The Hunger Games: Catching Fire

After winning the 74th Hunger Games, Katniss Everdeen (Jennifer Lawrence) and Peeta Mellark (Josh Hutcherson) return home to District 12. President Snow visits Katniss at her home. The two make an agreement to not lie to one another, and Snow explains that her actions in the Games have inspired rebellions across the districts. He orders her to use the upcoming victory tour to convince him that her actions were out of genuine love for Peeta, not defiance against the Capitol, otherwise Katniss's loved ones will be killed. He shows her the clip where Gale kisses her as a warning that they are watching her.
As the tour begins, Haymitch Abernathy, Katniss and Peeta's mentor, warns them that the "show" of their relationship must continue for the rest of their lives. Katniss suggests a public engagement between herself and Peeta, which is carried out and approved by Snow at his mansion in the Capitol.
Returning home, Katniss warns her friend Gale Hawthorne of Snow's threat to kill both their families. Peacekeepers crack down on District 12, and Gale is publicly whipped after attacking new Head Peacekeeper Romulus Thread. Snow announces that the upcoming 75th Hunger Games, the Third Quarter Quell, will feature tributes selected from previous victors. Katniss immediately devotes herself to ensuring Peeta's survival, and recruits Haymitch's help to do so. At the reaping, Effie Trinket draws Katniss and Haymitch's names, but Peeta immediately volunteers to take Haymitch's place.
As they prepare, Haymitch reveals that all the tributes are angry about being returned to the Games and advises a reluctant Katniss to make allies. In the pre-Games interviews, Katniss wears a wedding dress, as ordered by President Snow, but her stylist Cinna rigs it to transform into a representation of a mockingjay. Peeta announces that he and Katniss have secretly married and are expecting a baby, which causes outrage, and the audience of Capitol citizens begins shouting for the Games to be stopped, to no avail. Just before Katniss enters the arena, Cinna is severely beaten by Peacekeepers, as punishment for the dress, and dragged away as Katniss watches helplessly.
In the Games, Katniss allies with Finnick Odair and the elderly Mags, from District 4. The arena's outer force field shocks Peeta, stopping his heartbeat until Finnick revives him. The group flees from an emerging poisonous fog, and Mags sacrifices herself so as to avoid slowing them down. When vicious mandrills attack, Peeta is saved by the unexpected sacrifice of a tribute from District 6. The group escapes to a beach and meets District 3's Wiress and Beetee, and District 7's Johanna Mason. Wiress discovers the arena is designed like a clock with regular hazards each hour, but is killed soon afterward by Gloss and Cashmere, the brother and sister tributes from District 1, who are in turn killed by Katniss and Johanna, respectively.
Beetee suggests using one of the hazards, a tree that is struck by lightning every 12 hours, and a coil of wire to electrocute the other remaining tributes. The group separates to prepare the trap, but once alone Johanna suddenly hits Katniss over the head, stunning her, and discreetly cuts the tracker from her arm. Katniss returns to the tree and finds an unconscious Beetee with the wire from the tree attached to a makeshift spear. Unable to find Peeta, Katniss almost attacks Finnick, but Finnick reminds her to remember "who the real enemy is", as Haymitch had advised her prior to the Games. Katniss attaches the remaining wire to an arrow and shoots it into the arena's force field as the lightning hits, causing a power failure that takes down the dome's force field as well as the Capitol's surveillance.
Katniss awakens in an aircraft to find Haymitch, Beetee, Finnick, and Plutarch Heavensbee, the head Gamemaker, who is revealed to be a rebel against Snow. He tells her they are bound for District 13, headquarters of the new rebellion, and that half the tributes were aware of the plan to escape with Katniss, who symbolizes the growing rebellion. They were unable to rescue Peeta and Johanna, as they still have their trackers on them. A distraught Katniss is sedated after attacking Haymitch. She later awakens to find Gale by her side, who reassures her that her family is safe but tells her that District 12 has been destroyed by the Capitol. The film ends with a close up of Katniss' eyes, as her expression changes from desperation and panic to one of steely resolve.

Question: Who suggests using the tree hazard?
Answer: Beetee