Opposing elevational patterns in taxonomic, functional and phylogenetic diversity
We detected several similarities in the elevational patterns of taxonomic, functional and phylogenetic diversity of the summit’s plant communities. Especially for the richness-based metrics, a sharp decline towards higher elevations was detected. This trend is unsurprising given that elevation often acts as an environmental filter that limits species occurrence at high altitudes to species tolerant of the frequently stringent climatic conditions (Odland and Birks 1999, Körner 2004). Not only temperature but also water and nutrient availability decrease towards higher elevations, leading to reduced plant productivity and lower species richness (McCain 2007, McCain and Grytnes 2010). Thus, only species with specific traits that allow them to tolerate these harsh conditions can survive at higher elevation, thereby also reducing the trait range of plant communities (de Bello et al. 2013). This is evident in our results, and was also reported by Asplund et al. (2022) who observed a persistent decrease in functional trait diversity of vascular plants along a 500-m elevational gradient in southern Norway. The environmental filter associated with elevation will therefore favour the coexistence of species with similar traits that allow them to survive in the same habitat (Pavoine et al. 2014). Consequently, plant communities at higher elevations will be characterized by a more converged trait distribution, which could result directly from the increased climatic harness or indirectly from lower competition and increased positive interactions among species (Callaway et al. 2002).
Remarkably, however, phylogenetic diversity increased significantly with elevation. This implies lower phylogenetic relatedness among species in high- compared to low-elevation communities. Even though few species are found at higher elevations, they thus represent a comparatively broad range of the evolutionary tree, suggesting the aggregation of phylogenetic lineages in low-elevation plant communities (near the tree line) and overdispersion in high-elevation plant communities (near the limits of vascular plant life). This result is somewhat contrary to our expectations given that it challenges the environmental filtering hypothesis on species numbers and functional traits described above. Indeed, previous studies have reported an environmental filtering effect along elevational gradients leading to phylogenetic clustering and decreased phylogenetic diversity of mountain plant assemblages towards higher elevations (Bergamin et al. 2021, Galván-Cisneros et al. 2023). It is possible, however, that trait adaptation necessary to cope with the harsh environmental conditions at high elevations happened independently in distantly related lineages (see also Bryant et al. 2008). This type of convergent evolution has been observed in alpine plants, and has been used to explain their widespread adaptations to the stressful conditions at high elevations (e.g. dwarf stature, smaller leaves, high branch density and specialized morphology such as leafy bracts, wooly coverings and cushion forms; Trewavas 2014, Zhang et al. 2023). This conjecture is further supported by the fact that our highest summit is dominated by species with a wide elevational distribution range (e.g. Empetrum nigrum, Luzula arcuata, Salix herbacea, etc. occur across the entire elevational gradient). These species are able to tolerate a broad range of temperatures, and could have obtained the ability to tolerate cold temperatures independently throughout their evolutionary past. Alternatively, the observed increase in phylogenetic overdispersion could result from the shift of competition at low elevations towards facilitation among species in the high-elevation assemblages. Facilitation is an important driver of plant community assembly in high-alpine environments where conditions are physically stressful (Callaway et al. 2002), and has been shown to increase phylogenetic diversity (Valiente-Banuet and Verdú 2007, Butterfield et al. 2013, Graham et al. 2014).
