48 MISC. PUBLICATION 257, U. S. DEPT. OF AGRICULTURE 



leaves occurs still sooner. Molisch explained on this basis the fall 

 of the leaves of young pines in times of drought. Autumnal abscission 

 may be explained similarly. In the spring the tender needles tran- 

 spire strongly when the roots are actively functioning and when there 

 is plenty of soil moisture. In late fall, the decrease in soil moisture 

 (associated with the temperature changes in the soil and air and the 

 resulting alterations in the normal water intake and water loss) is one 

 of the deciding factors in the autumnal drop of leaves. 



The effect of forests on climate is still an unsettled question. While 

 there seems little doubt that the local climate in forests is different 

 from that outside, the extent of these effects on the large masses of 

 surrounding air has been much disputed. Although this question 

 lies outside the scope of this discussion, it may be mentioned that an 

 extensive literature is available to the student of this subject. 



Soil Moisture 



Soil moisture, which depends upon rainfall and other climatic 

 factors, in turn affects transpiration. It is to be expected that the 

 more water is available the more will be absorbed and transpired by 

 the local vegetation. Halbfass (82) estimated that in northern and 

 central Germany at least 50 percent of the annual precipitation is 

 accounted for by plant transpiration. Geinitz, according to Halbfass, 

 had stated that 65 percent of the annual precipitation was transpired 

 by the plants in Mecklenburg, but Halbfass found that only 54 

 percent could be thus accounted for in Sachsen- Weimar and thus 

 derived his 50-percent figure from these two sets of data. 



Hartig (90) was among the first workers to point out the increased 

 transpiration with increased water content of the soil. Broad-leaved 

 and coniferous trees, he found, decreased their transpiration as the 

 soil moisture decreased. His work, however, as has been mentioned 

 before, has little quantitative value. Von Hohnel (100), in his 

 classic experiments, likewise found that after every watering his 

 plants transpired more heavily the following day. 



According to Stenstrom (209), it can be accepted as a general rule 



that, extreme cases aside, y=T for any given plant, in which J5=soil 



moisture, L= relative humidity of the air, and T= transpiration. In 

 other words, the transpiration varies directly with the soil moisture 

 and inversely with the relative humidity. This general rule has been 

 made somewhat more inclusive by Gain (65), who states it thus: 

 If the soil-moisture optimum is not exceeded, the amount of water 

 transpired per unit surface is always greater in moist soils than in dry 

 ones, but beyond the optimum water content transpiration decreases. 

 In moist soils the daily transpiration curve goes relatively much 

 higher than in dry soils; but plants on dry soils transpire relatively 

 more at night. 



Recent research papers in this field can be grouped either as eco- 

 logical or physiological, depending on whether they rest on observa- 

 tions in the field or on experimental work in the laboratory. In the 

 former category should be included the work of Cribbs (40), who 

 found that the transpiration of Tilia was not much influenced on the 

 various sites that he studied unless the soil moisture content was 

 decreased to near the wilting coefficient. The daily decrease in tran- 

 spiration which occurred in the afternoon was not due to low soil 



