1921] Smiley: Flora of the Sierra Nenada of California 51 



than the actual elevation of the place of collection. This influence of 

 small bodies of water on local climates has been considered by Bart- 

 lett^*^ and Abbe." Bartlett's conclusions were deduced from obser- 

 vations made about Madison, Wisconsin, and perhaps have no other 

 significance for our subject than to give evidence that even small lakes 

 exert an appreciable control. Abbe's studies on mountain lakes led 

 him to think that they caused an amelioration of temperature about 

 them, especially when the adjacent slopes are at such an angle as to 

 catch the reflection from the water surface, and he was also of the 

 opinion that evaporation tends to produce fogs which may prevent 

 or minimize frost damage. My own observations about the high 

 mountain lakes of the Sierra tend to a different conclusion, for the 

 facts of plant distribution about their borders seem to indicate that 

 the influence of the lakes is always toward a lowering of the tempera- 

 ture of the surrounding terrane and this inference from the results 

 of plant collecting appears to be made more probable by a comparison 

 of the data available concerning the climate of Tahoe and Summit. 

 If we compare the mean summer temperature of Tahoe with that of 

 Summit, we find that the two places have practically the same amount 

 of heat (Tahoe, mean monthly temperature, May-October, 51.4° ; 

 Summit, 51.6°, data of 1911-17) ; though Tahoe is 800 feet less in alti- 

 tude than Sununit, its summer temperature is no higher than that of 

 the more elevated station. According to Woeikoff's law, Tahoe, as a 

 high mountain valley station, should have wanner siunmers than a 

 ridge station such as Summit, yet we find the temperature of summer 

 the same. The only reason for this depression at Tahoe, which is 

 apparent, is the lake. Evaporation from a water surface must pro- 

 duce a cooling effect; unfortunately no data are at hand to gauge 

 how far the smaller high mountain lakes may affect their local climates, 

 but for Tahoe we possess some information. Measurements made at 

 Tahoe indicate that evaporation from the lake surface may reach 

 several inches in the summer months : June, 3.80 ; July, 4.0 ; August, 

 6.5 ; September, 4.12 ; October, 2.65.^^ It appears extremely doubtful 

 that the high mountain lakes can ever modify the local climate by rais- 

 ing the temperature, as lakes in lowlands are known to do, since the 

 water of the boreal lakes is always cold. There never accumulates a 

 reserve of heat in the water of these lakes such as lowland lakes acquire 

 in the course of a summer. 



A certain effect upon zonal limits is exerted by topography in the 

 protection afforded by bold relief to snow banks and snowdrifts against 



