Savanna trees do not have thicker outer bark than forest trees of two tropical species

Abstract

Although the outer bark is important for protecting the inner tissues of stems of woody plants, especially from fire, the variation of outer bark in environments with different water availability, wind intensities, and solar radiation is little understood. Here, we examined the outer bark structure of Moquiniastrum polymorphum (Asteraceae) and Zanthoxylum rhoifolium (Rutaceae), co-occurring species in cerrado sensu stricto, cerradão, semi-deciduous seasonal forest and rainforest. Cerrado sensu stricto is a savanna with relatively intense solar radiation and winds. Cerradão, semi-deciduous seasonal forest and rainforest are progressively more closed and humid. We tested if trees in cerrado sensu stricto have relatively thicker outer and total bark than trees in the other three environments. For individuals of each species from each habitat, we described the morphological and anatomical features of the outer bark, including the origin of the first periderm. We compared the relative thickness of outer and total bark among habitats by means of ANOVA. In both species, we observed no qualitative differences in the morphological and anatomical features of the outer bark; therefore, these features are diagnostic for each species. For all habitats, the deep-seated origin of the first periderm and formation of a rhytidome in M. polymorphum produce a grooved bark with a loose appearance, whereas the subepidermal origin and formation of a single periderm in Z. rhoifolium produce a more uniform and smooth-appearing bark. Zanthoxylum rhoifolium trees in cerrado sensu stricto and cerradão have a thinner periderm, whereas M. polymorphum trees do not vary in rhytidome thickness among the habitats. In summary, trees from cerrado sensu stricto did not have relatively thicker outer bark and total bark than trees in cerradão, semi-deciduous seasonal forest and rainforest, suggesting that seasonal drought, intense solar radiation and winds do not seem to affect the total thickness of the outer bark.