Longleaf pine restoration on hydric sites: Understorey plant community responses to site preparation through 15 years

Question Longleaf pine (Pinus palustris) restoration is an important management objective throughout the southeastern United States. Site preparation prior to planting longleaf pine seedlings is often required to overcome challenges to seedling establishment on hydric sites. We know site preparation improves growth and survival of planted longleaf pine on hydric sites, but what are the impacts of site preparation on the understorey plant community? Location Marine Corps Base Camp Lejeune in Onslow County, NC, USA. Methods This study tested eight site preparation treatments, including an untreated control, six combinations of two vegetation control treatments (chopping or herbicide) with three soil manipulation treatments (mounding, bedding, or flat-planting [no treatment]), and a chopping-herbicide-bedding treatment. We collected data on understorey plant abundance, diversity, and composition through the first three and at 15 years after plantation establishment. We used ANOVA procedures and multivariate ordination to test for differences in understorey responses among the study treatments and through time. Results Site preparation had lasting impacts on the understorey plant community. The chop-only treatment was the only treatment that resulted in dominance of herbaceous vegetation and was among the treatments with the highest diversity and greatest abundance of Aristida stricta (wiregrass). Herbicide produced lasting reductions in Aristida stricta abundance and apparent shifts in community composition. Reduced graminoid abundance was particularly pronounced when herbicide was combined with soil manipulation treatments. Conclusions Understorey responses to site preparation through 3 years differed from responses observed in year 15, highlighting the importance of long-term monitoring. For example, herbicides controlled shrubs early, but by year 15, herbicide treatments were dominated by shrubs. Restoration scenarios often must balance ecological objectives (e.g., maintenance of a diverse understorey community) with forestry objectives (e.g., timely establishment of overstorey trees), and our findings can be used as a guide to balance these trade-offs.