Effects of climate change on the distribution of plant species and plant functional strategies on the Canary Islands

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 and 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.


| INTRODUC TI ON
Oceanic islands are nature's laboratories often having unique floras and faunas, owing to their ontogeny, remoteness and evolutionary potential (Whittaker et al., 2017;Whittaker & Fernández-Palacios, 2007). However, oceanic island biodiversity is considered to be disproportionately threatened by causes directly or indirectly related to human activities Macinnis-Ng et al., 2021;Tershy et al., 2015;Veron et al., 2019), particularly climate change .
To date, climate change research on land has mainly focussed on continents, even though changing temperature and precipitation patterns on oceanic islands will have particular relevance for island biota (Harter et al., 2015). Given the disproportionately large contribution of islands to global biodiversity Kier et al., 2009), the implications of climate change for oceanic island biodiversity are globally important.
The United Nations Intergovernmental Panel on Climate Change (IPCC) states that global surface temperatures will rise, leading to severe alterations in precipitation patterns in the 21st century (IPCC, 2021). These climatic changes could have severe impacts on oceanic island floras (Harter et al., 2015). Most island species can only retreat to potential refuge habitats within their island, or neighbouring islands, if they are within reach (Gillespie et al., 2008). In addition, oceanic islands are restricted in area, which limits the range size of endemic species. A limited range is often associated with small population sizes and higher vulnerability of species to environmental changes, natural hazards and demographic stochasticity (Barton & Fortunel, 2023;Lande, 1993). Many island endemic species are already under pressure from habitat loss, intensification of land use and the introduction of invasive alien species. Consequently, many island species are listed as threatened on the IUCN Red List (www. iucnr edlist.org; Romeiras et al., 2016). Furthermore, according to 21st century climate change scenarios, ongoing climate change will exacerbate the threat levels for island plants (Fortini et al., 2013;Gillespie et al., 2008). However, climatic alterations and their consequences on an entire archipelago's diversity and floristic composition (e.g. species richness, endemism and functional strategies) have not yet been assessed.
Under changing climatic conditions, species populations need to track their climatic niche (Chen et al., 2011;Lenoir & Svenning, 2015) or adapt to the novel climatic conditions for survival (Bradshaw & Holzapfel, 2006;Hoffmann & Sgrò, 2011). The tracking of climatic niches requires that climatically suitable habitat is still available within the dispersal range of a species (Zanatta et al., 2020;Zurell et al., 2016). However, many island species are bound to their islands and have narrow climatic niches (Fernández-Palacios, Otto, et al., 2021), which can increase the risk of extinction under changing environmental conditions (Fortini et al., 2013;Thuiller et al., 2005). For example, species assemblages in high-elevation areas on oceanic islands are disproportionately rich in endemic species (Steinbauer et al., 2016) and alpine plants on islands have been found to be particularly vulnerable to both changing precipitation patterns (Marrero-Gómez et al., 2007;Sperling et al., 2004) and Editor: Marta Carboni 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 (x = −0.36) than archipelago endemics (x = −0.28) or nonendemic native species (x = −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. By contrast, we found that succulent single-island endemics and nonendemic 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.

