Dense branching and spines are common features of plant species in ecosystems with high mammalian herbivory pressure. Combining dense branching with spines probably produces the most effective herbivore defences, as dense branching translates directly into higher spine densities. However, how cage architecture evolved under mammalian pressure has remained unexplored.
In a study published in Nature Plants, researchers from Xishuangbanna Tropical Botanical Garden (XTBG) and their collaborators analyzed how two traits, branching density and spine presence, evolved through deep time and in response to herbivory regimes in Combretaceae, a globally widespread plant family. Employing a phylogenetic comparative approach, they examined the emergence of dense branches and spines as a two-step evolutionary process for defensive cage structures in mammalian-driven ecosystems.
 The researchers firstly identified species developing in areas with high herbivory pressure on the basis of the richness of large browsing herbivores and pre-Anthropocene herbivore biomass. They then described branching density and spine presence in 132 Combretaceae species, and compared how the traits were expressed across herbivory regimes and biomes.
Afterwards, the researchers estimated ancestral herbivory regimes and the timing of trait acquisitions through biogeographical inference, identifying potential congruent events. Lastly, they used State-dependent Speciation and Extinction models to evaluate how dense architectures and spines influenced species evolution rates under different herbivory pressures.
The results revealed that modern plant architectural strategies evolved through a step-wise process. First, dense branches emerged under moderate herbivory pressure and then spines supporting higher species formation rates were acquired under high herbivory pressure.
"We can say that the combination of dense branches and spines forms a strong defensive cage, which is essential for resistance to herbivory by mammals,” said Kyle W. Tomlinson of XTBG.
The study highlighted the adaptive value of dense branches as part of an herbivore defense strategy and confirmed that large mammalian herbivory was the main selective force shaping the overall plant architecture of woody plants.

Contact
Kyle W. Tomlinson Ph.D Principal Investigator
Center for Integrative Conservation, Xishuangbanna Tropical Botanical Garden, Chinese Academy of Sciences, Menglun, Mengla, Yunnan 666303, China
E-mail:  kyle.tomlinson@xtbg.org.cn
First published: 04 March 2024