During a survey by the National Tropical Botanical Garden drone team, an enigmatic Schiedea was observed in December 2021on steep, rocky cliff faces of the Waiahulu Valley in the Waimea Canyon of Kaua'i. Subsequently, another survey was conducted in March 2022 and, by use of a remotely controlled cutting device suspended below the drone, the first herbarium specimen was collected, as well as a seed collection of an undescribed cliff-dwelling species of Schiedea. Detailed study of the collections and plants grown at the University of California, Irvine greenhouse showed that it had enlarged, somewhat whitish sepals similar to those of cliff-dwelling S. attenuata (the sole species in sect. Leucocalyx), yet differed significantly from all other species in the genus. It also shares with S. attenuata a woody habit, hermaphroditic flowers, coloured nectar and styles 5 to 7 or 8. We describe it here as S. waiahuluensis given the only known localities are on the cliffs of this valley and place it in an enlarged sect. Leucocalyx. With the discovery of this new species, there are 36 species in this Hawaiian endemic genus.

Caryophyllaceae, conservation, drone exploration, Hawaiian Islands, Kaua'i, Schiedea

Drone technology is advancing quickly and has become an effective tool for botanical surveys of cliff environments (Nyberg et al. 2023). Drone imagery with very high resolution has greatly increased our understanding of the distribution and abundance of many rare plant taxa. In Hawaii, researchers have been developing drone-based sampling systems capable of collecting plant material remotely. These platforms are being used to reach inaccessible areas to monitor the spread of forest pathogens (Perroy et al. 2022) and to conserve critically endangered plants (La Vigne et al. 2022).

While monitoring the walls of Waimea Canyon, Kaua'i for rare plant taxa and using a drone to photograph selected study sites, staff at the National Tropical Botanical Garden (NTBG) documented an enigmatic Schiedea in December 2021. On first examination of our images, we speculated that the unique shrub could possibly represent the extinct Kaua'i species, Schiedea amplexicaulis H.Mann, last observed in 1855, or perhaps a new species. Subsequently, we returned in March 2022 and used a remotely controlled cutting device suspended below the drone to make the first herbarium specimen and seed collections of the Schiedea. On close examination of our herbarium specimen, it became immediately clear that we had discovered a new Schiedea species, bringing the total number of this Hawaiian endemic genus to 36 species (Wagner et al. 2022), with twelve species occurring on Kaua'i as single-island endemic taxa (Wagner et al. 2005). This discovery is likely the first time an undescribed species has been located and collected via drone, demonstrating the profound significance of unmanned aircraft systems in the conservation and prevention of plant extinctions.

For the survey portion of this work, we deployed a DJI Phantom 4 pro quadcopter drone. This is a consumer-grade platform that was chosen for its portability and high-resolution image sensor (20 megapixels). Still photographic images were collected approximately 5 m from the cliff surface to allow adequate resolution for the location and identification of small plants.

Photographs taken by the drone were reviewed and classified by species in Adobe Lightroom software. Image classification is a manual process that relies on visual acuity and expert field knowledge. The high-resolution imagery is examined for species identification and phenology, to track plant distribution and abundance and also to guide field operations. GPS location information is embedded into each image file which assists in locating access points and re-finding specific individual plants.

Once the collection target was selected, we deployed an Outreach Robotics Mamba, suspended under a DJI Matrice 300 drone. The Mamba is a robotic sampling manipulator designed to grab, cut and collect plant material in vertical cliff habitats using propellers to advance the device towards an adjacent cliff in a swinging motion, while keeping the carrying drone clear of obstacles.

The distinctive morphology of this remarkable new species places Schiedea waiahuluensis into sect. Leucocalyx W.L.Wagner & Weller, previously represented by a single cliff-dwelling species, S. attenuata W.L.Wagner, Weller & A.K.Sakai. Besides both being cliff-dwellers with a woody habit, they are also both hermaphroditic, with enlarged greenish-white sepals, coloured nectar and 5 to 7 or 8 styles. Schiedea attenuata occurs in Kalalau Valley, around 7 km to the north of S. waiahuluensis and is restricted to dry/mesic vertical cliffs. Morphological characters of these species and S. viscosa are compared in Table 1. The enlarged, somewhat whitish sepals of S. waiahuluensis (Fig. 6), glandular pubescence and coloured nectar are features also found in S. viscosa H.Mann and S. lychnoides Hillebr. (sect. Nothoschiedea H.Mann). Ongoing analyses (McDonnell et al., in prep.) of our study of the phylogenetics of the genus Schiedea using Hyb-Seq showed that addition of a single sample of S. waiahuluensis placed it as sister to S. attenuata. This clade was, in turn, sister to a clade consisting of sect. Nothoschiedea + sect. Alsinidendron. Additional samples of S. waiahuluensis that will be added to the analyses should help with further understanding of the phylogenetic placement of Schiedea waiahuluensis and other details of the diversification of the genus.

In contrast to the facultatively self-pollinating species in sect. Nothoschiedea, S. waiahuluensis appears to be outcrossing, based on strong protandry and production of ca. 16,000 pollen grains per flower, a value typical of outcrossing species (Weller et al. 1998). The large, spreading sepals of this species suggest biotic pollination, although field studies to test this possibility would be extremely challenging. Schiedea waiahuluensis is unique amongst Schiedea species for the discovery of a native, probably new species of Engytatus (Heteroptera, Miridae; Polhemus, pers. comm.) on several individuals surveyed on the cliffs of Waiahulu Canyon (Fig. 6A). These are the first observations of a potential native herbivore on a Schiedea species.

Increasingly, drones are being used to assess and inventory cliff-dwelling plant taxa (Strumia et al. 2020; Reckling et al. 2021; Gao et al. 2024). The addition of drone-based sampling tools has the potential to transform plant conservation efforts in these hard-to-reach cliff environments (La Vigne et al. 2022). Hidden floristic diversity is likely to emerge as we embark on this new era of exploration and documentation of cliff ecosystems. The discovery of S. waiahuluensis after over 40 years of intense interest in this genus on Kaua'i indicates the potential for new discoveries using drone technology in studies of other endemic plant genera in the Hawaiian Islands.

We are grateful to Alice Tangerini for her excellent illustrations of this new species, constructed from limited collections and photographs and for assembling the photo plates of the habitat and features of this unique species. This work was supported by National Science Foundation grants DEB-1753659, DEB-1753664, DEB-1750373 and DEB-1752785: Collaborative Research: Unlocking the evolutionary history of Schiedea (carnation family, Caryophyllaceae): Rapid radiation of an endemic plant genus in the Hawaiian Islands (PIs: A. Sakai, S. Weller, N. Wickett, M. Moore, W. Wagner) and the National Tropical Botanical Garden Drone programme. We thank Outreach Robotics for development and deployment of the Mamba tool that made the drone collections possible. We appreciate the review of the manuscript by Matt Knope. We thank Javier Rodarte for care of plants at the University of California, Irvine greenhouse. For their constant support of fieldwork on Kaua'i, we thank the State of Hawai – Division of Forestry and Wildlife and the Plant Extinction Prevention Program, especially staff that assisted with this study: Adam Williams, Scott Heintzman and Susan Deans.