Impacts and Management of Foliar Pathogens of Eastern White Pine (Pinus strobus) in the Northeastern United States

White Pine Needle Damage (WPND) is a complex of foliar fungal pathogens currently impacting forests in the Northeastern US. Since ca. 2009, chlorosis and defoliation caused by WPND has been observed in stands of eastern white pine (Pinus strobus L) throughout the region. A changing climate, notably warmer temperatures and higher than average spring precipitation in the region are thought to be exacerbating the establishment and spread of these native pathogens. The goals of this research are to enhance the understanding of the timing and magnitude of WPND-induced defoliations across infected stands, assess the physiological response of trees affected by chronic needle loss, quantify the resulting growth reductions, and explore management options using silvicultural tools. This research builds upon previous work that has examined the epidemiology of the diseases’ causal agents and is the first to address aspects of WPND as it relates to tree and forest health. The first chapter of this work investigates methodology regarding sap flux measurements used for estimating whole-tree transpiration rates, which are later applied in the third chapter of this dissertation. I assess the variability of physical properties of the sapwood, consisting of the estimates of the sapwood area based on six different geometric models, as well as the spatial and temporal variability of sapwood water content, density, and thermal diffusivity. Results indicate that the commonly applied circular model used for estimating total sapwood area tends to over-estimate by 10% or more depending on the degree of replication used in determining sapwood thickness at a given radius. Additionally, it was found that sapwood water content and thermal diffusivity are sensitive to changes over time, resulting in significant error rates when assuming a single seasonal value. This data highlights the need for adequate replication of core samples used to derive parameters relating to the sapwood for estimating sap flux. Chapter two presents research findings of a multi-year litter trapping study and the results of a growth analysis using tree ring data obtained from sites throughout WPND-infected stands in northern New England. It was found that defoliations in June and July within diseased stands accounted for 47% of the total annual litterfall between 2014 and 2016. Foliage cast prior to the natural abscission in October was found to have significantly higher concentrations of nitrogen, thus fundamentally altering the timing and amount of nutrient deposition in diseased stands. Repeated defoliations were shown to induce significant reductions in wood growth initiating 2007-2009, resulting in declines in basal area increment of 25-73% compared to pre-outbreak growth rates. Chapter three address several aspects of tree physiology as it relates to WPND-induced defoliation. A subtle, yet significant decline in transpiration was measured in trees of high-infection severity compared to reference low-severity trees. However, disease severity was not found to influence leaf level gas exchange nor nonstructural carbohydrate allocation to various storage tissues throughout the growing season. These findings indicate that diseased trees are not likely to be carbon limited, thus are compromising wood growth at the expense of storing of photosynthate for other uses. Chapter four applies the current knowledge regarding the spread and dispersal of WPND fungi and tests the use of thinning as a silvicultural tool for mitigating the negative impacts of disease. Replicated plots of different residual densities (14 and 25 m2 ha-1) are used to evaluate the impacts of defoliation on common tree metrics to develop an index score for monitoring treatment response over time. Within the first two years This thesis is available at University of New Hampshire Scholars' Repository: https://scholars.unh. edu/dissertation/2397 following stand thinning, residual trees were found to increase in vigor and a reduction in WPND severity of 35% was noted in plots thinned to 14 m2 ha-1. Based on these findings, it is recommended that infected stands or stands at risk of infection should be maintained at densities equivalent to or below to B-line of the eastern white pine stocking guide.