190 Forestry Quarterly. 



end of the mesa. The outermost parts of the mesa have sparse 

 growth of small trees of various sizes. 



In accounting for these well-defined groups of age classes with 

 few intermediate ages, the author points out that the trees are 

 in the tension zone between forest and grassland where seedlings 

 in particular are sensitive to slight changes in environmental con- 

 ditions. Favorable conditions for growth are not present every 

 season. Thus during some seasons and even for a series of years, 

 the climate may be comparatively mild and moist. If this period 

 happens to coincide with an abundant seed year, trees will be 

 established on the mesa far beyond their ordinary range. Then 

 may follow a series of dry and cold winters in which only well 

 established seedlings, those that are at least five years old, can 

 persist. 



The pines do not invade the plains below the mesas because the 

 soil may be too dry for the seedlings to get a start. On the mesa 

 the seedlings often establish themselves under the protection of a 

 rock and regeneration is most abundant on the north slopes 

 where the soil moisture is most favorable. Competition with the 

 grass is another important factor. The coarse soil of the mesa 

 does not lead to such complete control of the grasses as does the 

 fine-grained soil of the plains and the trees establish themselves 

 on the mesa in the open places where the seeds can get down to 

 the mineral soil. While the mean temperatures of mesas and 

 plains do not differ greatly, yet the daily extremes are consider- 

 ably greater on the plains. It is very probable that tree seedlings 

 on the plains are killed by late spring frosts. 



A bibliography of Colorado botany is appended which con- 

 tains references to twenty-nine papers relating to the forests of 

 Colorado. 



The thoroughness of the preliminary work in Geology, Topo- 

 graphy and Climatology as well as the method of treatment of the 

 distributional studies make this work a model which future plant 

 ecologists would do well to follow. 



C. D. H. 



Studies in Mesa and Foothill Vegetation. I. The University of Colo- 

 rado Studies. Vol. VI, No. 1. 



