Xylem anatomical traits determine the variation in wood density and water storage of plants in tropical semiarid climate

Wood density (WD) is a functional trait that integrates hydraulic safety and efficiency, water and carbohydrate storage, and biomechanical properties of plants. The global economic spectrum of wood shows that average WD tends to be higher in dry regions than in humid regions. In this study, we investigated the factors determining WD variation, the maximum water content in wood (WCmax), and their possibly consequences to hydraulic functioning of plants under tropical semiarid climate. Fourteen xylem anatomical traits related to mechanical support, water storage, and hydraulic efficiency and safety were measured in 39 woody species commonly found in a seasonally dry tropical forest in the Brazilian semiarid region. We tested the phylogenetic signal of all traits. The range of WD was 0.24-0.85 g cm-3 (average WD = 0.62 g cm-3). Fractions of vessel wall, fiber wall, fiber lumen, and fiber type highly influenced WD. Septate and gelatinous fibers decreased WD, whereas libriform fibers contributed to the increase in WD. Axial and radial parenchyma, fiber lumen fractions, and septate fibers presence determined WCmax variation. Xylem anatomical traits showed a weak phylogenetic signal. Wood density had a phylogenetic antisignal, indicating a functional convergence independent of phylogenetic prox-imity, despite species varying in their anatomical patterns. We conclude that the increase in the average WD in the analyzed species was associated with the investment in vessel wall, fiber wall fraction, and libriform fibers (mechanical support structures), which possibly increase hydraulic safety. Thus, we suggest that fiber types are important determinants of WD and WCmax and should receive more attention.

Automated summary

Wood density is a functional trait that incorporates various aspects of plant physiology and mechanics. This study investigated the factors determining wood density variation and its implications for hydraulic functioning of plants in a tropical semiarid climate. The results showed that investment in certain fiber types and mechanical support structures contribute to higher wood density and potentially enhance hydraulic safety.