Climatic niche lability but growth form conservatism in the African woody flora

Abstract

Climatic niche evolution during the diversification of tropical plants has received little attention in Africa. To address this, we characterised the climatic niche of >4000 tropical African woody species, distinguishing two broad bioclimatic groups (forest vs. savanna) and six subgroups. We quantified niche conservatism versus lability at the genus level and for higher clades, using a molecular phylogeny of >800 genera. Although niche stasis at speciation is prevalent, numerous clades individually cover vast climatic spaces suggesting a general ease in transcending ecological limits, especially across bioclimatic subgroups. The forest biome was the main source of diversity, providing many lineages to savanna, but reverse shifts also occurred. We identified clades that diversified in savanna after shifts from forest.The forest-savanna transition was not consistently associated with a growth form change, though we found evolutionarily labile clades whose presence in forest or savanna is associated respectively with climbing or shrubby species diversification.

Precipitation is the main axis of tropical phylogenetic turnover across space and time

“…Under these models, overly high PNC limits distributions resulting in high extinction; conversely, if niche evolution is too prevalent this results in smaller numbers of more widespread species ( 15, 34 ). We suggest that our observed 20:1 PNC to niche evolution ratio, i.e., repeated individual niche colonizations followed by in-situ diversification of niche-conserved and often species-rich clades, ultimately resulting in c. 3,400 species across the global tropics, provides a compelling empirical estimate of optimal PNC, corroborating theoretical model predictions ( 15, 34 ) and other empirical observations ( 31, 33 ). Thus, while potent examples of PNC or niche evolution can be derived from studies of small clades ( 30, 33, 35 ), PNC to niche evolution ratios need to be assessed across species-rich, ecologically-diverse clades to test the ubiquity of this 20:1 ratio.…”

“…1). Remarkably, our 5% estimate for niche shifts through time closely matches estimates from continental floras, such as 4% shifts in the Southern Hemisphere (33) and 7% in tropical Africa (31). Theoretical models of diversification suggest that intermediate levels of PNC and niche evolution result in the highest species richness (15,34).…”

“…Just how prevalent PNC is across the tree of life, and how it influences diversity patterns, are heavily debated ( 3, 6, 9, 32 ). An important factor is phylogenetic scale which clearly influences assessment of PNC ( 32 ), yet almost all estimates of PNC are from studies of small lineages or geographically confined floras ( 30, 31, 33 ). Here, using a large pantropical clade of c. 3,400 species, we show that the evolution and distribution of Mimosoids were profoundly shaped by PNC, demonstrating that conservatism does indeed structure larger, older clades that are ecologically diverse, even when their constituent lineages vary substantially in how niche-conserved they are (Fig.…”

“…The prevalence of PNC related to precipitation demonstrates that precipitation gradients strongly influence the evolution and distribution of diversity within continents ( 3, 13, 31 ), yet Mimosoids were able to overcome adaptive barriers to occupy all tropical lowland climates and diversify as ubiquitous components of dry and wet biomes (Figs. 1, 3) ( 18, 19, 28 ), showing considerable climatic niche evolution.…”

Evidence of the niche expansion of crofton weed following invasion in China

“…For instance, according to a large‐scale test across Eurasia, North America, and Australia, a great majority of 50 terrestrial IAPs occupied conserved climatic niches (Petitpierre et al, 2012). Similarly, niche conservatism is documented in African woody flora (Gorel et al, 2022). Our findings showed that the climate niche of A. adenophora has been conserved during its early invasion process in China.…”