Afromontane forest expansion and contraction
Eastern giant lobelias evolved in Afromontane forests at elevation ranges from 1000 to 2500m. Through frost tolerant adaptations, some taxa colonized the inhospitable Afroalpine elevations from 3000 to 5000m (Hedberg 1969). We found that middle elevations are the most likely habitat for L. columnaris . Only Pico Basilé’s (North Bioko) population extends its range from middle elevation up to >3000m. At this altitude, the habitat is composed of shrubs and subalpine meadows.
The paleoclimatic scenarios of the Afromontane vegetation during the Pleistocene in West Central Africa are not well known. However, it is known that climatic fluctuations changed the past vegetation patterns dramatically. With pollen analysis is possible to reconstruct historical processes (Kadu et al. 2011) like the presence of forest bridges that connected sky islands and highlands (Kebede et al. 2007) and, as a result, facilitated periodic expansions of the Afromontane forest through lowland elevations (Mairal et al. 2017). This dynamic allowed species to disperse and to be isolated in adjacent mountains (Zimkus & Gvoždík 2013).
Under this scenario, L. columnaris probably had changes in population size during the Pleistocene, mainly caused by repeated climatic oscillations and Afromontane fragmentation. Also, environmental fluctuations caused the constriction of populations into forest refugia. Later, populations expanded during warm and humid environmental alternations (Gao et al. 2015). Therefore, the current disjunction ofL. columnaris in sky islands probably was a Pleistocene product of retraction and isolation of Afromontane forest and continuous post-Pleistocene dispersal to empty niches.
Although not explored in the study, active volcanism and land use have to be considered in a holistic view of the current distribution of this giant lobelia.