A wood functional perspective on the low success of savanna trees under woody encroachment

Woody plant encroachment is a global phenomenon, particularly observed in mesic savannas, likely driven by reduced fire frequency, lowered water tables, and/or elevated atmospheric CO₂. This results in a replacement of savanna specialists by generalist and forest species, leading to a fire-free ecosystem with greater light competition. Associated changes in community composition likely drive parallel shifts in species' wood properties and anatomical traits. We test whether wood traits underlie ecosystem changes during woody encroachment, predicting trait convergence among generalist and forest species that contrasts with savanna species. Such divergence could help explain the limited success of savanna species in encroached savannas.

We measured xylem cell-type allocation and wood density in branches of 75 woody species classified as generalist, savanna, and forest species.

Wood density is similar across groups, suggesting that it is not a reliable proxy for habitat preference in Cerrado species but rather an integrative functional trait. Specialist savanna species show a higher proportion of parenchyma than generalist and forest species, reflecting the prioritization of storage for savanna species. Generalist and forest species invest in a higher fraction of fibers, reflecting the investment in mechanical support, which will consequently allow individuals to grow taller and compete for light. Wood volume occupied by vessels is constant across different habitat preferences.

Savanna specialist species are functionally different from both generalist and forest species, which are more similar in the pattern of wood volume allocated to xylem cells. Such divergence may contribute to the limited success of savanna specialists in encroached mesic savannas.