Demography Of The Narrow Endemic Mint Dicerandra Thinicola: Patterns, Drivers, And Management Recommendations Based On 18 Years Of Data From Its Largest Wild Population

The mint genus Dicerandra is the rarest of any plants in the southeastern USA, and the narrow endemic D. thinicola H.A. Mill. is restricted to one wild population on public land and a few unprotected populations on private lands. From 2001 to 2017, we studied similar to 9,000 plants and 90,000 annual transitions in permanent plots in Florida scrub and roadsides to assess the health of this population and potential drivers of demographic change. Plant numbers have fluctuated widely, largely due to variably large pulses of winter seedling recruitment, but the overall trend has been upward, especially along sandy roadsides, in scrub gaps, and in chopped/burned scrub. Across the data set, annual survival (mean 67%) varied among habitats (being highest along roadsides) and among years (with decreased survival in some recent years). Nearly half of surviving vegetative plants advanced from vegetative to reproductive each year and most plants, once flowering, continued to flower each year. Growth in number of branches was consistently positive; only one-quarter of plants had reduced size each year. Relative growth rate was higher along roadsides than in other habitats. Reproductive output was lowest in scrub gaps and varied among years. Seedling recruitment was concentrated in winter months, varied widely among years, and was lower in the scrub matrix than in scrub gaps or roadsides. About half of seedlings died before their second year, the maximum observed life span was 13 yr, and fewer than 6% of plants survived 10 yr. Flowering began as early as age 2 (rarely as seedlings) and by age 4-7, 90% of surviving plants were reproductive. Land management (roller chopping and fire) in 2007 had profound effects on the population. Chopping killed 91% of plants and chopping followed by burning killed 100%. However, recruitment in these treated plots was 2.5 5.5 times higher in the 6 yr after treatments than in the 8 yr before treatments; no such differences were seen in untreated plots. Posttreatment plants grew faster and flowered earlier than other plants. Consequently, after several years, plant numbers in the treated areas had increased 4-8 times, whereas plant numbers in untreated areas changed little. This state-endangered plant is short-lived and depends heavily on disturbance (ideally fire) for recruitment and population growth. It is threatened not only by its narrow distribution but by insufficiently aggressive land management.