Comparative Studies of Growth in Deciduous and Evergreen Trees

Radial changes throughout the years 1944 and 1945 were studied in seventeen species of trees at Moscow, Idaho, in a region originally covered with prairie. Subsequent to their establishment in an arboretum the trees have not been irrigated, and all have become sufficiently adjusted to the environment to show approximately normal growth. 2. The groups of eight evergreen conifer (Abies concolor, Picea engelmannii, P. pungens, Pinus monticola, P. ponderosa, P. strobus, Pseudotsuga taxifolia, and P. t. var. glauca) and eight deciduous dicots (Acer pseudoplatanus, A. saccharophorum, Fagus grandifolia, Fraxinus americana, Juglans nigra, Quercus borealis, Robinia pseudoacacia, and Ulmus americana) that were studied began to grow at approximately the same time in both years. During the early part of the grand period of growth the evergreens grew more rapidly than the deciduous dicots, but in the later part of this period the relation was reversed. The evergreens shrank the most in response to drought in summer. Very slow enlargement was observed during autumn and winter in nearly all species, but this was frequently interrupted by periods of shrinkage. In a few instances winter swelling was in part definitely a rehydration of tissues that had suffered marked shrinkage during the preceding summer. In other cases winter swelling probably represented belated hydration of cells produced in summer. The radii of evergreens were less stable during winter than were those of deciduous trees, a phenomenon which is probably related to needle retention, as this habit makes the plants relatively sensitive to transpiration stress in winter, 3. In most respects, radial behavior of the deciduous conifer Larix occidentalis did not consistently parallel either that of the other conifers or that of the deciduous dicots, but in its radial stability during winter and in its earliness of foliation in spring it resembled the deciduous dicots closely. 4. The inclusion of Pinus ponderosa, Pseudotsuga taxifolia var. glauca, and Picea engelmannii in the series of trees provided excellent opportunity ,to compare the behavior of species native to low, intermediate, and high altitudes in the Rocky Mountains. No consistent differences in behavior were found that could be definitely related to altitudinal distribution. Pinus ponderosa exhibited a pronounced autogenous, retardation of growth that began early in summer. Subsequently this species did not seem as much affected by the normal summer drought as were the species from higher altitudes, but this behavior appears to be more a peculiarity of the species than a fundamental adaptation to the vertical gradient in climate on mountain slopes. 5. Radial changes in Pinus monticola paralleled those of P. strobus very closely, except that the former regularly began to grow a few days earlier. 6. Pseudotsuga taxifolia native to the Pacific slope, responded in a manner that indicated a greater degree of xerophytism than possessed by trees representing P. t. var. glauca of the Rocky Mountain region. The growth of the trees from the coast slowed down earlier in summer, and these trees shrank less in response to late-summer drought. Although there is no record of the exact source of the seed,this difference in behavior may reflect ecotypic specialization, for the driest habitats supporting Pseudotsuga along the coast are drier than the wettest habitats in which the species may be found in the interior. 7. No consistent relationship was found between xylem structure and earliness of cambial activity in spring. 8. In general, the leaves of the deciduous trees were nearly fully developed before the new foliage of the evergreens made its appearance. 9. The growing seasons of the trees began before the frost season had ended, and growth stopped well in advance-of the first frost in autumn. A drop in temperature to 0⚬ C. on June 14, 1945, at about the middle of the grand period of growth, temporarily produced either a reduction in the rate of growth or shrinkage in about half of the species of evergreens and in half of the species of deciduous trees. 10. Robinia pseudoacacia is unique in exhibiting a marked shrinkage, amounting to about 30% of its net annual increment, which took place abruptly after the grand period of growth had ended in both summers. In 1945 this shrinkage was not made up until the following February or March.