IX, A, 6 Brown and Mathews: Dipterocarp Fm^ests 521 



These changes in groAvth coincide with climatic changes, and 

 are very probably dependent upon them. The first period of 

 slow growth occurs during the dry season. We have found that 

 the percentage of soil moisture is not greatly decreased at this 

 time, but the rate of evaporation is high. This high rate of 

 evaporation not only checks the rate of growth, but very prob- 

 ably also causes a shrinkage in the trunk of the tree due to 

 loss of water. Daily changes in the girth of trees growing in 

 the open can be very readily detected by measurements taken at 

 intervals throughout a period of twenty-four hours. Thus a 

 change of nearly 0.5 millimeter has been noted in the diameter 

 of a Parashorea, 1.14 meters in diameter, growing in the open 

 on the grounds of the College of Agriculture at Los Baiios. The 

 change in girth takes the form of an increase during the latter 

 part of the night and a decrease during the latter part of the 

 day. This decrease is probably connected with the excessive 

 evaporation- rate occurring late in the afternoon. Since changes 

 of as much as 0.5 millimeter can be detected in the course of 

 twenty-four hours, it is not unreasonable to suppose that as great 

 or greater changes may take place as the result of variation in 

 the seasonal rate of evaporation. 



In the same manner the increased rate of growth which we 

 have noted in the early part of the rainy season may be due in 

 part to a lower evaporation rate accompanied by a consequent 

 swelling of the trunk. It is probable, however, that the increase 

 is due more largely to the favorable conditions for growth 

 obtaining at this time. The evaporation at this time is not 

 so excessive as during the dry season, while at the same time 

 the light conditions remain favorable. During the height of the 

 rainy season the sky is overcast during a large portion of the 

 time, and this probably accounts for the decreased rate of growth 

 which is noted from July to October. In the period following 

 this, moisture conditions continue to remain favorable and the 

 amount of light increases. This increase in light intensity ap- 

 parently accounts for the second period of rapid growth occur- 

 ring from October to December. Beginning in December, when 

 the dry season sets in, there is a slight but decided drop in tem- 

 perature, and the rate of growth decreases. As stated before, 

 this decrease culminates in April when the dry season is at its 

 height. As might be expected, the tall trees, which are most 

 exposed, show relatively greater changes in rates of growth than 

 do the smaller ones under the main canopy where the environ- 

 mental changes are not so great. 



