Vessel Redundancy: Modeling Safety In Numbers

We examined the concept that high vessel number provides xylem safety and also show that under certain circumstances high vessel number may increase rather than decrease the probab ility of mortality. The independent variable was the number of vessels per organ (redundancy). The dependent variable was the probab ility of organ death for which we set three thresho1ds for catastrophic runaway embolism (50, 75 and 90% embolisrn). Results were calculated based upon the probability that any particu1arvesse1would become embolized (P). When the modeled p was below the runaway embolism thresho1d, the safety benefits (decreased probability of organ death) increased dramatically in going from one to ten vessels and approached maximum levels of safety in organs with lOO or more vessels.Vessel redundancy conferred the greatest advantage when p approached, but was less than, the runaway embolism threshold of the organ. However, when p exceeded the runaway embolism threshold the redund ancy relationship was reversed and safety was greatest in organs with lower vessel numbers. Having greater vessel redundancy increased the likelihood of an "average" result, i.e., mortality if p is above the threshold, and survival when p is below the threshold. Model predictions are discussed in terms of redundancy segmentation, stern splitting and various other ecological and evolutionary strategies for plants exposed to different environmental conditions.