The composition, geography, biology and assembly of the coastal flora of the Cape Floristic Region

Abstract

The Cape Floristic Region (CFR) is globally recognized as a hotspot of plant diversity and endemism. Much of this diversity stems from radiations associated with infertile acid sands derived from sandstones of the geologically ancient Cape Fold Belt. These ancient montane floras acted as the source for most subsequent radiations on the Cape lowlands during the Oligocene (on silcretes) and Mio–Pliocene (on shales). The geomorphic evolution of the CFR during the Plio–Pleistocene led to the first large-scale occurrence of calcareous substrata (coastal dunes and calcarenites) along the Cape coast, providing novel habitats for plant colonization and ensuing evolution of the Cape coastal flora—the most recent diversification event in the Cape. Few studies have investigated the CFR’s dune and calcarenite floras, and fewer still have done so in an evolutionary context. Here, we present a unified flora of these coastal calcareous habitats of the CFR and analyze the taxonomic, biological and geographical traits of its component species to gain insights into its assembly. The Cape coastal flora, comprising 1,365 species, is taxonomically dominated by the Asteraceae, Fabaceae and Iridaceae, with Erica, Aspalathus and Agathosma being the most speciose genera. In terms of growth-form mix, there is a roughly equal split between herbaceous and woody species, the former dominated by geophytes and forbs, the latter by dwarf and low shrubs. Species associated with the Fynbos biome constitute the bulk of the flora, while the Subtropical Thicket and Wetland biomes also house a substantial number of species. The Cape coastal flora is a distinctly southern African assemblage, with 61% of species belonging to southern African lineages (including 35% of species with Cape affinity) and 59% being endemic to the CFR. Unique among floras from the Cape and coastal Mediterranean-climate regions is the relatively high proportion of species associated with tropical lineages, several of which are restricted to calcareous substrata of the CFR. The endemic, calcicolous component of the flora, constituting 40% of species, represents 6% of the Cape’s regional plant diversity—high tallies compared to other biodiversity hotspots. Most coastal-flora endemics emerged during the Plio–Pleistocene as a product of ecological speciation upon the colonization of calcareous substrata, with the calcifugous fynbos floras of montane acid substrata being the most significant source of this diversification, especially on the typically shallow soils of calcarenite landscapes. On the other hand, renosterveld floras, associated with edaphically benign soils that are widespread on the CFR lowlands, have not been a major source of lineages to the coastal flora. Our findings suggest that, over and above the strong pH gradient that exists on calcareous substrata, soil depth and texture may act as important edaphic filters to incorporating lineages from floras on juxtaposed substrata in the CFR.

Late Cenozoic environmental changes drove the diversification of a weevil genus endemic to the Cape Floristic Region

“…Sea level oscillations similarly impacted the diversification of several CFR lineages, such as coastal legless skinks for which cladogenesis also occurred between the late Pliocene and the Pleistocene (Engelbrecht et al, 2013). In plants, the role of sea level oscillations in the diversification of coastal lineages is particularly important, especially for the dune floras (Grobler et al, 2020), or for the species associated with calcareous substrates (Grobler & Cowling, 2021; Hoffmann et al, 2015). The fact that the inland group only consists of one species ( P. xerophilus ) tends to support the hypothesis that allopatric speciation likely did not play as a significant a role inland; a possible explanation for the latter is that the landscape was stable since the late Pliocene ( ca.…”

“…Most studies carried out on the origin of the CFR biodiversity have focused on plants, particularly on the richest families found in the CFR ( i.e. , Asteraceae, Ericaceae, Fabaceae, Iridaceae, Proteaceae, Restionaceae; Linder, 2003; Hoffmann et al, 2015; Pirie et al, 2016; Grobler & Cowling, 2021). During the late Miocene and the Pliocene, rapid climate changes associated with the availability of new habitats resulting from geomorphic uplifts were likely instrumental in spurring the radiation of numerous plant lineages (Cowling et al, 2009; Richardson et al, 2001).…”

“…Fynbos is by far the dominant vegetation type in the CFR (Allsopp et al, 2014); it is a sclerophyllous, fire‐prone shrubland characterized by specific plant assemblages dominated by Ericaceae, Proteaceae and Restionaceae (Cowling & Procheş, 2005). In the renosterveld, members of these three families are mainly absent, and this biome is mostly dominated by plants from the Asteraceae, Fabaceae, Iridaceae and Poaceae families (Grobler & Cowling, 2021); it is also characterized by the extraordinary diversity of geophytes (Cowling, 1990; Procheş et al, 2006).…”

“…The high levels of environmental and topographical heterogeneity of the CFR (Cowling & Lombard, 2002; Linder, 2005; Procheş et al, 2009; van Santen & Linder, 2020; Verboom et al, 2015) effectively translate into diverse ecological niches that favoured the evolution of numerous endemic plant lineages with narrow ranges and poor dispersal abilities (Dynesius & Jansson, 2000; Linder, 2003). The oldest plant radiations likely occurred in mountain ranges whereas most recent radiations were hypothesized to have occurred more recently during the Pleistocene (Grobler & Cowling, 2021; Linder, 2008; Linder & Hardy, 2004; Verboom et al, 2009). Some authors also suggested that speciation processes in the CFR differ among lowland and montane plant lineages, with lowland taxa being more prone to ecological speciation whereas montane taxa are more subject to geographic (allopatric) speciation (Verboom et al, 2015).…”

Canopy plant composition and structure of Cape subtropical dune thicket are predicted by the levels of fire exposure

“…This is also true of other systems such as forest-savanna ( Higgins et al, 2007 ; Smit et al, 2010 ; Ryan, 2002 ) and forest-fynbos mosaics ( Kruger & Bigalke, 1984 ; van Wilgen, Higgins & Bellstedt, 1990 ). At high fire exposure, thicket species comprise a diverse array of hedge formers characterized by many relatively thin and low shoots arising from robust underground stems ( Cowling, 1983 ; Grobler & Cowling, 2021 ; Fig. S8 ).…”