Date of Award

2019

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Natural Resources

First Advisor

Paul G. Schaberg

Abstract

Northeastern forests are in a period of immense change. While forests are inherently dynamic ecosystems, a range of environmental challenges may cause unique and uncertain transformations within forests moving forward. How trees in northern forests respond to these environmental and anthropogenic changes remains uncertain; reductions and increases in the growth of various species and shifts in current species’ ranges may take place.

I analyzed associations between tree growth (assessed using xylem increment cores) and a range of site, climate, and pollution deposition variables for seven major tree species in Vermont. First, I looked at red oak (Quercus rubra L.), a species that is projected to expand into the northern hardwood forest. Summer moisture metrics were consistently and positively associated with growth, while higher summer temperatures were often negatively associated with growth. Several pollution metrics were associated with reduced growth – a surprising result for a species not known to be sensitive to inputs of acid deposition.

Second, I examined growth of three tree species emblematic of the northern hardwood forest: sugar maple (Acer saccharum Marsh), American beech (Fagus grandifolia Ehrh.), and yellow birch (Betula alleghaniensis Britton), plus a fourth species, red maple (Acer rubrum L.), whose abundance has increased within the region. Winter moisture (likely snow) was important for all species except for red maple, and either summer moisture (positively associated) or temperature (mostly negatively associated) variables were important for all species, with some differences in relationships based on the elevation or time period evaluated. Pollution metrics were also negatively associated with growth for all species except beech.

Third, I studied correlations of growth and environmental parameters for two temperate conifers: eastern white pine (Pinus strobus L.) and eastern hemlock (Tsuga canadensis (L.) Carrière). For both species, growth was positively associated with warmer spring temperatures but negatively associated with summer temperatures. Negative relationships between pollution deposition and growth for white pine, and to a far lesser extent, eastern hemlock, were also detected. If growing seasons lengthen as projected, temperate conifers may be at a competitive advantage for increased growth if changes in summer temperature and moisture regimes do not add excessive stress that offset possible increases in spring growth. These three studies highlight the importance of considering the distinct seasonal projections for moisture availability and temperature when evaluating the likely growth trajectories of these keystone species.

Language

en

Number of Pages

205 p.

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