White Pine Needle Damage (WPND) is a complex of foliar fungal pathogens that have established and become a chronic disease impacting forests in the Northeastern US. Eastern white pine (Pinus strobus L) is a major softwood component of Northeastern forests, and annual summer defoliations caused by WPND have garnered widespread concern from regional foresters and landowners. With long term ecological and economic impacts in mind, it is critical to develop management recommendations for moderating the negative effects of WPND. This study uses litterfall measurements and tree ring analysis as a means to quantify the magnitude of defoliation severity and associated growth declines across study sties in the Northeast. The work presented here also evaluates the effectiveness of mechanical tree removal (thinning) as a means to both enhance growth and mitigate the severity of WPND defoliations. During the summer of 2015 two replicated experimental blocks were established in Hillsborough and West Ossipee, NH. During the subsequent winter, one-acre treatment plots were thinned to low density (60 ft2 ac-1) and high density (100 ft2 ac-1) residual stocking. Growth and forest health metrics were measured in the pre- and post-treatment years in order to evaluate the effectiveness of the silvicultural thinnings over time.
We found the WPND induced defoliations contributed to >50% of the total annual litterfall of eastern white pine, with the most foliar biomass cast during the month of June in each year of the study. Additionally, the nitrogen content of litter captured during the summer months was found to be over twice the amount found in litter captured during the natural needle abscission (October), suggesting that resorption and internal cycling of N is compromised due to WPND. Furthermore, the timing of the defoliation event represents a 4-5-month period of the growing season in which infected trees have a reduced leaf area, and thus, reduced capacity for carbon assimilation. Since the initial outbreaks of WPND ca. 2010, dendrochronology has revealed that basal area increment has declined significantly, with growth reductions measured between 23-65% across study sites in the region. Preliminary results from the first year of WPND infected stands to silvicultural thinning exhibited a positive response, as both the low and high density treatments had a mean reduction in WPND severity of 34% and 17% respectively. The results of this research will ultimately be used to inform regional forest management in the context of WPND.