The effect of added nutrients on current year circumference growth of ponderosa pine under varying levels of soil moisture

Abstract

During the summer of 1963, the effects of added nutrients on the growth of dense stands of ponderosa pine (Pinus ponderosa Laws. under varying levels of soil moisture were studied. Measurements of tree circumference growth, foliage moisture, foliage nutrient content, precipitation, air temperature, relative humidity, soil moisture, and soil temperature were made on six one-twenty-fifth acre split plots located near McNary, Arizona. The findings can be summarized as follows: 1. Growth in circumference did not start in the summer until soil moisture was sufficient. This was indicated by the beginning of growth on the watered plots before that of the unwatered plots. Growth on the unwatered plots did not start until after summer rains increased the soil moisture level. 2. Although a statistical analysis showed that the addition of nutrients did not significantly increase growth the first season after application, a graph showing the accumulative growth in circumference suggested that the nutrients with additional water would give larger increases in circumference growth in future growing seasons. 3. Growth in tree circumference was related to increases in the water added to the soil beyond that supplied by precipitation. 4. The foliage nutrient contents of both nitrogen and phosphorus were higher for the fertilized plots than for the unfertilized plots. This indicated that the added nutrients were absorbed by the trees. 5. Foliage moisture was not related to the soil moisture levels used in the experiment. A different time of sampling as indicated by foliage moisture data collected over a period of 24 hours might be a better indicator of soil moisture levels. 6. Foliage moisture was related to relative humidity changes. A more detailed study on foliage moisture changes in relation to relative humidity could be used to further verify this. 7. The end of the growing period must have been correlated with decreases in temperature or other environmental factors as soil moisture was plentiful.