The Utility of Precipitation-Effectiveness Formulas for Plant Ecology

Abstract

ANEW formula for precipitation effectiveness has recently been presented byJose Setzer.' fact that the effectiveness of precipitation depends on the evaporation, and this in turn on prevailing temperatures, necessitates a thorough study of the ways in which the several factors may interfere. Since Penck's first attempts, considerations of the interrelationship of the factors involved have been continually improved, and undoubtedly Setzer's suggestion regarding the greater effectiveness of temperature in the higher part of the scale is adequate to characterize the climatic situation of our country (Brazil). Generally, as in the present case, such data are immediately used as an explanation of vegetation types. However, we cannot rely solely on such formulas if we wish to understand the regional distribution, for example, of xerophytic and hygrophytic vegetation. There are other important factors that cannot be omitted, as a closer look at Setzer's exposition will bring out: The towns of Pirassununga, Sorocaba, Agudos, Aragatuba, and Itapeva possess the least humid climate in the state [of Sao Paulo]. Their annual effective precipitation is below the critical value of 64 on the Thornthwaite scale. natural vegetation is revealing. From Pirassununga to MogiMirim, from Sorocaba to Piramboia, around AraSatuba, and between Agudos and Campos Novos there are poor lands of savanna type. Shrubs are widely scattered and low (I2 to 212 or 3 meters when fully grown). There are many subxerophytic elements, and also some xerophytic ones (p. 254). At this point Setzer refers to a paper by the present writer and his colleagues.2 facts given by us, however, are different. vegetation found in the regions mentioned can be called neither xerophytic nor subxerophytic; such elements as Cactaceae and Bromeliaceae are almost completely lacking. plants that retain their foliage in the dry season are provided with broad and often delicate leaves, without heavy protection against excessive