Re: Where are the trees.
There is actually still no evidence either way....
What evidence there is is that Mars lost its magnetic field early, if it ever had one strong enough to fend off solar wind, and that it had active volcanism and seas of liquid water. Basically, between the volcanism and the solar wind, it literally boiled itself dry...
The big question is: How long was the intermediate period where Mars did tick the boxes we currently understand are needed for Life to appear? ( And please bear in mind that those boxes amount to a set of circumstances that are utterly lethal to us.) And did those circumstances last long enough for life to proliferate and leave a mark we can recognise?
Remember.. early life on earth did not need light or oxygen. In fact, the latter was, and is, utterly lethal to even current anaerobic organisms running the same processes those early ancestors did. Early earth life consisted of chemotrophs living in a hot and acidic environment, exactly like the black smokers and other underwater volcanic vents nowadays on Earth. And their telltale deposits are the oldest fossils we have found.
So did Mars have a period where there were oceans with volcanic vents/fissures? yes. Did it last long enough to let early/proto life appear? Probably, especially since we're finding out that life on Earth started way earlier than previously thought, and that that start in and of itself is ridiculously "simple".
Will we ever find proof of this? If life started and lasted long enough, there should be some fossil telltale somewhere, if Mars' relentless erosion hasn't destroyed the evidence already. So good luck finding it.
Would it be possible that there's still life on Mars? Only underground if there happens to be enough water deep-surface. Just as here on earth, stuff could have survived deep underground. Again.. good luck finding it.. Only an Armageddon-style drill setup can get that deep.
So you can't say life never existed on Mars, since the conditions to let it appear certainly were there, and possibly long enough to leave a telltale mark. Have we found any of those marks? No, so there's no proof-positive either.