ECOTYPIC VARIATION IN PHOTOPERIODISM OF TREES WITH SPECIAL REFERENCE TO PINUS RESINOSA AND THUJA OCCIDENTALIS

A certain daylength treatment inhibited the growth of northern seedlings of Pinus resinosa and Thuja occidentalis more than it inhibited southern seedlings of these species. These treatment – seed source interactions were small, but otherwise similar to those found earlier in 29 northern tree species. The smallness of the interaction was caused by the overruling effect of the endogenously determined growth pattern in P. resinosa, and by the lack of large photoperiodic response in T. occidentalis.The broad subject of ecotypic variation in the photoperiodism of trees is discussed. The evolution of dormancy mechanism suggests that photoperiodism should tend to be overruling in the truly northern trees, and the endogenous rhythm in trees from the middle latitudes.