Effects of chronic ammonium sulfate treatment on basal area increment in red spruce and sugar maple at the Bear Brook Watershed in Maine

Responses in basal area increment (BAI) of sugar maple (Acer saccharum Marsh.) and red spruce (Picea rubens Sarg.) to chronic ammonium sulfate ((NH4)(2)SO4) treatment were examined at the Bear Brook Watershed in Maine. The Bear Brook Watershed is a pair-watershed forest ecosystem study with West Bear watershed treated with (NH4)(2)SO4 at a rate of 1800 equiv.-ha(-1).year(-1) since 1989, while East Bear watershed serves as a reference. Following 10 years of treatment, BAI was significantly higher for sugar maple trees growing in the treated watershed, with yearly increases relative to the reference watershed ranging from 13% in 1999 to 104% in 1996. The increase in sugar maple radial growth was attributed to a fertilization effect from the (NH4)(2)SO4 treatment. A reduction in BAI in sugar maple growing in the treated watershed observed in 1998 and 1999 was attributed to internal stresses and growth allocation to crown recovery after the severe 1998 ice storm. Red spruce showed no BAI growth responses to the treatment. Lower foliar Mg and Ca concentrations in red spruce in the treated watershed and lower soil responses to N enrichment in treated softwood stands compared with treated hardwood stands could explain the lack of BAI response in red spruce.