We've seen a number of cases where Smokie could not access people when living trees of earth were in the way. In the beginning of the episode Jack lands in a bamboo field and Christian (aka Smokie) sends in Vincent to wake him up because it seems for some reason he can't go into the Bamboo forest. In other cases, we have seen characters hide within Banyan tree roots.
This has remained an unopened question for the entire season and does not appear to be that its going to be reconciled.
I believe the reason that Smokie cannot access people hiding in these living things is because Jacob controls the Earth. He rules over the living objects/ the living objects of Earth on the island.
So Smokie basically can't cross this Element border....because its controlled by his brother.
The same is true for water. However, Jacob does not control water. Water will be controlled by someone else. This is all goes back to the fundamental description of classical elements by the ancient Greek philosopher of Aristotle, et. al. This is described in this thread
Besides the philosophical reference point to these elements, there is all the wiccan and pagan mythological reference points to these elements. In either case, it is my belief, that smokey cannot cross over classical elements for which he has no control over. When the living trees are very close together such as is the case of the banyan tree or the bamboo, it becomes a fence/border for which he cannot penetrate to the other side.