K E Y W O R D S
climate change, climatic niche, endemism, functional strategies, oceanic island flora, potential habitat, range shift increasing temperatures (Giambelluca et al., 2008). Moreover, species occurring in arid areas may be particularly threatened, despite water conservation strategies and drought adaptations, because arid areas may experience greater aridification than humid areas (Huang et al., 2017), and aridification can have critical effects on biodiversity (Cartereau et al., 2023;Shi et al., 2021). In addition, islands have a higher proportion of keystone species than mainland regions, and their shift in space or possible extinction may dramatically affect entire ecosystems (Olano et al., 2017). Hence, understanding the effects of climate change on the potential distribution of island plants is vital for informing conservation efforts for endemic and native floras.
Insular woodiness is a key syndrome in island endemic plants (Burns, 2019;Carlquist, 1974;Lens, Davin, et al., 2013) and describes the evolutionary transition from herbaceous to woody species on islands. There are several hypotheses regarding the origin of insular woodiness, but one particularly well-supported hypothesis suggests that insular woodiness may be induced by drought stress, which requires better protection of root-to-shoot water transport against hydraulic dysfunction (Dória et al., 2018;Hooft van Huysduynen et al., 2021;Lens, Tixier, et al., 2013;Zizka et al., 2022). Therefore, insular woody species may be better protected from increasing drought frequencies under future climatic conditions, giving them a more prominent role than herbaceous and noninsular woody species in island floras. However, owing to dispersal limitations, long generation times and longevity of many woody plant species, a time lag in the response of woody plants to climate change is expected (Kissling et al., 2010). Therefore, it is unclear whether insular woody species have advantages or disadvantages under future climatic conditions. Succulent plants are drought tolerant because they store water to sustain their metabolism when hygric stress occurs (Griffiths & Males, 2017). Additionally, succulence is accompanied by a crassulacean acid metabolism (CAM) photosynthetic pathway in some clades, leading to higher water use efficiency due to a shift in CO 2 fixation from day to night (Griffiths & Males, 2017). Hence, succulent species could have an advantage over nonsucculent species if hot and dry climatic conditions increase because of anthropogenic climate change. However, previous studies have been ambiguous regarding the resilience of succulent species to climate change; they have shown a high susceptibility of succulents to drought intensity (Midgley & Thuiller, 2007;Young et al., 2016), no effect (Thuiller et al., 2006;Schmiedel et al., 2012) or a lesser impact than for nonsucculent species (Hoffman et al., 2009). Nonetheless, as arid environments are predicted to expand due to ongoing climate change (Seneviratne et al., 2012;Zscheischler et al., 2018), succulent species may be able to track their climatic niche, while nonsucculent species may lose climatically suitable habitat. In any case, the role of succulence in the response of oceanic island floras to changing climatic conditions is yet to be assessed.
In this study, we aim to assess how much potentially climatically suitable area will be lost or gained for endemic and nonendemic native seed-plant species of the Canary Islands, and their associated plant functional strategies, under different climate change scenarios. We divided the endemic species group into single-island endemics and archipelago endemics (see Hanz et al., 2022) to test the following three hypotheses: (1) Single-island endemics are the most susceptible to changing climatic conditions, as they may have narrower climatic niches and smaller range sizes than other species and limited potential for range shifts.
(2) High-elevation and arid areas have greater loss of potentially climatically suitable area than lower-elevation and humid areas across all floristic groups. The loss of climatically suitable area might be driven by difficulties for species to track their climatic requirements, particularly under distinct environmental pressures. (3) Herbaceous, noninsular woody and nonsucculent species will experience greater climate change-related reductions in potentially climatically suitable areas than insular woody and succulent species. This is because we expect increases in temperature and decreases in precipitation across the Canary Islands-conditions that are less favourable for these plant functional strategies.

| Plant occurrence data
We used occurrence data from the Banco de Datos de 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 nonendemic native species (https://www.biodi versi dadca narias.es/biota/ docum entos).
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.
However, we acknowledge that range-restricted species are often particularly threatened by climate change (Ohlemüller et al., 2008) and that our models are, to some extent, biased against smallranged SIEs and nonsucculent herbs within AEs and NENs (see Appendix S1). Furthermore, we restricted the frequent species to 500 random occurrences to avoid sampling bias (seven AEs and six NENs). We excluded the frequently cultivated Phoenix canariensis from the dataset because its occurrence was overrepresented in the database (n = 4446 occurrences) and the current species distribution does not reflect its climatic niche. A list of the number of occurrences of all species is provided in the Appendix S2.

| Plant 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 noninsular 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 of trait data (Cutts et al., 2021).

| Climate data
We implemented species distribution models focussing on 19 climatic variables with potentially direct or indirect impact on species occurrences (Xu & Hutchinson, 2013). Climate data were retrieved from Patiño et al. (2023), who generated bioclimatic variables based on a bias-corrected downscaling from 30-arc-second to 100 m resolution of climatological data (1979-2013) from CHELSA V1.2 (Karger et al., 2017(Karger et al., , 2018, using observations from meteorological stations . Specifically, these data comprise mean, maximum and minimum daily near-surface air temperatures and precipitation. Bias correction was applied to the 30-arc-second resolution and the subsequent downscaling was achieved by applying an atmospheric lapse rate correction following the approach described in Karger et al. (2017). High-resolution climate data for the future (2071-2100) were generated by a Delta change anomaly interpolation . This computed and downscaled the anomalies between present and future monthly climatic maps at 30-arc-second resolution, resulting from a downscaling of Global Circulation Models (GCMs) from the 6th phase of the Climate Model Intercomparison Project (CMIP6) using the CHELSA CMIP6 module (https://gitla bext.wsl.ch/karge r/chelsa_cmip6; Karger et al., 2023).
The anomalies were then downscaled using B-spline interpolation to 30-arc-seconds and applied to present maps at 100 m .
We used three Shared Socioeconomic Pathways (SSPs) to represent a wide range of future socioeconomic conditions: from sustainable development and equality (SSP1 or 'sustainability'), a world of resurgent nationalism (SSP3 or 'regional rivalry'), and rapid and unconstrained growth in economics and energy use (SSP5 or 'fossil- Intercomparison Project (ISIMIP) were considered for this study (Lange, 2019;Lange & Büchner, 2021). All the climatic maps were then aggregated for this study to a resolution of 500 m to match the occurrence data resolution in R, using the 'raster' package (Hijmans, 2019). Hence, we analysed an ensemble of 15 different future potential distributions of spermatophytes native to the Canary Islands for 2071-2100. Differences between projected future (ensemble means from five GCMs under SSP3) and current MAT and annual precipitation are mapped in Figure 1b,c (see also Appendix S1 for mean differences per island).

