You may be responsible.There are two issues: the first is whether the obligation to "water the lawn" would be reasonably considered to have taken in the obligation to water the tree as well; the second is whether your failure to water more caused the tree's death.If the tree was not in a separate location, but was on the lawn which you were supposed to water, a court could conclude that the tree had to be watered, too--not that a court would have to find that, but it's possible. And if that were the case, and you're watering was inadequate, then you could be liable.(The reason a more definitive answer cannot be provided is that so much depends on the specific facts and the specific language of the lease.)Since you could possibly be responsible, it may be better to pay this cost rather than go to court to try to fight it.