Vessel tapering is conserved along a precipitation gradient in tropical trees of the genus Cedrela

Abstract

Key message

The rate of vessel tapering is highly conserved across a precipitation gradient in tropical trees, pointing to a limit on tree height determined by a maximum basal vessel diameter.

Maximum tree height in the tropics decreases strongly with decreasing precipitation. The role of hydraulic architecture in controlling this variation in tree height remains unclear. The widening of conducting xylem vessels from the apex to the base of trees, also known as tapering, is important for maintaining the hydraulic conductivity along the tree stem. If in contrast vessel diameter were constant, then resistance would increase with path length constraining flow rates as tree height increases. Whilst previous research has shown that vessel diameter scales with tree height at a similar rate (similar power law exponent) across biomes and taxa, knowledge on these relationships across precipitation gradients within a single species is incomplete, especially for the tropics. Here we report how vessel density and diameter at the tree base differ for two tropical Cedrela species across four sites varying in precipitation from 1014 to 2585 mm year⁻¹. We find that maximum tree height decreases with decreasing precipitation across sites from 42 to 13 m. Despite the strong differences between sites in maximum tree height and water availability, tapering is indeed remarkably conserved and close to published scaling with height based on multi-species analyses. Thus, for a given tree height, basal vessel diameter does not vary between sites, whilst the maximum basal vessel size is two times smaller at the drier site (with the shortest trees) compared to the wettest site (with the tallest trees). This suggests a possible limitation of tree height determined by a maximum basal vessel diameter that can be sustained, given increasing embolism risk with increasing dryness. Our results show no hydraulic adaptation across this wetness gradient and reveal a clear relationship between maximum tree height and maximum basal vessel size.