One way that you could do it would be to coat the leaves with a material that emits light.
Always remember as a rule of thumb that, in the CG world, you do not have to actually mimic reality; you only need to do something that convincingly appears to do so. The viewer is, of course, thoroughly familiar with the real world and will interpret what he sees in terms of the real world, as long as you do not present him with any obvious inconsistency.
Consider, for example, how they create the illusion in the theatre that a scene is in sunlit woods. The quality of the light … provided by a simple gobo on the lens of a spot … carries the effect. Actual trees are not required. It can be very informative to assist a professional studio photographer when s/he’s setting up a catalog or food shot. If you knew what that “tasty food” actually was, you would not eat it! “Models cry glycerin tears.”
You do not have to simulate the total effect perfectly. You only have to do it well-enough to be convincing, for this shot. There might be several light-qualities (or whatever) that might be present in the “real” scene, but only one or two of them are most noticeable. Do only those.
Every shot is different. For instance, any shot where translucency of leaves is very apparent is probably an extreme close-up. Okay, what is the quality of this shot that is going to make your eye say, “oh, looky, there’s sunlight shining through the leaves!” Now, try to list at least six different ways that you could “cheat” to get that.
When you “CUT TO” the next shot in the same picture, maybe a medium shot covering some bit of action, then you do not actually have to carry the effect consistently throughout. Having seen the effect the first time, the viewer remembers it and “sees” it even if it is not actually there. (Case in point: Star Wars Episode I actually shipped to theatres with a crowd-shot in the Pod Race sequence where the crowd actually consisted of painted Q-tips. No one noticed until a subsequent “Making Of” documentary purposefully exposed the trick.) As long as the play of light along the ground (gobos again…) is consistent with what the trees have been shown doing, you can safely cheapen the rendering.
Remember that in CG you can also do things in “layers,” shooting more than one strip of film and superimposing it seamlessly. Maybe you could do a general “bright wash of color,” maybe (literally) in Gimp or Photoshop, make that wash very translucent (low Alpha value), then sandwich that layer on top of everything else using Blender’s Sequence-Editor tool or a third-party tool like After Effects, Premier, or even iMovie.
There is no rule that says that a finished image must pop-out of the initial rendering pass, just as there is no rule that says you must simulate physics. Be creative in your cheating. It’s not cheating, after all.