Monitoring rare plant populations is often challenging because the entire population may not be easily quantified, and the population dynamics of the quantifiable fraction may be highly variable. This is the case for the limestone glade bladderpod, Physaria filiformis, and the western prairie fringed orchid, Plantanthera praeclara, two federally threatened species in which populations are monitored annually. Flowering plants may be enumerated in both, but vegetative plants are difficult to observe in the orchid, and in both the size of the seed bank is unknown. In populations monitored >20 years, the number of flowering plants has varied from zero to many hundreds (orchid) or tens of thousands (bladderpod). Given such variability, the practical relevance of monitoring flowering plants is questionable, and quantifying the seed bank is usually impractical. We evaluated three alternative lines of evidence that can be used in evaluating the status of such imperiled plant populations: (1) We quantified spatial synchrony in six populations of the bladderpod over an eleven-year period. (2) We evaluated the use of monitoring appropriate habitat as a proxy for directly monitoring bladderpod populations. (3) We evaluated the use of climatic models that predict when conditions are favorable for the orchid.
Results/Conclusions
We found evidence for strong spatial synchrony in the number of flowering plants among the six populations of the bladderpod. Thus a decline in the number of flowering plants of one population may indicate no need for concern if other populations in the region are also declining, but should be cause for alarm if other populations are not declining. An experimental reduction in the cover of Eastern redcedar (Juniperus virginiana), which encroaches upon bladderpod habitat, was associated with an increase in the number of flowering plants. A revised monitoring protocol includes measurement of redcedar abundance and photosynthetically active radiation, which is a way of monitoring habitat suitability as a proxy for the imperiled plant species. Climatic modeling in the orchid revealed the most sensitive periods in the phenology of the orchid in relation to precipitation. Thus low numbers of flowering plants in years in which precipitation is deficient in the relevant phenological periods would be expected, but declining populations in years with adequate precipitation would be cause for concern. Direct monitoring of imperiled plant populations may yield data that are irrelevant for conservation planning. The use of complementary data and analyses may provide greater insight into the appropriate management actions.