Effects of fire and herbivory on an endangered terrestrial orchid in a Texas post oak savanna

Background/Question/Methods

Fire and herbivory are known to strongly influence the structure and dynamics of numerous plant populations in savanna ecosystems.  While other terrestrial orchid species have demonstrated a wide variety of responses to fire and herbivory, little is known about the effects of these disturbances on Spiranthes parksii, a federally listed endangered orchid endemic to the post oak savanna of Texas.  We designed in a full-factorial, completely randomized field experiment to assess the effects of these disturbances on S. parksii.  Thirty-two 1.5x1.5m plots were established in areas of high orchid abundance.  Treatments included i) burned and fenced to exclude large vertebrate herbivores (eg, rabbit-sized or larger), ii) burned, but not fenced, iii) fenced, but not burned, and iv) unburned, unfenced controls.  Pre-treatment data were collected and 2-m tall fences were erected in May 2009.  Fences consisted of 10-cm wire mesh affixed to steel t-posts.  Fires were conducted in July 2010 using a steel burn-box with dried hay added to ensure uniform and complete combustion.  We routinely collected data on orchid rosette and flowering stalk abundance and measured a variety of plant demographic parameters.  We also measured herbaceous plant community cover, PAR availability and soil moisture content for each plot.

Results/Conclusions

During the year following fire, S. parksii rosettes emerged a week earlier in the growing season in burned plots (p<0.0001), potentially allowing for more photosynthesis and subsequent carbon storage.  Unexpectedly, unburned plots averaged one flowing stalk per plot, whereas only one orchid flowered overall in the burned plots.  Flowering stalk herbivory was similar in burned and unburned plots (p=0.9671), however with very few stalks emerging, it is difficult to interpret whether fire may affect herbivore activities.  Excluding large herbivores did not have an effect on rosettes (p=0.0713), but flowering stalk numbers were lower in unfenced plots prior to initiating the fire treatment (p=0.008).  In 2011, the second year following fire, no flowering stalks emerged, but this was likely due to historically extreme drought conditions throughout much of Texas prior to flowering season.  While fire suppression and overgrazing threaten many endangered plants more data are needed in order to determine the response of S. parksii to fire and herbivore disturbance. Species-specific responses must be more clearly understood to develop conservation and management strategies related to these disturbances.