Forum:Game Mechanics vs. Negate?

Here's the situation. My friend (Crystal Beast Deck) and I (Plant Deck) were having a duel. He had Ancient City - Rainbow Ruins on the field and it was my turn. I played Forest but then he activated the 3rd effect of his field card. (You can send 1 "Crystal Beast" monster you control to the Graveyard to negate the activation of a Spell or Trap Card and destroy it.) But the official rules/rulebook states that "There can only be 1-face up Field Spell Card on the field at any given time between both players. When a Field Spell Card is activated, the previous spell card is destroyed automatically."

I also looked at the Game Mechanics page and it said the effect could be negated if it's chained with a card that destroys it but in this case, can you negate a Field Spell Card (Forest) with another Field Card (Ancient City - Rainbow Ruins) that is automatically destroyed?