Filling the tree
In United States Senate procedure, filling the tree is the process in which a piece of legislation in the Senate has all of its possible opportunities for amendments filled by the majority leader. It is not a new tactic, but has seen an increase in prevalence over the past few decades. It is also not always a surefire tactic: there have been cases where filling the tree has caused controversy and conflict on pieces of legislation.
The Senate majority leader has a traditional right to be recognized first for the purposes of offering a sufficient number of amendments on legislation. The term itself is a colloquial name for the diagram used to show the shaping of amendments to a bill. The trunk of the tree represents the bill, while the branches reflect the corresponding amendments.
Majority leaders fill the tree to introduce first- and second-degree amendments that block other senators from offering further amendments because the Senate cannot move on to another amendment without unanimous consent or overcoming a filibuster on the motion to put the other amendment before the body. Depending on the particular bill, one of four trees may be used: the first tree has room for three amendments, the second and third trees have room for five amendments, and the fourth tree has room for 11 (12 in rare instances) amendments. To fill the tree, none of the slots may be left available.
The majority leader must assess the risk in deciding to fill the tree. Some senators will reject a bill if they feel they have not been given an adequate opportunity to offer amendments. For example, Senator Susan Collins of Maine voted against the 2010 Defense Authorization Bill although she largely supported the substance of the bill, citing the filling of the amendment tree by Senate Majority Leader Harry Reid of Nevada.. Reid used this tactic during the Consolidated Appropriations Act, 2014 Senate floor debate, preventing amendments that would have removed the Wall st provisions that rolled back Section 716, (derivatives guarantees by the FDIC) of the Dodd-Frank legislation. 
Frequency of use
While filling the tree is not a new convention, it is becoming more and more prevalent according to a recent study. The study also concluded that filling the tree occurred more during and after the 109th Congress than in any previous Congress.
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