The blind men and the elephant: the impact of context and scale in evaluating conflicts between plant hydraulic safety and efficiency.

Given the fundamental importance of xylem safety and efficiency for plant survival and fitness, it is not surprising that these are among the most commonly studied features of hydraulic architecture. However, much remains to be learned about the nature and universality of conflicts between hydraulic safety and efficiency. Although selection for suites of hydraulic traits that confer adequate plant fitness under given conditions is likely to occur at the organismal level, most studies of hydraulic architecture have been confined to scales smaller than the whole plant, such as small-diameter branches and roots. Here we discuss the impact of the spatial and temporal contexts in which hydraulic traits are studied on the interpretation of their role in maintaining plant hydraulic function. We argue that further advances in understanding the ecological implications of different suites of plant hydraulic traits will be enhanced by adopting an integrated approach that considers variation in hydraulic traits throughout the entire plant, dynamic behavior of water transport, xylem tension and water transport efficiency in intact plants, alternate mechanisms that modulate hydraulic safety and efficiency, and alternate measures of hydraulic safety and safety margins.