530 CORRELATION OF DISTRIBUTIONAL FEATURES. 



The ecological distribution of two species, Liriodendron tuUpifera 

 and Bulhilis dactyloides, has been investigated, as exemplifying the 

 methods that it would be highly desirable to extend to a much larger 

 number of species if the data were available for doing so. The aim 

 of securing the climatic data for the areas of relative abundance has 

 been to determine the optimum conditions for these species, as con- 

 trasted with the conditions existing where they are not so well repre- 

 sented in the vegetation. The relatively narrow ampHtude of condi- 

 tions exhibited by the central areas of ecological distribution points in 

 each case to the conditions of these areas as the optimal ones. 



In all five of the following cases our effort has been the same, whether 

 concerned with the ecological centers for growth-forms or for individual 

 species. The former have been determined entirely from floristic data 

 in the case of the evergreen broad-leaved and microphyllous trees, from 

 floristic data on ecologically important species in the case of the decid- 

 uous trees, and from purely vegetational data in the case of Lirioden- 

 dron and Bulhilis. 



Evergreen broad-leaved trees. — Owing to the comphcated nature of 

 the areas in which different numbers of trees of this group are found, 

 it has been impossible to construct a satisfactory figure to represent 

 graphically the limiting conditions for the several areas. By a com- 

 parison of the map showing the cumulative distribution of this group 

 of trees (plate 3) with the various climatic maps, it is possible, however, 

 to determine some of the conditions upon which an abundant repre- 

 sentation of evergreen broad-leaved trees is apparently dependent. 



Both in the West and the Southeast these trees are seen to be ahnost 

 wholly confined to the region with an average frostless season of more 

 than 180 days, and with no cold days in our sense. The Desert region 

 between Texas and southern California is nowhere occupied by more 

 than 10 species of evergreen broad-leaved trees, and extensive stretches 

 of it are occupied by less than 5 species or by none at all, although the 

 temperature conditions are analogous to those of the adjacent regions 

 to the east and west in which there are 10 or more species. The 

 eastern boundary of this group of trees is mainly formed by the limit 

 of Ilex opaca, while the boundary for 5 or more species is formed 

 by several intersecting limits. The position of the latter boundary 

 corresponds roughly with the line for a frostless season of 240 days, 

 while in the West and the East the areas with 5 or more species are 

 so situated as to have a daily mean of 40° or more for the coldest 14 

 days of the year. 



The physiological summation of temperature appears to have a 

 slight correlation with the cumulative distribution of this group of 

 trees in the Southeastern States, but such correlation is not borne out 

 on the Pacific coast, where the region with 10 or more species encounters 

 the same values of the sunmiation as those found in the Northeastern 



