288 AMERICAN FORESTRY 



Comparison of wind in the open with that prevailing at a point distant 

 from the windbreak on its leeward side five times the height of the trees 

 sliowed the velocity of the wind reduced 80% by the most efficient wind- 

 breaks. The most obvious effects of this great reduction of wind velocity 

 are to prevent damages to crops by storms and to save the soil from being 

 borne away. Before the general planting of Osage orange hedges in Chase 

 County, Kansas, about 30 years ago a storm caused soil drifts to form 

 several feet deep which still can be seen. The reduction of evaporation due 

 to a decreased velocity of the wind is proportional both to the density and 

 height of the windbreak, and the protective influence increases with increased 

 wind velocity. The zone of greatest protection moves outward from the wind- 

 lireak with increase of wind velocity and the width of the protected belt 

 becomes wider. The belt of efficient protection averages two times the height 

 (;f the trees to windward and ten times to leeward. If the trees are 50 feet 

 tall they will protect a belt 600 feet wide. The percentages of moisture saved 

 varied in the observations from 12 to 40 per cent. 



It was discovered wholly unexpectedly that windbreaks possess great 

 value as heat regulators. All temperatures studied ranged upwards from 

 the initial temperature for growth, namely from 41° F. The highest daily 

 temperatures and the lowest nightly temperatures occurred at the place 

 where the windbreak retarded the movement of the air the most. The super- 

 heating and cooling effects were increased by increasing wind velocity, and 

 decreased in cloudy or rainy weather. It is probable that the effect at night 

 (luring the growing season is always beneficial by retarding transpiration and 

 thus checking the cooling effect of evaporation upon the leaves. Since 

 photosynthesis does not take place in the absence of light, transpiration during 

 the night could have no beneficial physiological effect; hence any influence 

 that will retard transpiration during the hours of darkness must be a bene- 

 ficial one. The superheating of the atmosphere in the daytime is most im- 

 portant in the spring and fall when the supply of heat is lowest. Soil 

 temperatures were highest in the zone of greatest protection during the 

 season of increasing temperature and lowest at the same point during the 

 season of decreasing temperature. The author concludes that the summation 

 of the diurnal and nocturnal effects during the growing season is a positive 

 quantity, and hence the average effect of a windbreak is to increase the 

 available heat to the plants growing in the protected zone. The effect of 

 the superheating of air and soil over the protected belt is to create hothouse 

 conditions on a large scale at the season when the plants need all the heat 

 they can get. Measurements of increased yields of corn attributible to super- 

 heating effect showed a gain in one field of 40 per cent in the zone of most 

 efficient protection and a gain of almost 15 per cent at a distance to leeward 

 of ten times the height of the protecting trees. 



Although some orchardists claim that windbreaks are harmful, it is cer- 

 tain that their effects in Nebraska in the season of 1908, were beneficial. 