| Modelling
We used Bayesian additive regression trees (BARTs) implemented with the R package 'embarcadero' (Carlson, 2020) to model the current and future distribution of plant species. BART is a method defined by a prior probability distribution and the likelihood of returning occurrence predictions that quantify the uncertainty around the projections (Carlson, 2020). BARTs have proven to be statistically powerful, excellent in performance and robust to changes in parameter choices (Baquero et al., 2021;Carlson et al., 2022;Dansereau et al., 2022;Pinto-Ledezma & Cavender-Bares, 2021).
Before modelling a species' distribution, we randomly sampled pseudo-absences across the study area. We tested the same number of pseudo-absences as unique presences (n pseudo-absences = n occurrences), avoiding spatial overlap with the presence data (Descombes et al., 2022). To identify the main subset of predictors, we ran an automated variable selection implemented in the R package 'embarcadero', following the recommendations of Chipman et al. (2012). The variables with the lowest average model root mean square error (RMSE) and, therefore, the highest accuracy, were selected (Carlson et al., 2022). The BART algorithm is insensitive to multicollinearity and can simultaneously model several predictors (Chipman et al., 2012). We ran final models separately for each species with the reduced variable set using default BART model settings To evaluate our models, we first fitted our models to 10 random subsets of 70% of the data and validated them against the remaining 30% of data. As we had many species with few occurrences in our dataset, which can lead to imperfect performance measurements (Collart & Guisan, 2023), we then pooled the suitability values of the hold-out data across replicates (Collart et al., 2021) to compute the Area Under the receiver operating characteristic Curve (AUC) and the Boyce index, which is used for presence-only data (Hirzel et al., 2006), using the R package 'ecospat' (Broennimann et al., 2022;Di Cola et al., 2017). The final variable sets, AUC and Boyce index for each species are provided in the Appendix S2.  probabilities of occurrences were averaged across GCMs and for each SSP scenario, resulting in three different climate change scenarios. Subsequently, we converted the projected probabilities of occurrence for the current and future distributions into a binary outcome according to the threshold that maximises the True Skill Statistic (maxTSS) for each species (Allouche et al., 2006;Liu et al., 2013).
Species for which models performed poorly (AUC < 0.7, Boyce < 0.4 or maxTSS < 0.4) were not included in the analyses (i.e. 26 SIEs, 15 AEs, 11 NENs), which left a total of 502 species. After the exclusion of species for which models performed poorly, the quality of models ranged from an AUC of 1.00 to 0.72 (mean 0.94 ± 0.05) and Boyce index of 0.4 to 1.00 (mean 0.82 ± 0.17), indicating an overall good model performance (Hirzel et al., 2006;Lantz, 2019; see Appendix S1). The ODMAP protocol of our analysis (Zurell et al., 2020) and a methodological flowchart are available in the Appendix S3.

| Statistical analyses
We quantified the gain and loss in a suitable climatic area between the current and future periods by summing the binarised numbers of gained and lost presences for each species, respectively. We then divided the gains and losses by the total number of occupied grid cells to obtain proportional gains and losses per species. We performed Kruskal-Wallis tests to test whether the relative difference in the area of potential climatic niche was significantly different between and within floristic groups. We further performed Kruskal-Wallis and Mann-Whitney U-tests to analyse whether the change in potentially climatically suitable area differed between herbaceous, noninsular woody and insular woody species, and between nonsucculent and succulent species within each floristic group. If necessary, post-hoc testing was performed using a Dunnett's test with Bonferroni adjustment for multiple comparisons. We repeated the analysis for each of the three SSP scenarios. All analyses were performed in R 4.1.1 (R Core Team, 2021).

