Cambial viability is reduced in water-stressed conifer saplings

In recent decades, heatwaves and droughts have accelerated tree mortality largely via xylem dysfunction and its downstream effects. Whether a tree succumbs to or recovers from drought depends in large part on viable cambial tissue, which is needed to produce new xylem and phloem in subsequent years. We conducted a drought-stress and rewatering experiment on saplings to examine how the vascular cambium responds to water deficit. Cambial viability was assessed using the Neutral Red stain, in addition to measurements of hydraulic function. Cambial viability was significantly reduced when saplings were exposed to moderate water stress, but adding water rehydrated the saplings and the cambium also recovered. By contrast, neither hydraulic nor cambial function recovered in saplings that were exposed to severely low water potentials followed by rewatering. Eventually, the leaves of the severely stressed saplings turned brown and the plants died. Our results suggest that a viable vascular cambium relies on functional xylem. The Neutral Red assay can assess cambial viability and potentially serve as a proxy for hydraulic function in ailing trees recently impacted by environmental stress.