Joshua M. Halman, Postdoctoral Associate
University of Vermont
Topic: Shifting Dynamics of Tree Growth and Biomass Accumulator
Abstract: Northeastern forests have long been exposed to environmental stressors that negatively impact tree growth and forest health. Decades of acid deposition have depleted soils of base cations and initiated various stages of decline for multiple tree species. However, as pollution has decreased and climates warmed, shifts in forest composition and competition have recently been observed. Work conducted at the Bartlett Experimental Forest (BEF; Bartlett, NH) found that over a 70-year period (1930-2000), some species abundances followed projected successional trends (e.g., increases in eastern hemlock (Tsuga canadensis) and American beech (Fagus grandifolia)), while others deviated from expectations. Red maple (Acer rubrum) and sugar maple (Acer saccharum) abundances decreased over this time period, largely due to reduced regeneration. Related work from Hubbard Brook Experimental Forest (HBEF; Thornton, NH) found that sugar maple experienced similar difficulty with regeneration, unless calcium (Ca) was applied to forest plots. In this particular study, the growth trends of mature sugar maple and American beech were also compared over a 12-year period, beginning when plots were treated with either Ca or aluminum (Al). Maple and beech growing in control and Ca-addition plots showed similar growth trends over this period, but on Al-treated plots, beech began to outpace maple growth shortly after application. Our data showed that Al-treatment resulted in increased tree mortality on these plots as well. Considering the historic sensitivity of sugar maple to acidic conditions, and the relative Al-tolerance of American beech, it appears that American beech has the potential to outcompete sugar maple for resources if acidification continues. However, the degree to which acidification will continue is uncertain – reductions in pollution and longer growing seasons may have reversed the decline of at least one forest species that was severely impacted by acid deposition: red spruce (Picea rubens). Whether or not these reductions in pollutants and changes to the climate will have similar effects on the northern hardwood forest has yet to be determined.