Physiological response of mature red spruce trees to partial and complete sapwood severing

Drought frequency and severity are projected to increase in many regions of the world, yet it remains unclear how mesic forest trees will respond to these novel climate conditions. Experimentally imposing drought in forests at the stand scale is logistically difficult, however, disturbing the hydraulic functioning of individual trees can induce severe water stress and may inform how large trees respond to future droughts. In this study, we manipulated water availability of red spruce (Picea rubens Sarg.) trees by partially or completely severing sapwood, and measured impacts on water relations and photosynthetic efficiency over four months. Trees with total sapwood severed (TOT) experienced a rapid 96.7 % reduction in daily sap flow whereas trees with only partial sapwood severed (PAR; estimated 2-5 % sapwood remaining) and no sapwood severed (CON; phloem girdled control) experienced average reductions of 74.3 % and 4.6%, respectively. Sapwood severing in TOT trees resulted in declining midday shoot water potential compared to PAR and CON trees, but TOT trees did not surpass the water potential indicative of 50 % loss of conductivity until approximately 12 weeks post treatment. At 7 weeks post treatment, TOT trees had 6.8 times lower midday stomatal conductance than PAR and CON trees. Furthermore, branch- and crown-wood water potentials reached extreme values (below measurement threshold of - 7.7 MPa) in TOT trees by the time of tree harvest at 18 weeks but remained high and did not differ between PAR and CON trees. Our results indicate that with minimal intact sapwood, PAR trees still had sufficient hydraulic functioning to avoid water stress, while TOT trees had temporary resistance to water stress likely associated with declines in carbon sequestration and growth. These findings advance our understanding of how red spruce may physiologically respond to periods of water stress in future climates.

Automated summary

This study investigated the effects of simulating drought conditions by disrupting the hydraulic system of red spruce trees on their water relations and photosynthetic efficiency. The results showed that trees with completely severed sapwood experienced rapid water stress, while those with partially severed sapwood maintained better water relations. These findings contribute to our understanding of the physiological responses of red spruce to drought conditions in the future.