Damage remains marked on creatures until the turn ends. If Fecund Greenshell’s second ability stops applying (because Fecund Greenshell leaves the battlefield or loses its abilities or because you no longer control ten or more lands), then any creatures that needed the toughness bonus to stay alive will die.

Fecund Greenshell’s last ability checks the power and toughness of a creature only at the moment it enters. If it enters with counters, those counters are included. Static abilities that modify the power and toughness of creatures you control (such as Fecund Greenshell’s own second ability) are also included. If that creature’s toughness is greater than its power when it enters, the ability will still trigger and resolve normally regardless of what happens to that creature after that.

If the top card of your library is a land card, you may choose to put it into your hand rather than onto the battlefield tapped.