Periodical Literature 135 



few years. These changes in humus contents naturally produce 

 changes in physical conditions, so that in the end, not only light 

 conditions but soil conditions have changed and influence the 

 soil cover. 



To study the influence of soil treatment, grass, and weed growth 

 on young plantations, a series of experiments was instituted, 

 which showed that while the loss of plants on soil under a leaf 

 cover during the first two years was from 2 to 18 per cent., the 

 loss on soil on which grass had been sown among the plants was 

 four times as great, namely from 8 to 74 per cent., and on all but 

 one bed over 30 per cent., showing what great importance the 

 character of the soil cover has and accentuating the necessity of 

 reducing the competition of weed growth to the utmost by 

 proper management of light and treatment of soil. 



A long series of observations on a number of areas thinned to 

 varying degrees and systematically observed for five to twenty 

 years, described in detail, gave the basis for studying the influ- 

 ence of light changes on soil flora and accretion. To determine 

 the degree of light which different densities of crown cover admit, 

 the Bunsen-Roscoe-Wiesner method of measuring by means of 

 photographic paper was employed (see Vol. Ill, p. 167). It con- 

 sists of exposing sensitive paper for a given time (gauged by a 

 chronometer which allows reading to 0.2 second and estimating 

 to o. I second) and comparing with a standard color which was 

 secured by exposing at the same time under the open sky, so that 

 relative intensities were ascertained. By choosing cloudless days 

 and making a long series of measurements within one and the 

 same hour, beginning in the open and returning to it at the end 

 of the series, the needful checks and points for interpolation were 

 procured. The results (averages of 6 to 10 observations) are 

 expressed in fractions, the light intensity in the open being taken 

 as I. For instance the relative intensities on a beech area of 

 which parts were thinned to varying degrees, namely to 0.8, 0.65, 

 0.5 of the cross-section area, and one still more severe, were 

 found respectively to be 0.26, 0.38, 0.43, 0.48 of the intensity in 

 the open. The results have shown that the method is sufficiently 

 correct for practical use, as is readily seen when the results are 

 plotted. Some of these have been briefed in Vol. Ill, p. 167. 

 They show that even severely thinned stands absorb remarkably 

 large quantities of light (60 to 80%), and they accentuate the 

 difference in the light-absorbing power of shade-enduring and 



