208 



MINNESOTA BOTANICAL STUDIES 



NORMAL PRECIPITATION AT STATIONS WITHIN OR NEAR THE SANDHILLS 



Eastern Border 



Station 



County 



Length of record Normal precipitation 



Central Portio.n 



Western Border 



Soil-moisture is a very important factor in regard to the local 

 distribution of species and plant associations. Notwithstanding the 

 fact of rather great differences in rainfall at widely separated sta- 

 tions within or near the sandhills, soil-moisture does not appear to 

 present nearly so great a range in quantity as might be expected. 

 Quite apart from the precipitation, it is uniformly the rule that at 

 a depth of but a few inches below the surface of the sand, even 

 in the most exposed situations, and in the driest portions of the 

 hills, the sand is moist ; so moist in fact that at a depth of one foot 

 or less I have seldom found soil in the sandhill uplands during my 

 extensive travel through the region, so dry that it could not be 

 moulded in the hand. Whether this is due to capillarity alone, or as 

 Jentzsch (42) has shown, in part to an internal dew formation 

 within the dune, the fact remains that moist sand is always found 

 at a depth of a few inches. The great variations of diurnal and 

 nocturnal temperatures may possibly effect periodical condensation 

 of the water vapor held in the upper layers of sand and thus aid in 

 the maintenance of a reservoir of water near the surface. 



As has been said precipitated moisture is absorbed at once by 

 the porous soil and such rain water descends to a depth varying with 

 the texture of the soil. Evaporation, as soon as the storm passes, 

 results in a drying of the surface with great rapidity, but this is only 

 to a slight depth. The loose sand acts as a dry mulch to check 

 evaporation from greater depths. The rate and ease of water 



