Woodiness and succulence of the Canary Islands flora

Aim: Oceanic islands possess unique floras with high proportions of endemic species. Island floras are expected to be severely affected by changing climatic conditions as species on islands have limited distribution ranges, small population sizes and face the constraints of insularity to track their climatic niches. We aimed to assess how ongoing climate change affects the range sizes of oceanic island plants, identifying species of particular conservation concern. Location: Canary Islands, Spain. Methods: We combined species occurrence data from single-island endemic, archipelago endemic and non-endemic native plant species of the Canary Islands with data on current and future climatic conditions. Bayesian Additive Regression Trees were used to assess the effect of climate change on species distributions; 71% (n = 502 species) of the native Canary Island species had models deemed good enough. To further assess how climate change affects plant functional strategies, we collected data on woodiness and succulence. Results: Single-island endemic species were projected to lose a greater proportion of their climatically suitable area (x̃ = ‑0.36) than archipelago endemics (x̃ = ‑0.28) or non-endemic native species (x̃ = ‑0.26), especially on Lanzarote and Fuerteventura, which are expected to experience less annual precipitation in the future. Moreover, herbaceous single-island endemics were projected to gain less and lose more climatically suitable area than insular woody single-island endemics. In contrast, we found that succulent single-island endemics and non-endemic natives gain more and lose less climatically suitable area. Main conclusions: While all native species are of conservation importance, we emphasise single-island endemic species not characterised by functional strategies associated with water use efficiency. Our results are particularly critical for other oceanic island floras that are not constituted by such a vast diversity of insular woody species as the Canary Islands.Occurrence data: We used occurrence data from the Banco de Datos de Biodiversidad de Canarias, an open-access database, for single-island endemic (SIE; n = 325), archipelago endemic (AE; n = 234) and definitely non-endemic native (NEN; n = 149) extant seed plant species (excluding subspecies), in a raster of 500 m x 500 m grid cells covering the Canary Islands (https://www.biodiversidadcanarias.es/biota/) [accessed 14/03/2022]. The database includes all species listed in the checklist of the Banco de Datos de Biodiversidad de Canarias, across 31,628 grid cell assemblages. Species range in occurrence from 1 to 4,466 cells. We only retrieved occurrences for which a species has been certainly observed or collected (precision level 1 of four levels). The Banco de Datos de Biodiversidad de Canarias provides presence-only information that is spatially biased by sampling effort (Hortal et al., 2007). However, the sampling bias of SIEs, AEs, and NENs is less than for species overall because studies incorporated into the database involved focus on, and extensive sampling of, endemic and non-endemic native species (https://www.biodiversidadcanarias.es/biota/documentos). We considered a species (pseudo-)absent if it was not recorded at a site, although we recognise that there is debate as to whether this truly represents absences. Functional strategies: We collected data on insular woodiness and succulence, which are relevant for species' responses to changing climatic conditions. As insular woodiness can be challenging to distinguish from non-insular woodiness (ancestral and derived woodiness) and herbaceousness, we mostly referred to literature sources from extensive studies on the woodiness of Canary Island plants (Hooft van Huysduynen et al., 2021; Lens, Davin, et al., 2013; Zizka et al., 2022). We defined plants as succulent if they displayed thickened leaves or fleshy stems. The thickness or fleshiness of plant organs indicates their ability to store water in their tissue (including moderately succulent species such as Rumex lunaria). We retrieved information on succulence from Muer et al. (2016) and taxonomic monographs, which have been shown to be reliable sources for trait data (Cutts et al., 2021)