That time of year thou mayst in me behold
When yellow leaves, or none, or few, do hang
Upon those boughs which shake against the cold,
Bare ruined choirs, where late the sweet birds sang.
In me thou see'st the twilight of such day
As after sunset fadeth in the west;

 If the speaker is comparing himself to a tree, then the “boughs” are a metaphor for his ageing body, which is slowly losing “leaves” in the way that he might be losing his faculties. 
 Note the clever word-play. A bare bough may be thought of as spear-like in appearance. It is described as shaking against the cold. So this could be a secret reference to Shakespeare himself. 
 Shakespeare enjoyed word-play and puns.  Compare this with other humorous references to his name in  Sonnet 135  and  Sonnet 136 .
Sonnet 73