Association of water depth and aquatic-plant competition in conservation of Isoetes taiwanensis in the Menghuan Pond wetland in Taiwan

Isoetes taiwanensis DeVol, discovered in 1971, is a critically endangered aquatic plant endemic to Taiwan. The only natural habitat of I. taiwanensis is the Menghuan Pond wetland in northern Taiwan. With competition of some aquatic plants and terrestrial plants, I. taiwanensis was close to extinction in 2006. The bottom of the Menghuan Pond wetland was compacted and fractures were filled along the shore to improve the average water depth in 2008. The I. taiwanensis population was restored, but factors crucial to its survival remain unclear. To analyze the factors, we monitored 24 plots in the Menghuan Pond wetland monthly from January to December 2010 to record the coverage rates of potential competitive plant species and hydrology data. The wetland was found to be an ideal environment for growth of I. taiwanensis with annual mean water depth 50 cm. The species did not grow in plots with annual mean water depth lower than 30 cm. The species had an advantage over other species in plots with annual mean water depth 46.8–185.6 cm. The coverage rate of I. taiwanensis was positively associated with annual mean water depth. With the deepest annual mean water level, >60 cm, more than 90 % of the area was covered. Fluctuation of the water level may reduce the competition among I. taiwanensis and other species, particularly Eleocharis congesta , E. dulcis , and Schoenoplectus mucronatus . These data may help in designing a protocol for sustained conservation of I. taiwanensis in Yangmingshan National Park.