Responses detected from lichen, understory vegetation, and tree crown forest health indicators in the Northeast

Background/Question/Methods

The impact of air pollution on forest health observed in the 1970’s and 80’s in the northeastern U.S. inspired the development of suites of specialized measurements to serve as indicators for forest health conditions. These suites, including lichen community composition, understory vegetation composition,  and tree crown condition, were collected on a subsample of national forest inventory plots in the 2000’s. Response to nitrogen deposition varies greatly by organism and can be tempered by factors such as precipitation, temperature, and soil conditions. Lichen species vary in their sensitivity to air pollutants and lichen tissue exposed to pollutants retain them, functioning as on-site monitors of deposition. Crowns of overstory trees are more directly exposed to air-pollutants than other components of forest ecosystems and sensitive species exhibit crowns with higher transparency, greater die-back, and lower density. However, the response of understory vegetation beneath a forest canopy is more complex and the metrics describing responses not well developed. Using data collected on 440 plots in the northeastern U.S., I first examine response metrics for plant communities. Then, responses of tree crowns, lichen communities, and understory vegetation are compared across a subset of 215 plots where soil chemistry measurements are also available.

Results/Conclusions

The suite of forest health indicators were designed to help us understand the conditions of the Nation’s forest resources. The understory vegetation data provided a number of indicators to test for correlation with the modeled nitrogen deposition (ranging from 3.5 to above 14 kg ha^-1 yr^-1) and climate estimates. These included proportion of introduced species and cover; proportion of species and cover by growth habit, Beta diversity, and the distribution of species with known or suspected response. Other predictor variables explored included variables collected on site: elevation, dominant tree species, crown conditions, stand age, total canopy cover, and ground cover characterization. Crown, lichen, and vegetation indicators provided diverse but complementary insights on the influence of N deposition in forests. Stronger relationships may be revealed where on-site pollution loading data are available to correlate with indicator response variables, especially with the compounding effect of climate variability.