Peperomia kauaiana (Piperaceae), a new alternate-leaved species from Kaua‘i, Hawaiian Islands and notes on two possibly extinct Hawaiian Peperomia

A new species of <i>Peperomia</i> with alternate leaves from Kaua'i, Hawaiian Islands, is described and illustrated, with notes on its conservation status, distribution and ecology. We present a dichotomous key to all five Hawaiian <i>Peperomia</i> species with alternate leaves and include notes on two possibly extinct Hawaiian <i>Peperomia</i> species, namely <i>P. degeneri</i> and <i>P. subpetiolata</i>. <i>Peperomia kauaiana</i> <b>sp. nov.</b> differs morphologically from its Hawaiian congeners by its unique combination of diminutive leaves 5-14(-18) mm long, 4-11(-14) mm wide, palmately 5- to 7-nerved, ovate to ovate-orbicular with margins revolute, petioles 2-5 mm long and spikes 11-17(-22) mm long. Plants have been documented in three distinct windward Kaua'i locations to date, including the southern ridges of Wahiawa, the central ridges of Wai'ahi and the north-eastern ridges of the Makaleha Mountains. <i>Peperomia kauaiana</i> represents a newly-described wet forest species endemic to the island of Kaua'i and is currently in need of conservation. Its discovery raises the total number of endemic Hawaiian <i>Peperomia</i> species to 24 and single-island endemic <i>Peperomia</i> on Kaua'i to three.