TEMrERATURES JK WO0I>ED AND TREELESS REGIONS. 89 



(1) Tliero is one season only of iiiarked forest action and that is the 

 wann season. The action is most marked in the early summer months, 



(2) In the forest durinj>' tliis season the maxima of temperatures are 

 lowered, the minima are rais(Ml, the mean is lowered by a degree or two, 

 and the (hiily range is reduced by several degri'cs. Tliis action decreases 

 slowly up to the level of the foliage, tlien rapidly, disappearing at a score 

 or two of feet above the foliage. 



(3) The forest litter plays an important part in preserving the tem- 

 perature of the forest soil. 



(4) The vertical temperature gradient through the forest is reversed ; 

 it is greatest at the level of the foliage, decreasing slowly downwards 

 but rapidly upwards. 



(5) The diurnal amiditude increases from the margin of the forest 

 outwards to a distaui^e of a score or so of rods, where it reaches a maxi- 

 mum. The amplitude is also greater in glades. Hence the extremes 

 of temperature are exaggerated just outside the forest. 



(G) As a result of the discussion of tree temperatures and the organic 

 sources of warming and cooling in the plant, it maybe added that — 



(7) The changes of heat due to organic pi'ocesses are not sensible ex- 

 cept, possibly, the cooling due to transpiration. The heat used in this 

 process is an appreciable fi'a(;tion of the heat from the sun's rays and 

 the cooling due to it may lower to the point of fiost a temperature 

 already falling from general meteorological causes. 



TEMPERATURES IN WOODED A:N^D TREEEESS TERRI- 

 TORIES. 



In passing to the subject of temperatures and forests in its broader 

 as[)ect, namely, in so far as it relates to differences in wooded and tree- 

 less territories that are situated widely apart, the strict comparability 

 of the stations is lost. The stations to be compared are distant from 

 each other at different elevations above sea level, in diftVrent latitudes, 

 an<l, possibly, members of different meteoi-ologieal services, with all the 

 differences in instruments and methods which that implies. These dif- 

 ferences in the stations can not be so ])erfectly neutralized by ap]dying 

 correcti(»ns to the observations as entirely to satisfy the critical reader, 

 because the corrections are themselves unceitain and they often sur- 

 pass in quantity (for instance, the corrections for altitude) the amount 

 of change which forests might be expected to make. Besides, it is often 

 impossible to say when a ditference is found that this is due to the forests. 

 It may be due to other causes, for instance, the coolness of eva])oration 

 in regions of heavy rainfall, and the existence of the forest may be due 

 to the very difference found. These dilticulties may be evaded more or 

 less completely iu certain definite cases, and it is the published cases 

 of this sort which will now be given. The case of Vienna and its sur- 

 roundings, discussed by Dr. Ilann, is one of the most satisfactory. It 



