The effects of experimental warming and irrigation on the water use of sugar maples (Acer saccharum) in a northern hardwood forest

Background/Question/Methods

Climate change can impact water supply in forests around the globe because atmospheric warming, due to increased levels of CO₂, may affect the amount and timing of precipitation.  Both mean annual temperature and precipitation are expected to increase in the Upper Midwestern USA, which may greatly affect forests in this region. The objective of our study was to investigate the effects of experimental soil warming and irrigation on the water use of sugar maple (Acer saccharum), in order to better understand how sugar maple-dominated hardwood forests in the Upper Peninsula of Michigan may react to climate change. We expected that water use would increase in the water addition treatment, decrease in the warming, and increase in the combination treatment as a result of the irrigation reducing the negative effects of the warming.  We measured sap flux density (g m^-2 s^-1), using heat dissipation sap flow sensors on 33 sugar maple trees in eight 15 m x 15 m plots with two replicates each of four treatments: 1) warming with infrared heaters (3-4 °C above ambient soil temperature), 2) water addition (20% above ambient precipitation), 3) combination of warming and water, and 4) control. 

Results/Conclusions

Mean daily and mean peak daily sap flux density values where greater in the water addition treatment (54% and 58%, respectively) and the combination treatment (37% and 32%, respectively) when compared to the control, which supported our original hypothesis.  This was likely a result of increased levels of soil available water, which increased transpiration rates.  However, mean daily and mean peak sap flux where also found to increase in the warming treatment compared to the control (25% and 25%), which was inconsistent with our original hypothesis.  We speculate that this was a result of either increased xylem water viscosity, which could increase transpiration rates by reducing resistance to sap flow, or increased root respiration rates which may have increased the capacity for ion transport and thus water uptake.  Overall, this study could indicate that sugar maple may increase its water use with future climate change, which could result in significantly reduced water yield in future northern hardwood forests.  More work is needed to investigate whether this effect will be sustained over time.