An interpolated biogeographic framework for tropical Africa using plant species distributions and the physical environment

Aim: Existing phytogeographic frameworks for tropical Africa lack either
spatial completeness, unit definitions smaller than the regional scale, or
a quantitative approach. We investigate whether physical environmental
variables can be used to interpolate floristically defined vegetation
units, presenting an interpolated, hierarchical, quantitative
phytogeographic framework for tropical Africa, which is compared to
previously defined regions. Location: Tropical mainland Africa 24°N to
24°S. Taxon: 31,046 vascular plant species and infraspecific taxa.
Methods: We calculate a betasim dissimilarity matrix from a comprehensive
whole-flora database of plant species distributions. We investigate
environmental correlates of floristic turnover with local non-metric
multidimensional scaling. We derive a hierarchical biogeographic framework
by clustering the dissimilarity matrix. The framework is modelled using a
classification decision tree method and 12 physical environmental
variables to interpolate and downscale the framework across the study
region. Results: Floristic turnover is related strongly to water
availability and temperature, with smaller contributions from land cover,
topographic ruggedness and lithology. Region can be predicted with 90%
accuracy by the model. We define 19 regions and 99 districts. We find a
novel arrangement of the arid regions. Regional subdivision within the
savanna biome is supported with minor variation to borders. Within the
forests of west and central Africa, our whole-flora gridded
regionalisation supports the divisions identified by a previous analysis
of trees only. Main conclusions: Physical environmental variables can be
used to predict floristically defined vegetation units with very high
accuracy, and the approach could be pursued for other inc ompletely
sampled taxa and areas outside of tropical Africa. Geographic coherence is
higher than in previous quantitative phytoregional definitions. For most
tropical African vascular plant species, we provide predictions of which
species will occur within each mapped district and region of tropical
Africa. The framework should be useful for future studies in ecology,
evolution and conservation.