Fire-induced divergence of the lifecycle of an endangered terrestrial orchid (Spiranthes parksii Correll)

Background/Question/Methods

Spiranthes parksii Correll is an endangered orchid endemic to the post oak savanna of east central Texas. Fire suppression in the post oak savanna has altered the structure and composition of these communities and may be a factor driving the decline of S. parksii through mechanisms such as heightened interspecific competition, decreased light levels, and excessive litter accumulation. Many terrestrial orchid populations respond favorably to prescribed burning. Additionally, herbivores interact with fire, often preferentially selecting recently burned patches on the landscape. We designed an experiment to determine the effects of fire, herbivory and their interaction on S. parksii.  In August 2009, we selected 32 (1.5m x 1.5m) plots in areas of known high NLT density and initiated four experimental treatments manipulating fire and herbivory. We burned and fenced eight plots, burned eight unfenced plots, fenced eight unburned plots, and left eight plots unburned and unfenced as controls. We burned in June 2010, between rosette senescence and flowering stalk emergence, using a burn box to control for variation in fire intensity and flame residence time. We measured and analyzed S. parksii demographic parameters to model the effects of fire and herbivory on the life cycle of the orchid.

Results/Conclusions

In burned plots, fewer orchids flowered (0 in burned plots, 0.43 in unburned plots), and rosettes emerged earlier (September 28 for burned plots, October 13 for unburned plots) and experienced increased herbivory (76% in burned plots, 23% in unburned plots) early in the growing season during the first year following the fires. Using stage-structured models parameterized with demographic data collected from individuals in the different treatments, we demonstrated that both fire and herbivory alter the timing and conditional rates of transition in the S. parksii lifecycle. The divergence in lifecycle pathways induced by the experimental treatments we imposed, suggests the potential for these two disturbances to alter the fitness of individuals. This alteration in individual fitness has implications for population dynamics, growth, and viability. The life-stage transition models derived from this experiment establish a baseline of information regarding population responses to fire and herbivory which can be used in the development of sound conservation strategies for this endangered orchid.