Effects of soil freezing stress on sap flow and sugar content of mature sugar maples (Acersaccharum)

Abstract

Mature dominant and codominant sugar maples (Acersaccharum Marsh.) at the Duchesnay Experimental Forest (Quebec, Canada) were subjected to deep soil freezing (DF), superficial freezing (SF), and superficial freezing with drought (SFD) during the 1990–1991 and 1991–1992 seasons. Soil temperatures below the DF trees reached lows of −6 °C at 20 cm depth and were significantly lower than the controls. Compared with the controls, unfrozen soil water content was lower (10%, v/v) below the DF trees. During the dry summer of 1991, soil water content remained high below the DF trees at 30% (v/v); controls reached 10% (v/v). Deep frost treated trees had significantly lower sap flow rates, total sap volume, and less total sugar per tree than other treatments for at least 2 years after treatment. These lower values were likely related to the condition of the DF trees (increased canopy transparency; branch dieback). Percent sugar was significantly higher in the DF trees in 1992. Drought did not seem to have a significant effect on sap flow rate as values obtained for 1991 to 1993 followed the same pattern as that for the control trees. Sap in SF trees tended to flow more than in control trees in 1991. Sap in DF-treated trees that did not show visual symptoms of dieback had a lower flow rate than control trees. Soil freezing, which is concomitant with severe reductions in soil water availability, had a negative effect on spring sap flow. A sufficient source of soil water at the moment of stem recharge seems to be critical during the conditioning period to maintain stem pressure and sap flow.