26 BULLETIN 1233, IT. S. DEPARTMENT OF AGRICULTURE. 



In a study of annual or perennial herbs and of deciduous trees 

 it may be possible to draw a fairly sharp line between the growing 

 season and the dormant period, if only on the basis of the period 

 each year in which the plant is in foliage or of that period which is 

 free from killing frosts. With conifers, however, or in fact any 

 evergreen vegetation, the problem is a difficult one; first, because 

 it is next to impossible to determine, in the field, when the period of 

 height or diameter accretion begins and ends; and, secondly, be- 

 cause it is by no means certain that evergreen plants do not lay up 

 carbohydrates necessary for growth whenever light, temperature, 

 and moisture conditions make photosynthesis possible, in winter 

 as well as summer. It is fairly apparent that photosynthetic activ- 

 ity in conifers may occur for short periods ol warm winter days. 

 Although we do not know the actual internal leaf temperature 

 necessary for photosynthesis in any plant, it is readily seen, from 

 the melting of the snow, for example, that the leaf tempera tare may 

 be several degrees higher than that of the air, and that the minimum 

 temperature for photosynthesis may be reached in bright sunlight 

 when the atmospheric condition seems very forbidding. The tem- 

 perature of light-absorbing bodies, with little wind, may well be as 

 much as 40° F. above that of the air. On January 10, 191S, at 

 2 p. m. snow was observed melting on the edge of a black shingled 

 roof, where there was reflected as well as direct sunlight, but which 

 was entirely removed from artificial heat, at an air temperature of 

 -6°F. (-21° C). 



From the consideration of such facts it becomes apparent that 

 any " growing season" for evergreen trees or herbs must be set 

 apart on a purely arbitrary basis. We may better speak of a period 

 of maximum or optimum growth, if it is desirable, rather than con- 

 vey the impression that growth is restricted to the warmer portion 

 of the year. In this bulletin the term "growing season" will be 

 used in this sense. 



In the preliminary report (3) on this study the writers did not 

 attempt to divide the data between growing season and dormant 

 period, but presented some facts to show that, considering a soil 

 temperature of 41° F. (5° C.) essential to the growth of any of the 

 Rocky Mountain trees, there is a marked difference in the length 

 of the growing season as between different slopes and different 

 forest types. The soil of a south exposure, for example, at a depth 

 of 1 foot was found to be above this temperature for about 220 

 days, and that of a north exposure for only 133 days. But while 

 there is no occasion for minimizing the importance of this difference 

 the assumption that growth is dependent upon a soil temperature 

 of 41° F. is not substantiated by the facts. The phenologies] ob- 

 servations of a number of years show that the first swelling of the 

 buds of Douglas fir may occur ;i( soil temperatures as low as 39° 

 F., or as high as 50° F. So far as can be determined by this crude 

 method of observation, soil temperature and moisture are strongly 

 interdependent, and it is the molecular activity of the soil water, 

 as determined by its temperature and amount, and as well by tho 

 solutes it contains, which controls the beginning of new growth. 





