SEASONAL BIOCHEMICAL-CHANGES IN CONIFEROUS FOREST CANOPIES AND THEIR RESPONSE TO FERTILIZATION

Seasonal changes in concentrations of total nitrogen, free amino acids . chlorophyll, starch and sugar were measured in foliage from fertiliz ed and unfertilized conifer forests in New Mexico and Oregon. In the N ew Mexico Douglas-fir (Pseudotsuga menziesii var glauca (Beissn.) Fran co) forest. fertilization resulted in elevated foliar nitrogen concent rations on all dates. from an average of 9 mg g-1 in unfertilized tree s to 14 mg g-1 in fertilized trees. In the Oregon western hemlock (Tsu ga heterophylla (Raf.) Sarg.) forest, fertilization increased total N by only 15%, from 13 mg g-1 in unfertilized trees to 15 mg g-1 in fert ilized trees. Foliar nitrogen concentrations on a weight basis were lo west in winter and spring, but did not vary seasonally when expressed on a leaf area basis. Chlorophyll concentrations increased with fertil ization and had greater seasonal variation than did total nitrogen con centrations. Chlorophyll concentrations were significantly higher duri ng the growing season than in the winter and spring months. Fertilizat ion did not result in major changes in the proportion of total nitroge n in chlorophyll at either the Oregon or the New Mexico site. Concentr ations of free amino acids varied with date and fertilization treatmen t; in New Mexico, amino acids were highest in the winter sample, where as in Oregon, they were lowest in winter and spring. At both sites, am ino acid concentrations were significantly higher in fertilized trees than in control trees on most dates and the ratios of amino acid-N to total N were also significantly higher in fertilized trees. For both s ites, starch concentrations were nearly zero for most of the year, but increased sharply just before bud break and initiation of new growth in the spring. Although fertilization resulted in increased nitrogen c oncentrations in foliage at both sites, the response in New Mexico was much greater than in Oregon. These results are in agreement with fore st productivity data that suggest that growth in the New Mexico site i s limited by nitrogen, whereas in the Oregon site it is not.