AIs that rely on MCTS - like AlphaGo - create their decision tree as the game progresses. Do they start from scratch each game and build a new tree or do they keep the tree and grow it from game to game further?
Besides the possible limitations of storage space for the search tree, I don't see any obvious drawback in keeping and growing the tree, which seems to me to be the preferred option. Are there other reasons to start from scratch each game?