Growth and nitrogen fixation activity of native legumes in response to light levels under longleaf pine canopies

Background/Question/Methods

Approximately five hundred thousand hectares of longleaf pine (Pinus palustris Michx.) plantations have been planted across the native range of the species in an attempt to restore longleaf pine ecosystems. Many of these plantations were started on former agricultural soils highly depleted in carbon and nitrogen (N). Introduction of native legumes into these plantations is a restoration tool used to increase soil organic matter and N pools, and to provide improved wildlife cover and food resources. Our objectives were to determine light levels needed for survival, growth, and N₂-fixation of native legumes in longleaf pine plantations and also estimate potential rates of N₂-fixation needed overtime to restore soil N pools on sandy N depleted former agricultural sites. This study assessed the survival, growth, and N₂-fixation capacity of native legumes using measures of stable isotope abundance under three light levels (8, 61 and 83% light penetration) in a fourteen year old longleaf pine plantation. Survivorship, growth, and N₂-fixation activity was assessed in six native legume species during their first year of establishment.

Results/Conclusions

 An average of twenty nine percent of the transplants from each species survived the first year. Growth, measured as aboveground biomass, was greatest in the open plots for most species with Desmodium ciliare (Muhl. ex Willd.) DC. (DECI) having the most aboveground growth in the open light treatment (31.1 grams per plant). N₂-fixation was not measured in the lowest light environment due to poor survival and growth and N₂-fixation activity was very low under intermediate light conditions (0-2.1 %N_dfa). The highest levels of N₂-fixation activity for most species was observed with eighty three percent canopy openness (44.5-79.7 %N_dfa), which corresponded to those plants with the greatest total biomass and highest N content (Lespedeza hirta (L.) Hornem., DECI, Lespedeza angustifolia (Pursh.) Ell., and Tephrosia virginiana (L.) Pers). Results from this study indicate that although these native legumes will grow and fix N in low light environments reestablishment of native understory legumes will be most effective after trees have been operationally thinned.