Shoot ( 5 rowth Patterns of Young LobEolZy Pine

Most shoot elongation on Pirr~y !oc& L. seedlings and saplings near Durham. North Carolina. was supplied by the lirst growth Ilush, which began about April I and ended in mid-May 1967. New growth per shoot decli’ned with distance I’rom the tree top. All leaders had three flushes and hall’had four. Variation in internode growth was dependent upon growth rate, not length of growth period, which was remarkably uniform among @I sampled shoots. Day-to-day shoot growth during the lirst Ilush was highly variable. During a~?17-day period that accounted for 42 percent of tot;tl elongation of the first Ilush, degree-hour heat sums above 50’F and so1a.r radiation in langleys per day accounted for 93 percent of the variation in total daily growth and 76 percent ol‘ the variation in diurnal growth. The effect of tempcrature was positive. and solar radiation negative. Heat sums alone accounted for 94 percent ol the variation in nocturnal growth. The threshold temperature for shoot growth was about 40’F at night, and averaged 50°F during the day. The apparent threshold temperature for diurnal growth increased with increasing intensity oE solar radiation. Growth per unit or hear above threshold temperatures was almost the same during the day as at night. forest Sci. 16: 472-482. Additional key words. Pi/~r:s ICX&~, temperature. relative humidity, solar radiation. THKOUGHOUT the 1967 growing season, selected shoots were observed on naturally s e e d e d l o b l o l l y p i n e (Pinus tcretkr L.) seedl ings and saplings near Durham. North Carolina. The time and rate of development of each successive growth flush and the amount and distribution of all new growth originating from each sample shoot were recorded. Shoot elongation was measured twice daily for a time during the first flush of spring growth in order to relate fluctuations in growth rate to recorded climatic variables. The first flush was chosen for these detailed observat ions because i t was common to and concurrent among al l sampled shoots. and because it accounted for most of the new growth. Observations were also limited to the peak period of shoot elongation, when growth rates are most susceptible to prevailing conditions of the external environment. Near the beginning and end of the period in which each flush is elongating, growth is governed more by physiologic conditions within each tree, and less by the immediate environment. Much of the present knowledge on variations in tree growth in relation to environmental variables, especially temperature, has been derived from small seedlings in greenhouses or growth chambers. This phase of the study was designed to explore the effects of important climatic factors on the shoot growth rates of larger trees tinder field conditions.