The present study represents an effort to explain so far as possible in the light of present knowledge the composition and occurrence of forest associations in the Harvard Forest. The Harvard Forest in Petersham, Massachusetts, has been maintained as an experimental area since 1907. The long-term research program, which has been centered here for more than forty years, is based upon the belief that the history and development of forest communities can best be understood by selecting a small area-some 2,400 A in this case-and recording at intervals the composition, volume, and other characteristics of each community as well as detailing all cutting operations and other disturbances. The study is restricted to the first 40-year period of University management, that from 1907 to 1947; and in general to those parts of the Forest which have been owned by Harvard University since the start. More specifically, it deals with Compartments I through VIII of the Prospect Hill Block, I through VIII of the Tom Swamp Block, and II through X of the Slab City Block. A total of 1,852 A of land are covered, of which 763 or 41% are in the Prospect Hill Block, 642 or 35% are in the Tom Swamp Block, and 447 or 24% are in the Slab City Block. Figure 1 shows the location of the portions pf the Forest included in this study. Elevations range from 700 to 1400 ft above sea level. The study is further confined to the composition aspects of the forest. The problem is exceedingly complex. The existing forest stands are small and highly variable. They owe their present-day existence to the interaction of a great many factors, among the most obvious of which are: the migration of tree species into the area; pre-colonial distribution of species as governed by site, fire, wind-damage, and other agencies; land-use history from the time of settlement; soil formations; depth of the water table; aspect and elevation of the site; variation in local climate between one point and another and between types of forest stands; the influence of insects and diseases such as the gypsy moth, the white pine weevil, and the chestnut blight; and the impact of fires and climatic agencies, especially the great hurricane of 1938. Many other factors could be named. . Because tree species are commonly identified by their common names, these names are used throughout. Scientific names are given in Table 4 or in the text. During the 9 years of this study, a number of facets relating to the composition of the Harvard Forest were examined (Spurr 1946, 1950). Aspects that have been previously published, and which consequently are outside the scope of the present paper deal with the vegetational significance of recent climatic changes (Spurr 1953), stand composition following the 1938 hurricane (Spurr 1956a), the relation of local climate to forest composition (Spurr 1956b), the effect of cutting practices on forest com-