Conflicting functional effects of xylem pit structure relate to the growth-longevity trade-off in a conifer species

Consistent with a ubiquitous life history tradeoff, trees exhibit a negative relationship between growth and longevity both among and within species. However, the mechanistic basis of this life history tradeoff is not well understood. In addition to resource allocation conflicts among multiple traits, functional conflicts arising from individual morphological traits may also contribute to life history tradeoffs. We hypothesized that conflicting functional effects of xylem structural traits contribute to the growth-longevity tradeoff in trees. We tested this hypothesis by examining the extent to which xylem morphological traits (i.e. wood density, tracheid diameters and pit structure) relate to growth rates and longevity in two natural populations of the conifer species Pinus ponderosa. Hydraulic constraints arise as trees grow larger and xylem anatomical traits adjust to compensate. We disentangled the effects of size through ontogeny in individual trees and growth rates among trees on xylem traits by sampling each tree at multiple trunk diameters. We found that the oldest trees had slower lifetime growth rates compared to younger trees in the studied populations, indicating a growth-longevity tradeoff. We further provide evidence that a single xylem trait, pit structure, with conflicting effects on xylem function (hydraulic safety and efficiency) relates to the growth-longevity tradeoff in a conifer species. This study highlights that, in addition to tradeoffs among multiple traits, functional constraints based on individual morphological traits like that of pit structure provide mechanistic insight into how and when life history tradeoffs arise.