Over the last century, tree growth has offset a large fraction of human carbon emissions. However, forests may not sequester this carbon indefinitely. Recent climate change has contributed to dramatic tree mortality in many regions. As these trees decay, they release stored carbon back into the atmosphere. Whether forests shift from net carbon sinks to sources depends on how climate change interacts with tree functional traits. One key trait that may influence both mortality and decay is xylem vessel length. Under drought conditions, vessel length influences stem hydraulic performance. Once trees die, vessels may provide colonization pathways for decomposing microbes. Despite their pivotal role, few studies have examined variation in vessel length, in part because this trait is relatively difficult to measure. In this study, we present a new probabilistic model for estimating vessel length from serial cross sections of silicone injected stems and evaluate how variation in vessel length among 18 woody plant species contributes to patterns of mortality and decay in the most extensive forest ecoregion in central North America.
Our model derives expected vessel lengths from simple assumptions about xylem development. This model explains changes in the proportions of open vessels at successive distances better than previously proposed models. As predicted, species with longer vessels experienced higher mortality following a severe summer drought in the northeast Ozarks. The vessel length effect was stronger than effects of all other anatomical and chemical functional traits with the exception of wood density. In contrast, the relationship between vessel length and decay was more complicated. Over intermediate ranges of vessel length, species with longer vessels decayed faster under common garden conditions. However, this pattern was reversed at extreme values of vessel length. Overall, these results suggest that vessel length may help predict patterns of tree mortality during climate-change related drought, but that patterns of decay depend more on other wood traits in addition to community dynamics of decomposing microbes.