| Change in potentially climatically suitable area between and within floristic groups
Overall, we found significantly smaller gains and greater losses in potentially climatically suitable area for SIEs than for AEs under cli-

| Inter-island comparison of change in climatically suitable area
The median proportional gain and loss in potentially climatically suit-

| DISCUSS ION
In this study, we investigated the effect of climate change on the potentially climatically suitable area of plant species native to the Canary Islands, an archipelago renowned for its exceptional plant endemism (Cai et al., 2022;Fernández-Palacios & Whittaker, 2008).
As hypothesised, we found that single-island endemic species that currently occur in predominantly arid regions would have the highest F I G U R E 2 Proportional gain and loss in climatically suitable area by 2100 in single-island endemic (n = 202), archipelago endemic (n = 194) and nonendemic native plant species (n = 106) on the Canary Islands, using three different climate change scenarios (SSP1, SSP3 and SSP5). SSP1 describes a world with strong economic growth via sustainability, SSP3 describes a future with high inequality between countries and SSP5 describes a world with strong economic growth via fossil fuel pathways. Single-island endemics have significantly lower gains and higher losses of potentially climatically suitable area than archipelago endemics under SSP1. Moreover, single-island endemics have significantly lower gains of climatically suitable area than nonendemic natives under climate change scenarios SSP1 and SSP5. Asterisks denote statistical significance (*p ≤ .05, **p ≤ .01, ***p ≤ .001).   (Andrew et al., 2022;Butt & Gallagher, 2018;Dudley et al., 2019). Indeed, our models suggest that species characterised by woodiness or succulence will be less negatively affected by climate change. Nevertheless, climate change is a substantial threat to most plant species native to the Canary Islands, and species vulnerable to environmental and demographic stochasticity, or species characterised by specific functional strategies, are exceedingly threatened.

| Single-island endemics are most susceptible to changing climatic conditions
We found that single-island endemic species are more vulnerable than nonendemic native species under a mild climate change scenario on the Canary Islands, possibly because of their small climatic niche. The loss in climatically suitable area indicates that species on islands will have limited opportunities to escape unfavourable climatic conditions and may be unable to track their climatic niches (Harter et al., 2015), which is further aggravated by the species' inherently small ranges and population sizes. The climate is predicted to become warmer and drier on the Canary Islands by 2100, under all three analysed climatic scenarios. In particular, precipitation seems to play an essential role for native Canary Island species, as the five most critical climatic variables across all species were related to precipitation rather than temperature. In general, the increase in temperature and decrease in precipitation were more severe under scenarios SSP3 and SSP5 than under SSP1. This indicates that a socioeconomic pathway favouring sustainability and equality might prevent many species from oceanic island floras from losing their climatic niche.

| Arid and high-elevation areas are disproportionately affected by climate change
The inter-island comparison indicates that endemic species on older and less elevated islands, that is, Lanzarote and Fuerteventura, will F I G U R E 6 Proportional gain and loss in potentially climatically suitable area (SSP3) for single-island endemic (n = 202), archipelago endemic (n = 194) and nonendemic native species (n = 106) on the Canary Islands when accounting for different functional strategies. (a) Herbaceous species have significantly lower gains and higher losses of climatically suitable area than insular woody species in single-island endemics. (b) Nonsucculent species have significantly lower gains and higher losses of climatically suitable area than succulent species in single-island endemics and nonendemic natives. Asterisks denote statistical significance (*p ≤ .05, **p ≤ .01, ***p ≤ .001). experience an above-average loss of climatically suitable area. These two islands already have water scarcity and predominantly arid climatic conditions (del Arco Aguilar et al., 2010), in which many species already experience physiological limitations due to drought stress. Species occurring on the topographically complex Famara cliff (Lanzarote) or Jandía massif (Fuerteventura) are predicted to be especially vulnerable. For example, the archipelago endemic Ferula lancerotensis occurs on these two hills and is predicted to lose almost its entire suitable climatic niche on the Canary Islands by 2100. Ferula lancerotensis depends on moist conditions and lower insolation on the windward slopes of the Famara and Jandía hills (Scholz & Reyes-Betancort, 2013). However, temperature is predicted to increase, whereas precipitation is predicted to decrease in these areas, likely making the climatic conditions unsuitable for many native species in the future.
Not only are species from the inframediterranean zone disproportionately affected by climate change; but species that occur in the supra-and oromediterranean zone (>2.000 m) may also be particularly vulnerable to changing climatic conditions, for example, Viola cheiranthifolia (Tenerife endemic) and Echium gentianoides (La Palma endemic). By contrast, species that currently occur across mid-elevations, for example, the lower Teno massif (peak at 1.345 m), may be able to increase their range size. For example, the potentially climatically suitable area for Sideritis cretica is projected to increase disproportionately in the future. These results indicate that upward range shifts might compensate for lost habitats where possible, but an upward shift to the highest elevational areas (e.g. Teide or Roque de los Muchachos) might not be possible. Disproportionate temperature increases at high elevations (Expósito et al., 2015;Krushelnycky et al., 2016;Sperling et al., 2004), water stress and area reduction with elevation can restrict upslope migration, making high-elevation species highly vulnerable to climate change (Costion et al., 2015;Dullinger et al., 2012;Rumpf et al., 2018;Steinbauer et al., 2018). Although floristic groups across all islands might lose climatically suitable area on average, we identified the Famara and Jandía hills, the summit broom scrub of Tenerife and La Palma and the Teide violet community (Tenerife) as the regions in which plant species are most vulnerable to climate change in the 21st century.