Functionally and phylogenetically novel plant communities that can not be detected from trends in species richness alone
Looking across surveys, the summit’s plant communities revealed different temporal trends in taxonomic, functional and phylogenetic diversity metrics. For instance, taxonomic richness and diversity did not increase on the studied mountaintops between 2001–2022. This is in contrast with many other studies, reporting an increasing plant species richness on European mountaintops over the past decades because of climate warming (Walter et al. 2005, Holzinger et al. 2008, Pauli et al. 2012, Wipf et al. 2013, Steinbauer et al. 2018). However, stable species richness was also found by Hagenberg et al. (2022) over the last two decades in the mountains of northern Sweden. Mountain ranges at higher latitudes (e.g. Scandinavia) are still strongly influenced by recent glacial retreat since the last Ice Age, and hence the upward migration of many alpine plants can be delayed by post-glacial dispersal constraints and slow primary succession (that is, the Holocene migration lag; Dullinger et al. 2012b). On top of that, many alpine plants have very specific substrate requirements (Ellendberg and Leuschner 2010), and their upward expansion may be hampered by unsuitable bedrock types or rugged rocky terrain (as, for instance, found on the highest two summits). Nevertheless, several new species from lower elevations where found on the summits since the first survey in 2001 (e.g. Epilobium angustifolium, Deschampsia caespitosa, Geranium sylvaticum) but this colonization was at least partly counterbalanced by the loss of some native alpine specialists (e.g. Artemisia norvegica, Draba fladnizensis, Draba glabella).
Functional richness of the focal mountaintop communities, on the other hand, did increase over the 20-year monitoring period, indicating that niche space occupied by species in the communities enlarged over time (Mason et al. 2005). According to Thakur and Chawla (2019), mountaintop plant communities may exhibit higher functional richness over time with higher resource use and increased niche differentiation because the filtering effect of low temperature will become less important in determining their functional composition under climate change. The increase in functional richness was, however, most apparent on the lowest mountaintop, where the environmental filtering effect is already less important but interspecific competition is a strong driver of plant community assembly (Callaway et al. 2002). The latter can also promote niche differentiation because it allows species to coexist through different patterns of resource use, thereby preventing competitive exclusion (Kikvidze et al. 2005, Zepeda and Martorell 2019). Besides, the lowest summit also received the highest inflow of colonizers from the treeline ecotone. These newly arriving species are often generalists which typically take up a larger niche space compared to the alpine specialists (Slatyer et al. 2013), hence also promoting functional richness of the local plant assemblages.
In accordance with functional richness, phylogenetic richness also increased between 2001–2022 but the increase was uniform across the four summits. Most likely, the new colonizers have added more phylogenetic diversity to the summit’s plant communities than was lost with the local extirpation of native alpine species. This is possible through immigration of lowland clades that are not yet present on the summits, particularly if these clades are also distant relatives, increasing the local pool of lineages (Swenson et al. 2006). Meanwhile, the few alpine specialists that were lost from the summit’s communities likely belong to clades that are sister to other clades with lower extinction risk and still persist on the summits. Once the effect of species number was removed, no temporal trend in phylogenetic diversity of the summit’s assemblages could be found. This implies that, while the number of phylogenetic lineages increased over time owing to immigration of lowland species, phylogenetic distances among species in the mountaintop communities remained virtually unchanged. Over time, immigration thus brings new lineages into the communities through the introduction of phylogenetically distinct species, but this process does not seem to affect the communities’ overall evolutionary relatedness.
To conclude, elevation, and the associated gradients in climate, nutrient availability and biotic interactions, are important drivers of plant diversity in our study area, but the effect differed for different biodiversity metrics. Environmental filtering may thus act differently on mountain plant communities depending on the aspect of biodiversity considered. Moreover, taxonomic, functional and phylogenetic diversity metrics also revealed different temporal trends in our study. While a species richness trend was potentially masked by high species turnover, immigrating species increased the occupied niche space (higher functional richness) as well as the number of lineages (higher phylogenetic richness) within the summit’s communities. Our study thus highlights the importance of looking beyond taxonomic diversity and including functional and phylogenetic diversity approaches to better understand plant community assembly and responses to environmental change on mountaintops. Finally, our results emphasize that future environmental changes may give rise to novel plant communities on mountaintops with different functional and phylogenetic properties. It is possible that the observed widespread trends in species richness (e.g. Pauli et al. 2012, Steinbauer et al. 2018) mask more complex, and currently unrevealed, trends in functional and phylogenetic diversity. Long-term monitoring programs integrating multiple aspects of biodiversity will thus become increasingly important to assess the vulnerability of mountaintop flora under climate change.