Trees propagated from cuttings of 22 clones of Populus deltoides were grown under unfertilized and nitrogen-fertilized conditions. Large increases in growth resulted from fertilization. A strong clone X fertilizer interaction was observed for diameter, height, and volume growth. Wood specific gravity was reduced by fertilization but no interaction between clones and fertilizers was noted for that character after 2 years.The presence of the genotype-environment interaction indicated that it would be unwise to select eastern cottonwood from a nutritional environment other than the one in which it will ultimately grow. Frequently a clone which performed poorly under unfertilized conditions excelled when fertilized and in several instances the reverse was true. This suggests the possibility that selection can be made specifically for fertilizer response in tree improvement programs. Caution should be exercised when interpreting fertilizer response of experimental populations which mfght contain genetic bias.

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This paper reports the results of fertilizer experiments conducted with thinned and unthinned natural stands of shortleaf pine. Nitrogen and phosphorus were applied singly and in combination on both thinned and unthinned plots. The plots were located on three soil series found in close association in the Great Valley Region of eastern Tennessee.Four-year response in basal area growth per tree was increased as much as 300% over the unthinned check by thinning and application of nitrogen. An increase of up to 40% over the thinned check was achieved with nitrogen alone. These responses were still evident 4 years after fertilization. Basal area growth response to phosphorus was erratic throughout the study period.The results indicate that stand density must be low enough at the time of nitrogen application to allow growth to proceed unhindered.