If all the plants and trees died tomorrow, would we run out of oxygen? --- Zachary, Baytown, Texas
If all the plants, grasses, trees, etc. on land were to die tomorrow, it would be a very sad day. But it is not clear that we would run out of oxygen. That's because most of the planet's oxygen comes from algae. According to pages like this, "70% to 80% of all the oxygen we breathe comes from algae." If all the algae died too then we would have a problem. But as long as the algae is alive, we are probably OK.
Here is an interesting look at algae and its importance (the first five minutes is intro and can be skipped):