Using dendroecological techniques to interpret the response of trees to environmental change at Vermont Monitoring Cooperative’s Mount Mansfield study site

Background/Question/Methods

A number of tree species in Vermont have undergone documented decline in the 20th

century, most notably red spruce (Picea rubens Sarg.) and sugar maple (Acer saccharum

Marsh.). However, there is also varied and conflicting evidence about how these trees are

responding to more recent changes in climate and atmospheric deposition. We examined

xylem increment growth, relative growth rates, and vigor of five key tree species in the

Northern Forest growing in the Vermont Monitoring Cooperative’s long-term study site

located in Mt. Mansfield State Forest (Underhill, VT): sugar maple, red spruce, red maple

(Acer rubrum L.), yellow birch (Betula alleghaniensis, Britton), and balsam fir (Abies

balsamea, [L.] Mill.). Target species were sampled along an elevational gradient to capture

trends in the three major forest cover types – deciduous, mixed, and coniferous forests –

replicated in three watersheds. We also assessed the nature and timing of growth trends

relative to local weather and pollution data to evaluate how these variables may have influenced

species relative growth rates and productivity.

Results/Conclusions

We found baseline differences in past as well as present growth; with red maple and yellow

birch having the highest mean basal area increment (BAI) growth, and sugar maple and

balsam fir, the lowest. The timing of maximum growth also varied among the species, with

sugar maple and yellow birch growing best in the 1960s-80s, and red spruce and red maple

showing the greatest growth only very recently. When growth was converted into a Z-score

that compares recent growth to mean growth for each species, red maple and red spruce

had growth above, yellow birch and balsam fir had growth that was indistinguishable from,

and sugar maple had growth below their respective means. Although many year-to-year

declines in growth were likely associated with specific (often localized) stress events,

protracted patterns in growth (e.g., the recent increases in red spruce and red maple

growth) were associated with broader climate or deposition trends. Based on our

assessments with other chronologies, growth at Mt. Mansfield for these species aligns with

regional trends and may indicate that patterns assessed here are indicative of the broader

region.