Basically, plant structural complexity could offer new habitats or resources for lizards. For example, more kinds of branches could provide niches at different heights to be occupied by different lizard species. This could be tested by surgically removing branches to see if lizard diversity declines, or by adding even more (artificial) branches to see if it rises.
Work Step by Step
The thing here is to be creative in thinking about how diverse plant form could provide opportunities for many lizard species. Testing this would be to remove or enhance whatever structures in plants were suspected of helping lizard diversity.