| The USDA Forest Service, the US Environmental Protection Agency (EPA), and other agencies jointly developed the Forest Health Monitoring (FHM) program to assess the health of forests in the USA. Among the FHM indicators that have been implemented at sites across the USA are forest mensuration, crown condition, damage & catastrophic mortality, lichen communities, and vegetation structure. The efficacy of these five FHM indicators plus two additional indicators, tree canopy gap fraction and tree seed production, were tested at Bear Brook Watershed in Maine (BBWM) to determine their ability to detect the impacts of enhanced atmospheric deposition on forest vegetation. BBWM is a paired watershed research site, established as part of the EPA's Watershed Manipulation Program. Since 1989, West Bear has been chemically manipulated with bimonthly ammonium sulfate applications.; Results from the FHM damage and catastrophic mortality and crown condition indictors reflected no incidence of foliar damage related to the ammonium sulfate additions at BBWM. Similarly, the FHM forest mensuration indicator reflected no influence of the ammonium sulfate additions on tree radial growth. There was a significant increase in the number of sugar maple seedlings in the manipulated watershed. Similarly the FHM vegetation structure indicator showed a significant increase in shrub abundance in the manipulated watershed.; The FHM forest mensuration indicator results reflected a slight trend towards decreased diversity in the number of sapling and seedling species present in the manipulated watershed. The FHM lichen communities indicator showed a similar trend of lower species diversity in the manipulated watershed. For lichens, the overall number of species and the average number of species per plot were both slightly lower.; Results from previous studies at BBWM have demonstrated changes in stream chemistry, soil chemistry, and tree and moss foliar chemistries all related to the ammonium sulfate manipulation. However, none of the five FHM indicators, or the two additional indicators, demonstrated decisively that the ammonium sulfate additions were affecting the vegetation at BBWM. A more accurate assessment of the linkage between acidic deposition impacts and forest vegetation is necessary to develop an indicator able to detect chemical perturbations such as acidic deposition. |
