CLIMATIC FACTORS 



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Picea excelsa; in southern Europe, Quercus ilex — all species with small 

 light minimum. In north central Utiited States (Minnesota) the 

 Ahies halsamea-Picea canadensis forest, giving the densest shade, tri- 

 umphs over all other plant communities as the climatic climax. 

 Somewhat farther south F. grandifoUa, Acer saccharum, and Tsuga 

 canadensis together form the beech-maple-hemlock climax forest. 

 The relative light intensity, a quantity related to the relative light 

 minimum, was graphically represented 

 by Lee (1924) for several forest types 

 in Minnesota (Fig. 60). 



The arrangement of forests according 

 to light intensity corresponds in general 

 to their order in the succession. e 



Brown (1919) studied the signifi- / 

 cance of the light factor in the origin 

 of plant communities in the Philippines. 

 MacLean (1919) made significant stud- 

 ies of the light relations in the virgin 

 lorests of southern Brazil, finding that 

 the Hght distribution was verj^ uneven. 

 The available light under the many- 

 layered canopy averaged about H40> 

 but sunny spots at times received 

 as much as }i. Usually the shrub layer cast the densest shadow, 

 since its close leaf mosaic intercepts more hght than the filmy upper 

 canopy in which the fine leaflets of the Mimosaceae predominated. 



In central European forests, plants may bloom in l^o of total Hght, 

 and fruit in } iqq (Kastner). Sterile plants and seedlings seem to 

 thrive in caves with much less light. Mosses extend their range to 

 1/2,000 (Leskeella) ; Algae such as Gloeocapsa and Protococcus, to about 

 1/2,500. Adiantum capillus veneris was found by Morton (1927) 

 in the Quarnero caves at 1/1,700. In that case it seems that higher 

 temperature can to some extent compensate for lack of light. 



Submerged vegetation of lakes and seacoasts shows a pronounced 

 zonation due to light (r/. p. 246). Pearsall (1917-1918) has traced the 

 hght zonation of the communities of higher plants in the English lake 

 district (Fig. 61) . In this region the Nitella flexilis association endured 

 the least illumunation. In Lake Constance, Zimmerman (1927) found 

 the following distribution of algae primarily dependent upon light: the 

 Spirogyra adnata association to a depth of 10 m.; the Cladophora 

 profunda-Chamaesiphon incrustans association between 10 and 20 m.; 

 and the Hildenbrandtia rivularis-Bodanella lauterhorni association from 



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Fig. 60. — Relative light inten- 

 sity in different forest types in 

 Minnesota: a, mature Pinus bank- 

 siana forest; b, mature P. resinosa 

 forest; c, mature P. strobus forest; d, 

 young P. resinosa forest; e, young 

 P. banksiana forest;/, pure deciduous 

 forest; and g, mature Abies-Picca 

 forest. {After Lee.) 



