Environmental and physiological control of needle conductance for bare-root black spruce, white spruce, and jack pine seedlings on boreal cutover sites

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Abstract
A study was conducted to examine the influence of environmental conditions of boreal cutover sites on plant water status and needle conductance of newly planted bare-root black spruce (Picea mariana (Mill.) B.S.P.), white spruce (Picea glauca (Moench) Voss.), and jack pine (Pinus banksiana Lamb.) seedlings. As absolute humidity deficit between the needles and air (AHD) increased, xylem pressure potentials (ψx) became most negative in black spruce, intermediate in white spruce, and least negative in jack pine seedlings. Needle conductance (gwv) was strongly related to AHD in all three species, with increasing AHD resulting in a decrease in gwv. However, at low levels of AHD, gwv values for black and white spruce seedlings were approximately 50 and 25% higher, respectively, than those for jack pine seedlings. For black and white spruce seedlings, gwv decreased as ψx became more negative, while jack pine gwv responded to more negative ψx with a threshold for stomatal closure at approximately −1.7 MPa. In all three species, increasing photosynthetically active radiation (PAR) resulted in greater gwv at all AHD levels. However, at high AHD levels, gwv, response to PAR was suppressed. The findings of this study indicate species differences in physiological response to atmospheric conditions under nonlimiting soil moisture conditions. The implications for successful reforestation are discussed.