ENVIRONMENTAL GRADIENTS IN A SIMULATION MODEL OF A BEECH - YELLOW-POPLAR STAND.

A simulation model for growth and succession of a hypothetical American-beech - yellow-poplar forest has been developed to study changes in response of this simple community under conditions of a slowly varying climate. As the temperature, through a model parameter equal to the number of growing degree days (GDD), is increased, a sharp discontinuity in response of the model is noted at approximately 4500 GDD. A similar discontinuity is observed at about 4800 GDD as the temperature is slowly decreased. This hysteretic response with width of 300 GDD can be compared for consistency against the rather sharp boundaries which occur between forest communities along smooth environmental gradients. Although many environmental gradients are responsible for transitions in natural systems, the model calculations described indicate that changes in a single environmental variable, temperature, can account for transition zones by affecting competitive ability.