US EPA

19308 

Book/Book Chapter 

Nitrate pollution: a particular danger to boreal and subalpine coniferous forests 

Waring, RH 

1987 

Nitrogen was a limiting nutrient during the evolution of boreal and subalpine coniferous forests in association with cool climates and slowly decaying litter. Today as a result of atmospheric pollution, many coniferous forests are accumulating nitrogen in excess of the growth requirement of the trees. An early symptom of excess nitrogen in these ecosystems is the appearance of nitrate in the soil solution. Roots of densely growing conifers are unable to metabolize large amounts of nitrate so a second symptom of excess nitrogen is the appearance of nitrate in the xylem sap. Conifers have restricted abilities to synthesize enzymes required to reduce nitrate nitrogen into amino acids and are unable to store nitrate. With growth restricted for much of the year, amino acids must be formed or else toxic compounds will accumulate and cause green leaves to be prematurely shed. The accumulation of amino acids in foliage is associated with reduced allocation of carbohydrates to roots. Excess nitrogen in subalpine or boreal ecosystems is likely to cause nutrient imbalances and make conifers more susceptible to drought, other pollutants, and pathogens. In the long run, a chronic excess of nitrogen will favor deciduous species over conifers. 

In: Fujimori, T.; Kimura, M., eds. Human impacts and management of mountain forests: [proceedings of a symposium]. Ibaraki, Japan: Forestry and Forest Products Research Institute; pp. 93-105.