No. 1, July, 1920] ECOLOGY, PLANT GEOGRAPHY 25 



tation, crops, etc. Four maps of Georgia are given showing geographical divisions, soil 

 types, tree (lis! ribul ion and leadin -, as an illusi rat ion of how I his met hod of mapping 



is to l>e applied. — /'. /;. Strausbaugh. 



188. Haiitzell, 1'. %. Comparison of methods for computing daily mean temperatures: 

 effect of discrepancies upon investigations of climatologists and biologists. New York Agric. 

 Exp. Sta. [Geneval Tech. Bull. 68: 1 35. PL 1 . <ig. 1-19. 1919.~UBing the data from a 

 complete thermograph record of the temperature at Fredonia, N. Y., for the >■ ar 1916, the 

 author has calculated mean daily, mean monthly, and mean annual temperature by different 

 methods and made a statistical study of the results. Means based on maximum and minimum 

 temperatures were found to differ more or less from means obtained by dividing the sum of 

 the hourly temperatures for each day by twenty-four (thermograph averages). This differ- 

 ence varied considerably with the hour at which the reading of maximum and minimum tem- 

 peratures was made, being greatest for 5 p.m. readings, less for 8 p.m. readings, and least for 

 midnight readings. Assuming the thermograph average to be the true mean, the error of 

 daily means calculated by the other method may be so large as to make them practically worth- 

 less for use in biological investigations requiring close comparison of temperature data from 

 day to day. Even monthly means so calculated may be so inaccurate as to introduce import- 

 ant errors into ecological work; but the error of the annual mean is negligible. The study 

 also brought out clearly the fact that temperature averages admit of comparison only when 

 calculated from readings taken at the same hour. — F. C. Stewart. 



1S9. Hesselman, Henrik. Om vara skogsforyngringsatgarders inverkan pa salpeter- 

 bildningen i marken och dess betydelse for barraskogens foryngring. [Influence of silvicul- 

 tural practice on soil nitrification and its importance in the reproduction of coniferous forests.] 

 Skogsvardsforen. Tidsskr. 16: 1-104. 1918. The lack of reproduction in many Swedish co- 

 niferous forests is due, not to any moisture relation, but to a lack of available nitrogen, the 

 transformation of nitrogenous compounds ceasing with the production of ammonia. These 

 soils are mostly acid in reaction, and are not favorable habitats for nitrifying bacteria, es- 

 pecially Azotobacter, which requires soils rich in certain salts, especially calcium. Various 

 lichens and mosses, so abundant in these coniferous forests, are indicators of nitrogen de- 

 ficiency, as are such plants as Airajlexuosa, Arctostaphylos uva-ursi, and species of V actinium. 

 Among the indicators of abundant available nitrogen are Epilobium angusti folium and Rubus 

 idaeus. [See Bot. Absts. 2, Entry 653, also full review by Pearson in Jour. Forestry 17: 69- 

 73. 1919.]—//. C. Cowles. 



190. Hutchinson, A. H. Limiting factors in relation to specific ranges of tolerance of 

 forest trees. Bot. Gaz. 66: 465-493. 7 fig. Dec, 1918. — From data accumulated in personal 

 contact with the forests of northern Ontario, supplemented by the records of such Canadian 

 explorers as Bell, Macoun and Low, the author considersthe factors efficient in limiting the 

 range of the principal trees composing the forests of eastern Canada. The northern limits of 

 20 species are plotted upon maps showing temperature and precipitation distribution, and 

 various peculiarities of these limits are discussed. Temperature seems to explain the north- 

 ern limits of a few species only, the principal ones being Picea mariana, Larix americana and 

 Bctula papyrifera, although for these species another factor seems to be involved in Labrador. 

 Water is regarded as the limiting factor in the western extension of Acer saccharmn, Tsuga 

 canadensis, Fagus americana, Thuja occidenialis and Ulmus americana, but other species show 

 irregularities which do not correspond in the least with the water supply. The most import- 

 ant conclusion regarding the time factor in relation to temperature and soil development is 

 embodied in the statement that "deciduous hardwood forest is encroaching upon the conif- 

 erous forest region, and that the progress of this encroachment has lagged behind tempera- 

 ture changes, being now dependent upon the rate of soil development." — There follows a 

 discussion of the ranges of tolerance for various species. In this discussion it is interesting 

 to note that Abies balsamca is regarded as possessing a wide water range but as seldom thriving 

 except in very moist soil on account of fungous diseases of the roots in drier soil. Picea cana- 

 densis and Picea mariana, possessing the same northward range, are frequently separated by 



