It's an amazing achievement, to unlock this space for this kind of exhibit. The crowds I saw were drawn to the sheer strangeness and hugeness of the shapes of the trees, which are supposed to link the ideas of deforestation and climate change. Angela Palmer has done something remarkable in persuading the Mayor's office to let her use this space for this work. Its scale and ambition makes the current occupant of the Fourth Plinth look rather irrelevant.
But, being honest, I'm not sure it works that well, either as a polemic or as art; I'm not sure it left people convinced. Palmer had originally envisaged the stumps as standing straight up, which would have made it easier to understand them as the leavings of human greed, rather than the lumber they look like. I'm guessing that it simply wasn't practical to display the stumps like that. And the huge text billboards seemed to be as much about Palmer's struggle to realise the work, with Antony Gormley saying "the project can't be done", as they were about the issue of deforestation and simply added a level of Fitzcaraldo-in-reverse hubris. (This is like dragging the rainforest to the opera-house rather than vice versa).
When artists create events like this why don't they let the art speak for itself and instead work closely with an NGO who can make the polemic explicit on site, and far more effectively?