332 NATURE STUDY REVIEW 



may rise very slowly, a few inches each year. Some trees are 

 capable of resisting such an approach. The cottonwood, one 

 of the most common trees of the living dunes, is capable of pro- 

 ducing secondary or adventitious roots along its stem. Most 

 people know that if a cottonwood twig is put into damp soil, 

 it will put forth roots and grow into a tree. When sand piles 

 up slowly over trees possessing this power, new roots are pro- 

 duced on the stem above the level of what was once the earthline, 

 As the primary roots of the tree are buried too deep for respir- 

 ation these new roots take their place. This may go on indefi- 

 nitely so long as the sand does not rise faster than the growth 

 of the tree upward. Only the tops of the buried trees appear 

 like climips of vigorous young trees growing in the sand. But 

 we know what is happening down below in the sand, for seedling 

 trees could not get a start here in the drifting sand. 



Some trees such as the birch and basswood are very tenacious 

 of life under adverse circimistances and may live with their 

 roots buried very deeply in the sand. One of the accompany- 

 ing pictures (fig. 5) shows a birch which had been partially buried; 

 then the sand was blown away revealing the story of its burial 

 and successful struggle for life. Though not capable as the 

 cottonwood of putting forth adventitious roots it had retained 

 life with its roots buried at one time more than ten feet under the 

 sand. 



When the dune has passed or has ceased to move forward, as 

 sometimes 'happens, the surface sand becomes more stable, 

 moving only in a small way. Dune grass {Ammophila) gradually 

 binds the sand with its creeping roots and the surface finally 

 becomes sufficiently responsive to allow the germination of seeds 

 of trees and shrubs. If seed trees of the forest are near enough 

 for the transportation of their seeds to such spots a new forest 

 is soon begun on the now dead sand dune. The sand dune has 

 done its destructive work but nature is resilient and forests rise 

 again to clothe the wounds with their verdure. 



