Disruption of photosynthesis and carbon transport due to damage of the tree crown and stem cambial cells, respectively, can cause tree mortality. It has recently been proposed that fire-induced dysfunction of xylem plays an important role in tree mortality. Here, we simultaneously tested the impact of a lethal fire dose on non-structural carbohydrates (NSCs) and xylem hydraulics in Pinus ponderosa saplings.

Saplings were burned with a known lethal fire dose. NSCs were assessed in needles, main stems, roots and whole plants and xylem hydraulic conductivity was measured in the main stems during 29 days post-fire.

Photosynthesis and whole plant NSCs declined post-fire. Additionally, all burned saplings had 100% phloem/cambium necrosis, and roots of burned saplings had reduced NSCs compared to unburned and defoliated saplings. We further show that, contrary to patterns observed with NSCs, water transport was unchanged by fire and there was no evidence of xylem deformation in saplings that experienced a lethal dose of heat from fire.

We conclude that phloem and cambium mortality, and not hydraulic failure, were likely the causes of death in these saplings. These findings advance our understanding of the physiological response to fire-induced injuries in conifer trees.