310 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1953 



the first community to invade the bare site changes that site, making 

 it unfit for itself, but fit for the invasion of new species which in turn 

 kill out the species of the previous stage. Thus, as grasses succeed 

 annual weeds, and forbs succeed grasses, and these are ousted by 

 slu'ubs, and those by trees, each grcup replaces, and is replaced by, the 

 plants of the community adjacent to it in the sequence of plant 

 succession. 



The selective application of herbicides gave us the first experimental 

 "tool" in history to test this hypothesis. Previously, we could only 

 set back the stage of development by physically removing, e. g., the 

 trees of the supposed last stage. Actually such removal involved the 

 baring of the soil at that spot, which in effect was returning that spot 

 to the very first stage of succession, not to the preceding one. Basal 

 spraying gave us a remarkably precise tool with which to kill a species 

 and leave it in situ. Actually, the decaying roots are an extraneous 

 factor, and yet the resulting treeless community is a reasonable fac- 

 simile of the supposed preceding shrub stage. When and as the prin- 

 ciple of relay floristics applies in nature, it can be seen from figure 2 

 that removal of the tree stage is a relatively ephemeral phenomenon 

 and that the area will again be invaded by trees. Maintenance will 

 thus be a repetitive process of removing these invaders. 



From experimental field studies at Norfolk, Conn., came the first 

 indication to me that many of the native woody plants had not learned 

 their ecology lessons. Of 65 woody species that were spot-sprayed out 

 of various kinds of nonforest associations which supposedly they 

 had previously invaded, only half a dozen showed any urge to return. 

 All these few were trees, not shrubs ; and those were returning in such 

 abundance as no respectable "old field succession" had previously 

 known. 



Initial floristic composition was developed as a working hypothesis, 

 and it remains a hypothesis, to account for the fact that most of the 

 woody plants seemed incapable of invading, as seedlings, the grass- 

 land and shrubland stages. This hypothesis, graphically expressed 

 in figure 3, assumes that the weeds, grasses, forbs, shrubs, and trees 

 were all present on or in the soil at the time of abandonment, last 

 grazing, last fire, or last destruction, as seeds, seedlings, or shoot- 

 producing roots. Development through successive stages is then a 

 matter of unfolding that which was determined at the start. Weeds 

 at first outgrow and overtop all others, but soon the perennial grasses 

 become visually predominant. Eventually the coarse forbs take over, 

 through which the shrubs, originally present, eventually make their 

 way. Finally the trees, there from the start, overtop the other plants 

 and kill them out or relegate them to an inferior status. 



The economic importance of this hypothesis is at once apparent. 

 If the trees had invaded at the very start of vegetation development, 



