Effects of understory seeding on ecosystem properties in planted longleaf pine forest

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Benju Baniya, School of Forest, Fisheries and Geomatics Sciences, University of Florida, Gainesville, FL; Forest ecology Lab, The Jones Center at ichauway, Newton, GA, Seth Bigelow, Forest Ecology, Forest Adaptation Research LLC, Leesburg, GA, Steven Brantley, Jones Center at Ichauway, Newton, GA, R. Scott Taylor, Joseph W. Jones Ecological Research Center, Newton, GA, Jason G. Vogel, School of Forest Resources and Conservation, University of Florida, Gainesville, FL and Ajay Sharma, West Florida Research and Education Center, University of Florida, Milton, FL

Background/Question/Methods
Restoration of the diverse understory of longleaf pine (Pinus palustris) ecosystems is an important land management goal in the southeastern United States, but little is known about the influence of restoration activities on ecosystem properties. We conducted a study in biennially burned and recently thinned longleaf pine plantations in southwest Georgia to examine the effects of seeding native species on understory characteristics and soil isotopes (δ13C and δ15N). In 2015, we seeded four plots (~ 4 ha each) with five warm season C4 grasses: Aristida stricta, Panicum virgatum, Sorghastrum secundum, Sorghastrum nutans, Schizachyrium scoparium and a legume: Tephrosia virginiana. Four untreated plots were kept as controls. In 2020, we sampled understory using 25 randomly laid out 1m2 quadrats in each plot and calculated species richness and diversity (Shannon Diversity Index and Simpson’s Diversity Index). Soil samples (0-10 cm, 10-20 cm, and 20-50 cm), forest floor and litter were collected to assess nutrient pools. Linear mixed models were used to examine the effect of seeding on understory species richness and diversity, and soil isotopic properties. Also, multivariate analyses on species cover composition, including NMDS (Non-Metric Multidimensional Scaling), PERMANOVA, and indicator species analysis were used to describe differences in community composition.

Results/Conclusions
We found a total of 121 species across all plots. Mean cover of the planted species was 15% with S. nutans representing the highest cover (%). Species richness (mean±se) of the seeded and control plots were 11.28±0.29 m-2 and 10.34± 0.29 m-2, respectively, and did not differ significantly (α=0.05). Shannon and Simpson’s diversity indices (mean±se) in the seeded plots were 1.74 ± 0.35 and 2.65±0.11, and 1.69 ±0.04and 2.64± 0.10 in control plots, respectively; but did not differ significantly. NMDS showed some overlap in species composition while PERMANOVA detected significant difference in species composition between seeded and control plots (p= 0.001). Ten species were associated as indicator species in seeded plots, including four of the seeded grasses (S. nutans, S. secundum, P. virgatum and A. stricta) whereas 11 indicator species were recorded in control plots. Soil δ15N and δ13C did not differ significantly after treatment. The findings suggest that the seeding is having minimal effect on herbaceous diversity, but the C4 grasses are dominant in the seeded plots mimicking natural longleaf savanna. Soil properties change slowly but we expect to see alterations in isotopic signatures over time.