How do you measure “efficiency” ? By money spent? Then yes, trees are currently cheaper. But trees are complicated (see other comments). Additionally, even if we cover the whole landmass with trees, there is still a catastrophic amount of Co2 left in the atmosphere.

I mean just looking at the amount of concrete in that picture, I get pessimistic. When will this particular site have dug itself out of the carbon “hole” created by its construction?

As for trees: That is really, really hard to measure and even harder to know in advance. Some factors appear to be:

• different tree species store different amounts of carbon
• tree plantation or actual forest?
• prior use of the site (e.g. meadows do store carbon too)
• development over time (most trees need to grow a couple years before they start storing significant amounts of carbon)
• failure of sites due to being planted in a bad way (e.g. a lot of Chinese Green Wall sites and quick-buck billion tree projects seem to be affected by this)