Growing Seasons of Cottonwood and Sycamore as Related to Geographic and Environmental Factors

Abstract

The hypothesis that variation in dates of the beginning and end of growing seasons of eastern cottonwood (Populus deltoides Bartr.) and American sycamore (Platanus occidentalis L.) can be attributed to variation in air temperature andphotoperiod was tested by using data collected during a single year (1963) from trees widely distributed throughout their natural ranges. Dates of bud breaking and winter-bud formation were used as the criteria for the beginning and end of growing seasons. Correlation and regression analyses were used to elicit the relationships of these dates with latitude, altitude, air temperature, photoperiod, and with the dates of frost occurrence at the locations of the sampled trees. The growing season in these two species was not confined to the period between killing frost in spring and fall. The lengths of growing seasons were inversely related to the latitude. With increasing latitude, growth began later and stopped earlier. Both air temperature and photoperiod at the time of growth initiation and cessation varied with latitude. At a given latitude between 30°N and 50°N cessation of growth occurred at higher temperature than that at initiation of growth, and growth cessation between 33°N and 50°N was associated with shorter photoperiods than those at growth initiation. Temperature and photoperiod, acting in combination, appeared to be important, although not exclusive, factors in controlling the initiation and cessation of growth of these species. The regression line of monthly mean temperature on photoperiod on the day of growth initiation appeared to delineate a boundary between the period of dormancy and the active growing season. The cessation of growth could not be as clearly defined.