A topic from AEG's L5R CCG Rules forum, copied for Oracle of the Void from a copy of the forum saved in October 2015, so pages it didn't save are missing. Players' names are left out; the rules officials are named. Rulings are as they stood on these dates; later rulings, the rulebooks and the Rules Archive may differ.
A player, 5 September 2012
Deep Roots says "Battle: Choose your performing personality with 7 or more base Force. Negate all current and new force penalties on him and effects which prevent him from contributing Force. He contributes Force even while bowed."
Hida Horu says "Horu does not contribute Force to his army in battle resolution while there are two or more units in his army."
If I were to use Deep Roots on Hida Horu while he was in an army with other personalities, would that allow Horu to contribute force? I suppose the hart of the question is, is a game-state change due to a trait considered an "effect" for the purpose of these type of cards?
Ben Higgins (rules official), 6 September 2012
An effect is anything, other than a cost, that creates a card or other thing in the game, changes the characteristics or state of a card, stat or other thing in the game, adds restrictions or permissions to the game’s procedures, or creates a new instance of a game procedure.
Since traits do produce effects, and Horu's trait changes the characteristic of a stat (the ability to contribute Force), Deep Roots will negate the effects of his trait, allowing him to contribute his 8F to his Army's Force total even if there are other units there.
Exactly how awesome that is might be subject to some discussion however.