Effects of Experimental Soil Warming and Water Addition on the Transpiration of Mature Sugar Maple.

Sugar maple (<italic>Acer saccharum</italic>), an economically important timber and syrup species, is not expected to flourish under projected future climates. The objectives of our study were to (1) tease apart the effects of warming and soil moisture availability on transpiration rates of mature sugar maple trees with a full-factorial soil warming × water addition experiment and (2) determine the primary environmental driver(s) of sugar maple transpiration in the upper Midwestern United States. Over three growing seasons, we monitored sap flux of 33 trees in eight 100-m² plots, two replicates each of four treatments: (1) heat (soil warmed +4°C), (2) water (1.3 × ambient growing season precipitation, (3) heat + water, and (4) control. As expected, sugar maple transpiration decreased under the heat treatments in all years and increased in water treatments in years 1 and 2, all mediated primarily by soil moisture. However, under the heat + water treatment, supplemental water compensated for the warming-induced soil evaporation only in year 1 (2011), which was the driest year. Despite clear evidence of a soil moisture-mediated treatment effect, light was the dominant driver of seasonal variation in sap flux in this sugar maple-dominated ecosystem with relatively short growing seasons. However, sap flux was reduced with decreases in soil moisture, and therefore, net C gain likely was as well. Overall, our results suggest that even though this northern temperate species is primarily limited by light, sugar maple productivity may be reduced by a warming climate on drier sites within its current range, if warming is not accompanied by a sufficient increase in precipitation. [ABSTRACT FROM AUTHOR]