Whereas the poet has compared his life to a ship's voyage, "Terminus" compares this life to a tree which has reached its limits of growth and must send out no new branches or root. In what ways do both comparisons represent a human life?

One of the meanings of the word "terminus" in the Merriam-Webster Dictionary is "a final goal, a finishing point." In the poem "Terminus," Ralph Waldo Emerson writes of old age and the end of human life. He advises old people who are approaching death to slow down and focus on accomplishing few goals instead of many because they have to make wise use of the time and energy that they have left. To illustrate his point, the poet compares life to a sailing ship and to a tree.
In using the analogy of life as a ship sailing the sea, Emerson writes that in old age the ship is near its port. In other words, it has come close to the final destination of its journey. When a ship is nearly in port, it has to slow down. The sailor who guides the ship takes in or reefs the sails, which reduces speed, and uses the rudder to guide the ship safely into port. Emerson is here saying that, when you are nearing the end of your life, you have to slow down and focus on what you are doing. It is important to "accept the terms" of the "Giver," which is a reference to God.
The analogy to a tree that must shoot forth no further branches or roots is similar in that it refers to the necessity of conserving strength and completing what has already been begun. Emerson alludes to this when he writes that the tree should "mature the unfallen fruit." In reference to human life, this counsels you to concentrate on completing things you have already started instead of beginning new things that you will not have time to finish.