BELATIVE RESISTANCE OF TREE SEEDLINGS TO EXCESSIVE HEAT 15 



While some injury was noted when the recorded temperatures were 

 as low as 120° to 130° F., this was only after long exposures, and 

 the fact that many seedlings lived through exposures of 170° to 180° 

 F. for from 6 to 8 minutes shows clearly, by comparison with the 

 results in moist air, that under ordinary atmospheric conditions the 

 internal temperatures of the leaves must be markedly kept down by 

 evaporation, as has been suggested by Bates (2, 3), in considering 

 the relation of transpiration rates to heat requirements. Since the 

 order of susceptibility of the species closely approximates that as- 

 sumed by Bates, it becomes fairly certain that a free rate of trans- 

 piration, as compared with a more restricted rate, offers a measure of 

 protection from the direct effects of very intense insolation. 



Under natural conditions, on exposed southerly slopes in the cen- 

 tral Rocky Mountains, maximum temperatures at the surface of the 

 soil as high as 154° F. haye been recorded, and this as late in the sea- 

 son as September 21. Temperatures as high as 160° F. may nor be 

 uncommon. In contrast, the maximum temperatures at the soil sur- 

 face on north exposures are little higher than the maximum air tem- 

 peratures, or from 100° to 110° F. From the mere fact of the occur- 

 rence of temperatures as high as 150° F. ? it is evident that extreme 

 heat, though of short duration and rare occurrence, may be an im- 

 portant factor in preventing reproduction generally and in drawing 

 a line between the natural habitats of the different species. TVTiere 

 soil drying accompanies high temperatures, as it almost invariably 

 does in nature, temperatures of only 130° F. may do- considerable 

 injury. With a moist soil there is some injury to the leaves of all 

 species at 130° F., but many individuals will escape injury until the 

 temperature reaches 150° F. or more. 



While lodgepole pine seedlings show great sensitivity at an early 

 age, their superior protection against direct heat injury to the needles 

 counterbalances in part any weakness which they may show in root- 

 ing, etc., if drying of the surface soil is involved. This is not suffi- 

 cient, however, to permit this species to occupy so warm and dry a 

 zone as that chosen by yellow pine. Spruce is quite sensitive from 

 either standpoint. Yellow pine is most capable of supplying itself 

 with water and is fairly immune to heat injury. Douglas fir is well 

 equipped to obtain its water^ but its leaves are most easily super- 

 heated. 



As between yellow pine and Douglas fir, whose habitats merge in 

 the central Rocky Mountain region, the ability of the latter to replace 

 the former depends almost entirely upon the critical temperature 

 conditions which the germinating seedlings must withstand at the 

 surface of the soil. It is known that Douglas fir and yellow pine 

 seedlings have almost equal root vigor, and the distinction between 

 the two is clearly due to greater tolerance of heat on the part of yel- 

 low pine. On south slopes, where the surface soil temperatures dur- 

 ing the dry periods of September and October often reach a maxi- 

 mum of 140° to 150° F., any survival of fir must be confined to spots 

 which are shaded at least a part of the day. 



The relatively great ability of lodgepole pine as compared with 

 spruce to withstand high temperatures is undoubtedly the deciding 

 factor in their relations on open, denuded areas. Lodgepole pine re- 

 produces splendidly on intensely heated sites within its range, pro- 

 vided only that its moisture needs are guaranteed by a lack of com- 



