Date of Award

Summer 8-20-2021

Level of Access Assigned by Author

Open-Access Thesis

Degree Name

Master of Science (MS)

Department

Forest Resources

Advisor

Jay Wason

Second Committee Member

Shawn Fraver

Third Committee Member

Laura Kenefic

Abstract

Projected changes in temperature and precipitation are expected to lead to declines in many forest tree species across the Northeastern United States. Red spruce (Picea rubens) and balsam fir (Abies balsamea) are two of the most common tree species in Maine that make up about 5.5 million acres of the state’s forest land. As such, future climate conditions pose a major challenge for management of these species, and it is unclear how management may mitigate or exacerbate the negative effects of climate change. Therefore, in this study we investigated how thinning to release suppressed red spruce influences understory microclimate, and quantified negative thinning shock effects on the physiology of residual trees. Additionally, we quantified the physiological response of mature red spruce to an experimentally imposed extreme drought induced by severing functional sapwood. Finally, we assessed how thinning influenced tree radial growth and climate-growth relationships from three long-term experimental thinning sites in Maine. We found that thinning profoundly altered the microclimate in the forests leading to increased temperature and vapor pressure deficit in the understory. These changes in microclimate were greater on hotter and drier days, and when coupled with a shift from low to high light environments, drove negative thinning shock responses in the trees that were thinned around. We observed temporary decreases in midday shoot water potential for 3.5 weeks, and prolonged decreases in photosynthetic capacity in the trees that were thinned around, relative to control trees. Therefore, our results suggest that these negative thinning shock effects may contribute to lagged post thinning growth responses, and that the effects of thinning shock may be more severe in future climates. During our experimentally imposed extreme drought, we found that severing the sapwood of mature red spruce only induced simulated drought when the sapwood was 100% severed. Partially severed sapwood trees, with as little as 2% intact sapwood exhibited no signs of water stress. However, the trees with 100% of their sapwood severed experienced slow declines in water potential and water storage that did not surpass physiological thresholds for ~12 weeks. The ability to transport sufficient water with as little as 2% of functional sapwood and the long desiccation time during this experimental extreme drought suggest that canopy red spruce trees may be hydraulically resistant to drought. However, this resistance likely comes with a strong cost of reduced carbon uptake and growth. Radial growth from the spruce-fir sites that experienced thinning in 2002 indicated that post thinning growth responses to the thinning differed significantly across treatment intensity and site. Although these strong site effects limited our ability to draw general inference across sites, in the post thinning period we did find a general reduction in high-frequency growth variability for red spruce suggesting less sensitivity of growth to climate. These results were supported by climate-growth analysis for red spruce that revealed a reduced number of significant climate-growth correlations for heavily thinned red spruce in the post thinning period compared to pre thinning. These results suggest that at some sites and in some conditions, thinning may reduce red spruce climate sensitivity by increasing access to light and other resources. Although our results are primarily limited to these three sites, this work provides a foundation for future larger-scale studies to determine how thinning impacts climate-growth relationships more generally. Our results suggest that spruce-fir forests are climate-sensitive and thus at risk due to climate change. While thinning has been shown to reduce tree climate-sensitivity in current climates, the negative thinning shock effects identified in this study may be more extreme in the future. Although red spruce may exhibit initial drought resistance, we quantified negative physiological effects of lasting drought, which are projected to become more frequent and severe with climate change. As such, our results suggest spruce-fir forests may benefit from management strategies that successfully reduce competition for resources, while also limiting the impact on stand microclimate.

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