) if this site preparation decreased the regeneration of species that are more tolerant of shade and reliant on advance reproduction. In 2016, the density and basal area of the shade-intolerant loblolly pine (Pinus taeda), shortleaf pine (Pinus echinata), and sweetgum (Liquidambar styraciflua) increased with increasing gap size (P < 0.05), supporting our first hypothesis. Compared to untreated gaps, white oak (Quercus section Quercus), red oak (Quercus section Lobate), and red maple (Acer rubrum) abundances were significantly reduced in gaps treated with chemical or mechanical site preparation, supporting of our second hypothesis. Based on these results, it appears possible to secure adequate pine regeneration with a relatively wide range of early gap-treatment option in similar pine-hardwood forest types. However, pines dominated nearly all gaps (regardless of treatment), so much so that many no longer qualify as mixedwood. This outcome suggests that managers interested in group selection for mixedwoods in this part of the coastal plain should consider creating larger gaps (≥0.25 ha) in areas with preferred hardwood advance reproduction, while limiting within-gap site preparation to help protect it.