| Insular woody and succulent species face lower climate change-related reductions
We found that insular woody species may gain more and lose less climatically suitable area than herbaceous species, among the single-island endemics. This finding can be linked to the theory that palaeodrought is a major driver of insular woodiness in Canary Island lineages (Hooft van Huysduynen et al., 2021;Lens, Davin, et al., 2013;Zizka et al., 2022). As there is evidence that in-situ wood development coincides with palaeoclimatic aridification (Hooft van Huysduynen et al., 2021), insular woody endemic species may be better adapted to drought than perennial herbaceous endemic species.
Therefore, adaptation to stable climates and long generation times could pose a possible conflict with rapid climate change-induced range shifts in insular woody species.
Our results suggest that succulent species are more likely to gain climatically suitable area by 2100 than nonsucculent species, among single-island endemics and nonendemic natives. Succulent plants may have an advantage over nonsucculent plants under more arid conditions because of their ability to store water and their water-efficient metabolism (Griffiths & Males, 2017;Vendramini et al., 2002). For example, Euphorbia canariensis, a keystone species in the succulent scrub, is predicted to increase its suitable climatic area by 78% under climate change scenario SSP3. Hence, succulence seems to be an effective strategy in the face of climate change, especially in native plants with preadaptations to arid conditions (e.g.

| Study limitations
Although our models provide robust predictions for the change in potentially climatically suitable area of species native to the Canary Islands, we must address the fact that our models did not include biotic factors, such as species interactions or dispersal. In particular, interspecific competition with succulent invasive species, such as Opuntia and Agave, may be favoured and accelerated by climate change (Arévalo et al., 2017). In addition to habitat destruction, grazing by introduced herbivores poses a massive threat to many native species on the Canary Islands. A poignant example is the Jandía peninsula, Fuerteventura, where feral cattle are found in high numbers most of the year (Scholz & Reyes-Betancort, 2013).
Moreover, in high-elevation areas, climate change-induced high rabbit densities are already threatening the persistence of native plant species (Cubas et al., 2018), as well as in most other ecosystems of the archipelago (Cubas et al., 2019). Therefore, additional threats, such as invasive species, habitat loss and resource overexploitation Morente-López et al., 2023), strongly influence whether species can shift their range to climatically suitable areas in the future. Additionally, we acknowledge that we analysed the native species' realised niches and not their fundamental niches. Hence, our results may underestimate the climatic niches of these species.

| CON CLUS ION
Although climate change is recognised as a growing threat to the outstanding biodiversity of oceanic islands (Harter et al., 2015;Macinnis-Ng et al., 2021;Veron et al., 2019), the impact of climate change on the functional strategies of island floras is less clear. We found that endemic species in aridity-dominated environments are particularly threatened by future decreases in precipitation. However, insular woody and succulent species may have an advantage in a climate that is warmer and drier than today on the Canary Islands.
Indeed, the Canary Islands are a hotspot of insular woody species and succulent plants ( Barajas-Barbosa et al., 2022;Irl et al., 2020;Zizka et al., 2022), which suggests that a large proportion of the Canary Islands flora could be able to cope with the predicted climatic changes. Nonetheless, with ongoing climate change, a net loss of species with unique functions seems inevitable, leading to functional homogenisation and impoverishment, and a possible deterioration of ecosystem stability.

ACK N O WLE D G E M ENTS
We thank Frederic Lens for his help in identifying insular, ances-

CO N FLI C T O F I NTE R E S T S TATE M E NT
The authors declare no conflict of interest.

PEER R E V I E W
The peer review history for this article is available at https:// www.webof scien ce.com/api/gatew ay/wos/peer-revie w/10.1111/ ddi.13750.

DATA AVA I L A B I L I T Y S TAT E M E N T
Data that support the findings of this study can be found in the Dryad data repository at https://doi.org/10.5061/dryad.cc2fq z6b4.