DNA barcoding has been largely successful in differentiating animal species, but the most effective loci and evaluative methods for plants are still debated. Floras of young, oceanic islands are a challenging test of DNA barcodes, because of rapid speciation, high incidence of hybridization and polyploidy. We used character-based, tree-based and genetic distance-based methods to test DNA barcoding of 385 species of native Hawaiian plants constituting 20 lineages at the nuclear ITS(2) locus, nine lineages at each of the plastid loci trnH-psbA and rbcL, eight lineages at the plastid locus matK and four lineages with concatenated data. We also incorporated geographical range information and tested if varying sample sizes within a lineage influenced identification success. Average discrimination success was low (22% maximum) with all methods of analysis across all loci. The character-based method generally provided the highest identification success, there were limited benefits from incorporating geographical data and no relationship between number of species sampled in a lineage and identification success was found. Percentages of identification success are the lowest reported in a DNA barcoding study of comparable scale, and multi-species groups that radiated in the Hawaiian archipelago probably cannot be identified based on current DNA barcoding loci and methodologies.

Rapid ʻ Ohiʻa Death (ROD), caused by the fungal pathogen Ceratocystis, is killing large numbers of ʻ ohiʻa trees (Metrosideros polymorpha) in Hawaiʻi. ʻ Ohiʻa are a dominant tree in Hawaiian forests, have a range that goes from arid to wet forest climates, and are important for endangered species habitat and ecosystem function. To test whether actively planting ʻ ohiʻa seedlings is a viable restoration strategy in areas with high ROD mortality, we planted ʻ ohiʻa in a ROD-affected forest and crossed this with weeding and fencing treatments to compare ROD mortality to other stressors. We also tested for viable Ceratocystis spores in soils around planting areas. We found that seedlings were more likely to die in unweeded and unfenced treatments than controls. Although viable Ceratocystis spores were found in soil, none of the 41 dead seedlings tested positive for Ceratocystis. This indicates that competition from exotic plants and exotic feral ungulate damage are more likely to kill seedlings than ROD within the first year after planting.

Summary Loss of local biodiversity resulting from abrupt environmental change is a significant environmental problem throughout the world. Extinctions of plants are particularly important yet are often overlooked. Drawing from a case in Hawai‘i, a global hotspot for plant and other extinctions, we demonstrate an effort to better understand and determine priorities for the management of an endangered plant (‘Ihi makole or Portulaca sclerocarpa) in the face of rapid and extreme environmental change. Volcanic heat emissions and biological invasions have anecdotally been suggested as possible threats to the species. We integrated P. sclerocarpa outplanting with efforts to collect geological and ecological data to gauge the role of elevated soil temperatures and invasive grasses in driving P. sclerocarpa mortality and population decline. We measured soil temperature, soil depth, surrounding cover and P. sclerocarpa survivorship over three decades. The abundance of wild P. sclerocarpa decreased by 99.7% from the 1990s to 2021. Only 51% of outplantings persisted through 3–4 years. Binomial regression and structural equation modelling revealed that, among the variables we analysed, high soil temperatures were most strongly associated with population decline. Finding the niche where soil temperatures are low enough to allow P. sclerocarpa survival but high enough to limit other agents of P. sclerocarpa mortality may be necessary to increase population growth of this species.

Aim Terrestrial plant species on islands have a long history of study to determine how they evolved and what explains their levels of endemicity, but studies on fungi are lacking. Here, we examine: (1) how percent endemism of non‐lichenized class Agaricomycetes; hereafter, ‘mushrooms’ compares to angiosperms, ferns, bryophytes and lichens from oceanic islands/archipelagos; (2) whether endemic mushrooms evolved from an ancestor diversifying into multiple species after island colonization (cladogenesis) or over time evolved into a single endemic species unique from its ancestral mainland counterpart (anagenesis); and (3) if mushroom percent endemism and cladogenesis are correlated to geographic variables that help explain these phenomena in other species groups. Location Hawaiʻi, Galápagos, Canary Islands, Madeira, Azores, Cabo Verde, Christmas Island. Taxon Mushrooms, angiosperms, ferns, bryophytes, lichens. Methods Checklists of mushrooms and other species groups from seven oceanic islands/archipelagos were compared. Having multiple endemic congeners from a single island/archipelago was used to infer cladogenesis versus anagenesis in endemic mushrooms. Pearson's correlation coefficients were calculated between an island/archipelago's percent endemism and percent cladogenesis, and their distance to the nearest mainland, area, maximum elevation and latitude from the equator. Results Hawaiʻi has the highest mushroom endemicity at 83%, of which 50% are potentially derived from cladogenesis. Considering mushrooms on all islands/archipelagos, speciation by cladogenesis leads to fewer endemic species (37%) than anagenesis (63%). We find positive and statistically significant relationships between distance to mainland and island area with percent endemism among mushrooms, and cladogenesis is positively and significantly correlated with island size and maximum elevation. Main conclusions Mushrooms show greater percent endemism on larger, more isolated islands/archipelagos. However, they display the lowest percent species derived from cladogenesis among the organismal groups examined. Both the lack of data and unreliability of data are impediments to studying fungal endemism on islands, making drawing decisive conclusions challenging.