Growth response of Sphagnum capillifolium to nighttime temperature and nutrient level: Mechanisms and implications for global change

Individuals of Sphagnum capillifolium were cultured for 2 mo under six diff erent combinations of nighttime temperature and nutrients. Low nighttime te mperature caused a five-fold reduction of growth. Growth was also reduced w hen the plants were cultured without any addition of nutrient in the growin g medium, but only when nutrient deficiency was coupled with high nighttime temperature. Growth reduction was associated with decreased rates of net p hotosynthesis, but was not accompanied by a degradation of photosynthetic p igments and/or variations in the pigment ratios, nor was the ultrastructure of chloroplasts significantly altered. The decline in the net photosynthet ic rate may be due to a limitation in the enzymatic reactions at unfavorabl e temperatures. Nitrogen and, especially, phosphorus appeared to limit grow th of Sphagnum capillifolium at optimal temperatures. A nighttime temperatu re of 5 degrees C was above the lower threshold triggering the synthesis of red wall-pigments, Known to be promoted by nighttime chilling. Climate war ming is expected to increase the growth rates of Sphagnum, but the conseque nces on the carbon balance of peatlands cannot be predicted because tempera ture rise may also enhance breakdown of peat.