Winter climate change and the hydraulic properties of the northern hardwood forest

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Christ, Jordan R.
Currently sugar maple (Acer saccharum) and yellow birch (Betula alleghaniensis) are in decline in the northeast U.S. This decline disease is caused by multiple stressors which weakens the tree, making it more vulnerable to acute stress events or disturbances. Stressors affecting the northern hardwood forest include atmospheric deposition, insect pests and pathogens, invasive species, fragmentation, and climate change. Of these stressors, much is uncertain about the effects of climate change on tree species physiology, health and future distributions in New England. Winter climate change is especially important because many trees species in the northern hardwood forest are not very cold hardy and rely on snowpack over winter as insulation which protects roots from soil frost damage. Furthermore, recent temperature increases have been greater over the winter season compared to summer, providing a greater emphasis on the winter season. A better understanding of the impacts of winter climate change will allow for improved land management practices for forest industries, like timber and maple sugaring, which can provide economic longevity for New England. Sap velocity can be directly linked to productivity in tree individuals as the flow of sap is closely correlated to the rate of transpiration, a by-product of photosynthesis. We present the heat dissipation method for calculating tree sap velocity in two experiments under the general theme of winter climate change and the hydraulic properties of the northern hardwood forest. The experiments in this document include 1) comparing root percent embolism and sap velocity in yellow birch stands over natural gradients and 2) the pre-bud break sap velocities of sugar maple trees in response to winter variability, air temperature fluxes and sapwood aspect and depth. Chapter 1 Results: Snowpack variation over natural gradients of aspect and elevation was greatest over winter 2010/2011 compared to 2009/2010 at the HBEF and differences in root percent embolism using a repeated measured ANOVA with elevation and aspect as factors were significant in 2011 (p=0.05488); however no significant differences were found in 2010. Overall snowpack depth (80-130cm) and duration (18 weeks) was greatest over winter 2010/2011 compared to 2009/2010 (40-70cm and 16 weeks) and root percent embolism was greatest at all plots in 2010, with the exception of the Kineo Mid plot. These results suggest that annual snowpack variation in depth and duration influences root embolism patterns in yellow birch trees. In comparing average root percent embolism to average sap velocity we found that within class of aspect all plots with the greatest average root percent embolism had the lowest average sap velocity and vice-versa for both years, with the exception of Kineo Low and WS 3 High in 2010, likely due to drought during 1 of the 2 days when sap velocities were compared. This trend suggests that root embolism may be limiting sap flow in yellow birch trees. Chapter 2 Results: Sap velocities of sugar maple trees during the early spring sugaring season in 2011 and 2012 were influenced by thaw-freeze cycling and temperature fluctuations, where these conditions were important in producing the greatest sap velocities. After two days of above freezing night temperatures in 2011 xylem pressure was greatly reduced. Sap velocity was greatest at 2cm sapwood depth and south aspect (up to 3,000 g/m²/s), where stem temperature fluctuation was likely the greatest due to sun exposure. Sapwood depth had a greater influence on flow rates than sapwood aspect. Sugar maple trees had later and greater pre-bud break sap velocities in 2011 (Julian day 96, 3000 g/m²/s)compared to 2012 (Julian day 80, 120 g/m²/s) where the winter prior to the sugaring season in 2012 was very mild with little snowpack compared to winter conditions prior to the sugaring season in 2011.