Effects of chronically elevated nitrogen and sulfur deposition on sugar maple saplings: Nutrition, growth and physiology

At the Bear Brook Watershed in Maine (BBWM), we examined the effects of long-term experimentally elevated N and S deposition on foliar chemistry, growth, and photosynthetic capacity of sugar maple (Acer saccharum) saplings. The BBWM is a paired watershed system; one watershed has been acidified bimonthly with granular ammonium sulfate ((NH4)2SO4) since 1989. The adjacent watershed is used as a reference. We observed a 56% increase in foliar Al and a 25% reduction in foliar Ca for sugar maple saplings on the treated watershed compared to reference. Foliar N (+15%), P (+10%), and K (+15%) were significantly elevated in treated saplings. Along with changes in foliar nutrients, there were significant differences in photosynthetic capacity.Photosynthetic capacity might have been affected by foliar nutrient imbalances induced by N and S deposition, though light-saturated net photosynthesis and sapling growth were not correlated with differences in foliar nutrient content. Sapling stratum growth and allocation appeared to be determined primarily by disturbances in the overstory canopy that enhanced light conditions in the sapling stratum. Photosynthetic capacity also appeared to be closely linked to sapling density which altered light environment at the leaf-level and increased foliar temperatures. In addition, lower carboxylation capacity was related to low Ca and high Al. Electron transport efficiency and triose-phosphate utilization appear negatively influenced by increased foliar Mn, Fe, and Zn content, although the levels found in this study are below those that provide obvious symptoms of toxicity.