A Discussion of the Application of A Climato‐Logical Diagram, the Hythergraph, to the Distribution of Natural Vegetation Types

Perhaps no problem is of greater interest to the ecologist than that of correlating natural vegetation with climatic factors. In spite of this extreme interest, no great success has attended the many efforts in this direction. This may be due both to complications which arise from the effect of edaphic conditions in nullifying those of climate, and also because climatic data as at present recorded may not represent the most important factors which control the distribution of vegetation. Because of their obvious and limiting influence in this relationship, the factors of temperature and rainfall have received the most attention. One of the first vegetational-climatic classifications was that of Mayr (Baker '34) in which the vegetation was delimited according to temperature. Though the divisions made by him show a general conformity, they are only broadly applicable. Koppen (James '22) later developed a scheme in which the moisture relationship received greater attention. Because each of these schemes was based on but one of these two important climatic factors, they fail in those areas where the factor used is not the limiting one. Perhaps the most common of the schemes for classification in America is that devised by Merriam (1898). In this the summation of temperatures during the growing season was used to define the limits of life zones. Because of the obvious effect of the moisture factor, the United States, to which it applies, was divided into a moist eastern and an arid western division. Though this scheme is not soundly based (Livingston and Shreve '21), the divisions correspond rather closely with natural vegetation units. For this reason the nomenclature is widely used even though the basis of it is not held in high repute. The work of Livingston and Shreve ('21) was undertaken in a somewhat different manner from other studies in this direction. The United States was first divided into provinces which seemed to be units of vegetation and then the climatic factors responsible for this division were sought. The writers conclude that a better interpretation of climatic factors together with a more thorough understanding of the effect that these have on the physiological processes are needed before there can be any adequate correlations.