Growth Studies in Forest Trees. 2. Pinus strobus L.

Abstract

1. The winter condition of the secondary cortex and cambium of white pine is similar to that of Pinus rigida. The marked differences which occur between the mature bark of white pine and pitch pine are occasioned by changes which take place in the outer cortex (periderm). 2. The cambium varies both in number of cell layers (2-10) and thickness in different parts of a tree. It is smallest in both these respects in the twigs and young branches, and increases gradually in dimensions from the apex downward, until that point is reached in the bole where the last annual ring is the thickest. Thereafter, the decrease in the diameter is not proportional to the falling off in the diameter of the last formed ring. 3. Phloem development continues until late in the autumn, much longer than xylem development. Sieve tubes in all stages formation occur between cambium and fully formed phloem. The seasonal growth of phloem exhibits little or no compression as late as October first. Subsequently contraction occurs, due to the extreme cold temperatures of winter. All the seasonal growth of phloem is crushed with the exception of the last 6 or 8 transitional tracheids. Compression is greater in the crown than below. 4. The processes of primary thickening and secondary thickening overlap, and both may be going on in closely neighboring spot sin the tree at the same time. 5. Growth in white pine is divisible into (a) growth without cell division and (b) growth with cell division. The first begins as early as March and the elements concerned (phloem) increase in radial diameter from 50 to over 100 per cent. The awakening of growth is due apparently to the rise of soil water with an accompanying increase in temperature. 6. Growth by cell division begins during the last half of April. At the start it is very rapid, and more elements are formed at the inside of the cambium than at the outside. The formation of new xylem elements follows, the same order as in pitch pine, that is, it begins first in the bole at some distance below the apical shoot and spreads upward and downward. As a result, growth at the base of a tree may begin several weeks later than in the crown. 7. The awakening and rapidity of growth is dependent on three factors, moisture, available food (reserve), and temperature. The first two are at an optimum in the spring; the amount of growth therefore is directly proportional to prevailing temperatures. 8. The intensity of growth is a variable factor which changes from day to day and even within a single day. Two periodic optimums of growth intensity occur, one during May and early June, the second in July and August. These vary from time to time at a given height in the tree and follow no definite law. 9. The amount of growth at a definite time and place in the tree is equal to the sum of the prevailing growth intensities by the time each was in force. It is very irregular at different heights in the tree, but the cambium tends to even up discrepancies as the season progresses. The irregularities of growth are manifested not only in the actual dimensions of the newly formed tissues, but also in the xylem elements. Wide discrepancies may occur in closely neighboring trees; in general, larger differences may be expected the greater the disparity in age. 10. Growth is first retarded in the upper portions of the tree; it may continue vigorously below for some weeks longer. 1. Xylem formation goes on very Sluggishly in all parts of the tree (the terminal leader excepted) until late September and early October, phloem development as long as temperature permits. 12. The total growth of white pine extends over a period of 5.5 months, growth by cell division between 4 and 5 months. 13. Late wood formation begins during the first half of August; it is associated with a decrease in growth intensity and begins first in the higher parts of the tree. 14. Elongation of new shoots and leaves is simultaneous and begins in early May; it manifests itself only after xylem formation has begun. Growth in length in the shoots ceases about July 1; needle growth may continue until August 15 or even later. 15. White pine has long roots and short roots. Only the first elongate to any extent and often are in symbiosis with mycorhiza. Growth in length begins during the last half of April, in some cases even earlier; no reliable data were obtained regarding its cessation. Secondary growth occurs during the first season and proceeds in the usual way.