Figure 4.— Average 3-year height of seedlings by germination period. 



2.0 



MAY JUNE JUNE JULY JULY AUG. AUG. AUG. SEPT. SEPT. 

 24 8 21 5 18 2 16 30 14 27 



DATES AT MIDPOINTS OF GERMINATION MEASURING PERIOD 



year, height growth on the two shadiest plots definitely showed the effect 

 of shading. However, on all plots, 3-year height of seedlings was strongly 

 related to the date they originated (fig. 4) . At the end of the third grow- 

 ing season, surviving seedlings that resulted from seed germinating the 

 first week in June were 70 to 75 percent taller than those that came up the 

 last week in June, and they were 210 percent taller than those that origi- 

 nated the last week in July. 



At the 3-year measurement, utilizing only the three sunny plots, we esti- 

 mated that those seedlings that were over 2.0 feet tall were in a good com- 

 petitive position and would have a chance to survive to maturity. Fifty-six 

 percent of those that came up during May were in this class (417 per acre) 

 while only 14 percent of the stems originating in June were over 2.0 feet 

 tall (1,833 per acre). None of those that appeared after 1 July had 

 attained this height. 



Summary and Discussion 



The results of this study indicate that survival and height growth of 

 yellow-poplar seedlings, in competition with other vegetation that develops 

 simultaneously in newly clearcut areas, are dependent on time of germina- 

 tion. A high proportion of those seedlings that appeared before mid- June 

 competed well with the flush of new vegetation. Most seedlings that came 

 up after 1 July lost out in the race for growing space. 



The practical implication of these findings is that the old stand should 

 be logged between the end of the growing season (preferably after yellow- 

 poplar seeds ripen) and 1 May of the following year to favor yellow- 

 poplar reproduction. This would promote maximum germination and 



5 



