History and evolution of the afroalpine flora: in the footsteps of Olov Hedberg

Abstract

The monumental work of Olov Hedberg provided deep insights into the spectacular and fragmented tropical alpine flora of the African sky islands. Here we review recent molecular and niche modelling studies and re-examine Hedberg’s hypotheses and conclusions. Colonisation started when mountain uplift established the harsh diurnal climate with nightly frosts, accelerated throughout the last 5 Myr (Plio-Pleistocene), and resulted in a flora rich in local endemics. Recruitment was dominated by long-distance dispersals (LDDs) from seasonally cold, remote areas, mainly in Eurasia. Colonisation was only rarely followed by substantial diversification. Instead, most of the larger genera and even species colonised the afroalpine habitat multiple times independently. Conspicuous parallel evolution occurred among mountains, e.g., of gigantism in Lobelia and Dendrosenecio and dwarf shrubs in Alchemilla. Although the alpine habitat was ~ 8 times larger and the treeline was ~ 1000 m lower than today during the Last Glacial Maximum, genetic data suggest that the flora was shaped by strong intermountain isolation interrupted by rare LDDs rather than ecological connectivity. The new evidence points to a much younger and more dynamic island scenario than envisioned by Hedberg: the afroalpine flora is unsaturated and fragile, it was repeatedly disrupted by the Pleistocene climate oscillations, and it harbours taxonomic and genetic diversity that is unique but severely depauperated by frequent bottlenecks and cycles of colonisation, extinction, and recolonisation. The level of intrapopulation genetic variation is alarmingly low, and many afroalpine species may be vulnerable to extinction because of climate warming and increasing human impact.

Plant evolutionary ecology in mountain regions in space and time

“…East African high mountains have long fascinated botanists as "sky islands" for alpine plants. Revisiting classical hypotheses concerning the Afroalpine flora in the light of recent findings from densely sampled molecular and niche modelling studies, Brochmann et al (2021) highlight the rather recent and only modest diversification of cold-adapted lineages that immigrated, often repeatedly, after the onset of harsh alpine conditions in that region.…”

The ghost of past climate acting on present‐day plant diversity: Lessons from a climate‐based delimitation of the tropical alpine ecosystem

“…The tropical alpine ecosystem is increasingly seen as an intriguing “natural laboratory” to address questions of rapid plant diversification and fast adaptation to extreme environments (McCormack et al, 2009; Tovar et al, 2022) due to its relatively young age, hotspots of plant diversity, and the often sky island‐like character of this ecosystem (Madriñán et al, 2013; Brochmann et al, 2021). Nevertheless, scientists have not yet agreed on a universal delimitation of what encompasses tropical alpine areas (Table 1).…”

“…While the high‐altitude area in the northern Andes is the largest and most continuous, in Africa and Hawaii the area is fragmented over scattered mountain peaks that are geographically and environmentally isolated by tropical lowland (Sklenář et al, 2014). TropalpHawaii and tropalpAsia are located on oceanic islands, while the Páramo and Afroalpine can be considered continental islands (“sky islands” in particular for the Afroalpine, but also partly for the Páramo [Madriñán et al, 2013; Brochmann et al, 2021]). The Páramo hosts by far the highest plant diversity of all four tropical alpine regions, with about 3026 species in 449 genera, while tropalpAsia comprises 1118 species in 226 genera, the Afroalpine comprises 521 species in 191 genera, and tropalpHawaii comprises 13 species in 10 genera (Gehrke, 2018).…”

The genomics and physiology of abiotic stressors associated with global elevation gradients inArabidopsis thaliana

“…Arabidopsis habitat in the Atlas Mountains in Morocco is similar climatically to Iberian mountains, but populations in Morocco and sub-Saharan Africa are older than most Eurasians with higher genetic diversity (Brennan et al, 2014; Durvasula et al, 2017; Fulgione and Hancock, 2018). Eastern African populations are among the highest recorded elevations (up to ∼4400 m) and though these represent unique environments for Arabidopsis (Brochmann et al, 2021) there have been no phenotypic studies.…”