14 



Combination No. 2. 



Norway spruce and white cedar, where conditions are suitable, might 

 be planted in alternate rows. After 30 to 35 years, the Norway spruce 

 begins to overtop the white cedar, but the combination makes one of 

 the most efficient shelterbelts that can be grown in Iowa. 

 Combination No. 3. 



Any broadleaf species suitable for the region might be used on the 

 interior rows of the plantation, these rows to be flanked by the plant- 

 ing of any evergreen suiting the locality, in the outside rows 1 and 9. 

 Combination No. 4. 



Alternating rows of European larch and Norway spruce. The 

 European larch when planted alone makes a rather open stand. The 

 tolerant Norway spruce, being of slower growth, will form an under 

 story which will keep out grass and weeds, benefiting both species. 



VARIATIONS *IN THE METHOD 



One block, making up from one-fourth to one-half of the planta- 

 tion, might be renewed in one year, and the balance of the plantation 

 in the years immediately following. This would distribute the plant- 

 ing over several years, which might be advantageous under certain 

 conditions. 



REGENERATION BY ALTERNATE ROWS 



The method of regeneration by planting alternate rows may well be 

 used in shelterbelts where the original spacing is wide. Many cotton- 

 wood plantations might be renewed by this system. Disadvantages 

 are to be found, however, in the fact that only the shade enduring 

 species can be used for planting between the old rows left standing 

 after the first cut and, later, when the remaining old trees are re- 

 moved it is practically impossible to fell the timber without damag- 

 ing the young trees which have been planted. 



Under this system the entire stand of old trees is removed in two 

 cuttings. The second cutting should generally follow the first after 

 about 10 years, but this period can either be shortened or lengthened 

 to suit conditions (see figs. 15, 16 and 17.) Fairly good windbreak pro- 

 tection will be afforded until time for the last cutting of the old tim- 

 ber. At this time, unless the period between cuts has been of con- 

 siderable length, the efficiency of the windbreak is likely to be impaired 

 for a number of years, inasmuch as the tolerant trees which are used 

 in replanting are generally of slow growth and will not be tall 

 enough to give much protection against the wind in a period less 

 than fifteen or twenty years. 



This system could be used in certain cases in cottonwood or willow 

 plantations which are rather open. As a rule, it could not well be 

 used in dense plantations of either soft maple, boxelder or other 

 species forming a very heavy shade. For the reasons already given, 

 this system is not as desirable as some of the others already described. 



SPECIES USED IN REPLANTING 



A list of trees suitable for replanting by this method will be re- 

 stricted, except under exceptional cases, to those which are tolerant 

 of shade. This list will include: 



Norway spruce Hard maple 



White cedar Soft maple 



Red cedar Basswood 



White spruce Boxelder 



In no case should trees intolerant of shade be used except where 

 it is evident that they will not become overtopped by the adjoining 

 trees. It might sometimes be possible to use a fast-growing, intoler- 



