Interactive effects of soil resources and light availability on the establishment and growth of an endangered orchid, Navasota Ladies’ Tresses (Spiranthes parksii)

Background/Question/Methods

Agricultural practices (e.g., grazing, mowing, and fertilization) are known to play a key role in orchid population performance and may serve as potential management tools for conservation.  Mowing/grazing can promote orchid populations by altering species composition, competition, and light availability.  Alternately, poor timing and intensity of grazing/mowing may negatively impact orchids through biomass accumulation, subsequent competition for light, orchid defoliation, and/or flowering stalk herbivory.  The problem is potentially compounded by anthropogenic nutritional inputs (i.e. increased N deposition or agricultural fertilization).  This has been documented in European systems where certain agriculture practices have contributed to orchid population declines.  Spiranthes parksii is a federally endangered plant species endemic to Texas post oak savannah.  We investigated the effects of fertilization and grassland grazing/mowing on S. parksii establishment, growth, and abundance using a randomized, split-plot experimental design.  We established thirty-two 5x5m plots with 2m buffers with four randomly assigned fertilizer treatments: control, nitrogen only, phosphorous only, and nitrogen/phosphorous combined.  Plots were divided to form two 2.5x5m subplots.  We manually clipped and removed herbaceous biomass during a period of orchid dormancy in one randomly assigned subplot of each pair.  Rosette and flowering stalk demographic data were collected for S. parksii and related Spiranthes species. 

Results/Conclusions

The number of Spiranthes rosettes and flowering stalks increased but did not differ significantly among treatments due to the large number of plots without plants (63%).  The majority of flowering stalks (80%), however, occurred in clipped subplots and in phosphorus additions plots (80% in both P and N/P plots).  Rosette area was slightly greater in unclipped subplots (p<0.1) and average leaf shape was significantly different with leaves longer relative to their width in the unclipped subplots (p<0.05).  Among the fertilizer treatments, leaf area was greater in P plots than control and N plots but did not differ from N/P plots.  Patterns in rosette size among clipped treatments are consistent with typical plant shade responses.  For instance, plots with lower light (i.e., unclipped plots) tended to have larger rosettes. These results correspond with similar findings in an experiment comparing closed shrub canopy versus removed shrub canopy.  Research will continue to evaluate the effects of light and other environmental variables on S. parksii and trends suggest that differences among treatments that are not apparent in the short term will become manifest over time.