Data from: 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) 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 seven 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.