Desert perennial species operate close to hydraulic failure primarily due to (i) low soil water potentials, causing runaway embolism, and (ii) irregular distribution of soil resources, crucial in plant establishment. Theory predicts hydraulic sectoriality (the preferential distribution of resources to specific modules of the plant) to be adaptive in deserts because it could compartmentalize cavitated modules, and maximize the foraging efficiency of the genet. Recent research shows that the abundance of sectored woody species increases along aridity gradients. However, we still lack an understanding of intra-specific variation in such hydraulic design.
We examined the variation in hydraulic sectoriality for the desert species Cryptantha flava (Boraginaceae). We collected basal stems for 10 individuals at different developmental stages in 16 populations in C. flava’s natural range of distribution, varying in aridity from Wyoming to Arizona. We measured anatomical traits that affect sectorial patterns of resource distribution within the plant (distance to xylem vessel, distance between neighboring vessels, distance between vessel clumps, etc). We then correlated the rate of increase in hydraulic sectoriality with age (slope of correlation: anatomical trait of interest vs. plant size) or the basal rate of sectoriality in juveniles (intercept) to the climatic differences reported for each population from long-term weather stations.
We found large intra-specific variation in the degree of hydraulic sectoriality in populations of C. flava. While all populations exhibited an increase in the degree of hydraulic sectoriality with ontogeny, the rate of increase was significantly associated with differences in mean temperature among populations, but not with average monthly precipitation. We also found differences among populations in terms of their hydraulic sectorial design: populations in cooler locations had a larger initial degree of sectoriality among juveniles, while the rate of development of sectoriality with ontogeny was greater in populations where temperatures were greater.
Our study suggests that the physical separation of modules (not just patchy xylem vessel distribution within modules) is the most effective form of hydraulic sectoriality in climates with high probability of embolism formation. This study adds to the hypothesized adaptive value of hydraulic sectoriality in desert plant species by demonstrating that a single species shows a large degree of variation in hydraulic design and can progressively increase their degree of sectoriality from isolation of individual vessels to physically splitting parts of the stem.