Distribution of calcium oxalate crystals in the secondary phloem of conifers: a constitutive defense mechanism?

•   Calcium oxalate (CaOx) crystals were studied in the secondary phloem of 46 conifer species from six families to characterize their distribution relative to defense. •   Scanning electron microscopy, and polarized light microscopy of stem tissue allowed analysis of the quantity, location, morphology and size of the crystals. •   Vastly different patterns of CaOx crystal deposition were observed in stems of Pinaceae and nonPinaceae lineages. The CaOx crystals were present in all species but the highest density occurred along the nonPinaceae lineage. In Pinaceae, all species accumulated crystals intracellularly in crystalliferous parenchyma, whereas all nonPinaceae species had only extracellular crystals. A possible relationship between the number of aggressive bark beetles species and the amount of CaOx accumulation was noted where increased crystal accumulation appears to be antagonistic to beetle attack. Mapping along with this trend was the presence of phloem fibers. •   We conclude that in conifer stems the patterns and frequency of CaOx crystals function as a constitutive defense and in combination with fiber rows, provides an effective barrier against small bark-boring insects.