Weak latitudinal trends in reproductive traits of Afromontane forest trees

Abstract

Background and Aims Is the increase in species diversity patterns towards lower latitudes, linked to reproductive traits? Plant reproductive organs influence reproductive isolation and hence species divergence. Abiotic differences between temperate and tropical regions can also directly impact on plant reproductive traits. Here we provide a novel overview of southern hemisphere, Afromontane forest tree taxonomical patterns and ask whether reproductive traits relate to latitude, while accounting for environmental (tree height) and evolutionary (biogeographical affinity) selective forces. Methods We compiled a novel dataset with a) flower colour, size and pollination syndrome and b) fruit colour, size and dispersal syndrome for 331 tree species found in six Afromontane forest regions. We categorised each species into latitudinal distribution using these six regions, spanning the southern Cape (34º S) to Mount Kenya (0º S). Additionally, we gathered maximum tree height (m) for each species and determined the global distribution of all 196 tree genera (Afrotropical, Palaeotropical or Pantropical). Key Results Species, genera and families showed a general decrease in richness away from tropical and subtropical forests towards warm temperate forests. Southern Afrotemperate forests (the furthest south) had the highest tree endemism. There was no relationship between latitude and the reproductive traits tested here. Biogeographical affinity related to fruit colour and dispersal syndrome, with palaeotropical genera showing relative increases in black-purple fruit colour compared to pantropical genera and palaeotropical genera showing relative increases in biotic seed dispersal compared to Afrotropical genera, which showed higher relative abiotic seed dispersal. Taller trees had a higher chance to be wind or insect pollinated (compared to bird pollinated) and had larger fruits. Conclusions Latitude explained patterns in Afromontane tree taxonomical diversity, however tree reproductive traits did not relate to latitude. We suggest that phylogenetic conservatism or convergence, or both, explain the reported patterns.

Generalist southern African temperate forest canopy tree species have distinct pollinator communities partially predicted by floral traits

“…In a New Caledonian rainforest, the dominant flower type is also small and relatively simple, mostly pale, or white in colour, indicating unspecialized flowers (Carpenter et al, 2003). Flower morphology of Australian, New Zealand and New Caledonian forests seem to bear noticeable resemblance to those of South Africa, including forests along the Afromontane archipelago (Griffiths & Lawes, 2006;Swart et al, 2023). These pollination syndromes are in fact very diverse in their advertisement and reward strategies.…”

“…It has been hypothesized that a driving force behind the present richness and floral diversification of the southern African flora is adaptation to pollinators (Johnson, 1996). African montane forest trees at first glance seems an exception to this floral diversification, being dominated by small, whitish and open flowers (Swart et al, 2023). Part of the broader Afromontane archipelago forests, southern African temperate forests are dominated by evergreen trees with little apparent flower morphological disparity.…”