A Climatic Index to Vascular Plant Activity

Major, Jack

SUMMARY: "Actual evapotranspiration" as used in Thornthwaite's treatment of climates is suggested as a measure of plant activity. It is the resultant be- tween the need for water, as expressed by calculated potential evapotranspiration, and current precipitation plus available water stored in the soil. The seasonal courses of potential evapotranspiration and precipitation at Cloverdale, California, are discussed as an example. Because precipitation and the need for water are just out of phase at Cloverdale, luxuriant growth of vegetation in spring is followed by drought in summer, by a brief period of fall growth, and by little plant activity during the cold of winter. This kind of Mediterranean climate shows both a large excess of water and strong drought, in winter and summer respectively. Other climates where lack of heat alone limits plant growth, where lack of water is limiting, and where seasonal drought alternates with seasonal plenty of water in a hot climate are discussed and diagrammed. The importance of local climates in modifying the effects of regional climates is pointed out. It is concluded that there is at least a. qualitative correlation between loss of water from ecosystems and luxuriance of plant growth.