A Bayesian model for xylem vessel length accommodates subsampling and reveals skewed distributions in species that dominate seasonal habitats.

Vessel  length  is  an  important  but  understudied  dimension  of  variation  in  angiosperm  vascular  anatomy. Among other traits, vessel length mediates an important tradeoff between hydraulic efficiency and safety that could  influence  how  plants  respond  to  extreme  weather  with  climate  change.  However,  the  functional significance  of vessel length variation within individual stems is poorly known, in part because existing data analysis methods handle uncertainty in a way that makes vessel length distributions difficult to compare. We provide a solution to this problem through a hierarchical Bayesian framework for estimating vessel lengths and we demonstrate the flexibility of this method by applying it to data from serial cross sections of dye injected stems. Our approach can accelerate data collection and accommodate  associated uncertainties by statistically correcting for bias and error that result from subsampling images. We illustrate our analytical framework by estimating and comparing vessel length distributions for 21 woody species characteristic of a North American forest.  The best-fit  model  corrected  for both bias due to secondary  growth  and sampling  error  within  and among  species.  Vessel  length  estimates  from  this  model  varied  by  almost  an  order  of  magnitude  and parameters  of these  distributions  correlated  with  point  estimates  derived  from  a different,  commonly  used method. Furthermore, we show how key contrasts can be estimated with the Bayesian framework, and in doing so, we show that the shape of the vessel length distribution differed between ring- and diffuse-porous species, suggesting that within-stem vessel length variation corresponds to water stress seasonality and contributes to landscape-level  habitat segregation. Our analysis method revealed the importance of within-stem variation in vessel length, and our results complement work on between-species variation in average vessel length, further illuminating how vascular anatomy can influence woody plants’ responses to water stress.