Changes in secondary xylem traits paralleled the evolution of allocation to bark in savanna species

Abstract

The evolution of thick-barked tropical savanna woody species from ancient forest lineages remains poorly understood. We compiled trait data for 91 Neotropical savanna, forest and generalist species, including bark, size, and xylem traits to test the hypothesis that bark production involved parallel evolution of morphologic- and anatomic- level traits. We used a Principal Component Analysis and pairwise correlations to understand trait covariation, and contrasted individual traits and strategy axes among habitat groups. We also tested for phylogenetic conservatism in strategy axes and individual traits. We found three leading axes of trait strategies. The first expressed a trade-off between fire resistance (allocation to bark and higher hydraulic segmentation) and light competition (allocation to wood), segregated shade-intolerant savanna from the remaining shade-tolerant species, and showed strong phylogenetic signal. The second axis did not segregate habitat groups and reflected a phylogenetically conserved hydraulic safety-efficiency trade-off. The third reflected an evolutionarily labile survival-growth trade-off axis and separated resource conservative savanna and acquisitive forest species; generalist species showed intermediary traits in this axis and did not differ from forest and savanna species. Our results support the hypothesis that the evolution of fire resistance in savanna species involved changes in multiple stem, twig and whole-plant traits that co-vary along strategy axes and contrast between savanna and forest species.