REVIEWS 731 



lion. With rainfall, the extent of erosion and run-off depends on the 

 rate at which rain falls, the steepness of slope, the presence of estab- 

 lished gullies, the character of the soil, and the density and character 

 of the vegetation. 



Studies of plant growth brought out that erosion is detrimental to 

 plant growth because of lack of adequate soil moisture and lack of 

 l)lant nutrients due to the reduction of soluble plant foods. On eroded 

 soil a new series of succession takes place, and to re-establish the more 

 desirable and permanent species that occupied the soil before depletion 

 requires years of time and good range management. The amount of 

 organic matter affects greatly the water-holding capacity and is shown 

 in the little erosion from fully vegetated lands except during intense 

 rainfall or prolonged heavy rain, and then erosion is not serious. De- 

 nuded or sparsely vegetated slopes may. after small storms, have both 

 run-off and erosion. General observations show moderate sheep-graz- 

 ing on sparsely vegetated range increases the run-off and erosion when 

 the physical factors are favorable to erosion and where erosion is al- 

 ready in the incipient stage. The seriousness of erosion is largelv 

 determined by the extent to which ground cover is maintained, and this 

 cover may be destroyed and serious damage result from overgrazing 

 or mismanagement of stock. Deferred and rotation grazing should be 

 practiced and the stock kept under control at all times, with slight 

 changes as erosion becomes manifest. \\'ith erosion once under way, 

 mechanical methods are necessary to assist nature. 



E. N. M. 



Limiting Factors in Relation to Spccitic Ranges of Tolerance of 

 Forest Trees. By A. H. Hutchinson. Botanical Gazette, vol. 66, pp. 

 465-493. 7 figures. December, 1918. 



The range of forest trees in Canada is correlated with available in- 

 formation on soil and climate. No quantitative data are given. Tem- 

 perature, moisture, and soil are regarded as the factors most commonly 

 limiting the distribution of trees ; but it is shown in a number of specific 

 cases that other factors, such as light, competition, and the time ele- 

 ment, may be of great importance. Thus, the southern range of Abies 

 halsamea is sometimes determined by competition with Acer and T^uga. 

 Larix americana is often forced by competing species into habitats 

 which they are unable to occupy. This is generally true of species hav- 

 ing a wide range. That the northern range of trees is not alwavs de- 

 termined by temperature is shown by the fact that the lines marking 



