It is well known that shading a solar panel greatly decreases its output
It can be difficult to observe how surrounding obstacles will effect a solar system at different times of the year. This is because the angle of the sun shifts, and will be considerably lower in winter than in summer, meaning that smaller obstacles can be an issue where they were not over summer. Trees also grow (stop the presses) so the increase in height should be factored into shade planning, as you don’t want to get called out for an underperforming system when a branch is now covering a panel.
A great way to work out shading potential is using 3D modelling software like Sketchup. These programs allow the designer to virtually build a solar system and plot tress and other obstacles in their relative positions, before running shading simulations.