Nice pic, Blue Spawn! You're off to a great start there!
Here's a few suggestions to take off some of the work that's obviously confronting you for that scene...
I've done a number of pics that entail vast amounts of detail over a large area, as yours seems to be. Now, if you were to literally try to place thousands of detailed trees over the extent of that landscape, unless you have a computer that makes HAL look like a pocket calculator, it's not going to happen - the voxel tree, or octree (the algorithm 3D programs use to compute visibility and perspective) is going to be measured in billions of gigabytes. If your leaf sizes are equivalent to millimetres, and you're spreading trees over an area measured in hundreds of square kilometres, the memory requirement is huge. Not to mention the VAST number of polygons the program has to compute to render the scene. And not to mention the Highlander-like lifespan you'd need to be able to place all the trees!

Next time you're at a lookout overlooking a sweeping view, try to picture how many cubic millimetres, or how many polygons, it would take to cover everything you see, and you'll get what I mean.
So, how to get around this problem? First, having positioned your camera and deciding where you're taking the shot from, surround only that immediate area with detailed trees. Looking at your pic, that would be just the hilltop in the foreground and the area at its base. Where you've put those trees on the left is as far as you need to go out with the detailed models, no further. Next, set up a separate project and render a few trees, one at a time, by themselves on a black background, save the pics, and crop them to the exact size of the tree on each one. In Photoshop or your favourite paint app, use the Magic Wand or Mask Select to mask out the back background, then save the mask as an alpha channel and save the pic. In your 3D program, create a flat plane and use the tree pic as a texture, using the alpha channel to create a transparency around the outline of the tree. Position and size the plane on the landscape so it's facing the camera and about the size you want for the distant tree. Do the same with different tree pics for variation. You only need about five or six different tree pics to make it look good. Copy the planes many times and scatter them about the middle distance - don't do this all the way to the horizon, it's still too much work though! Stick to placing them just on that middle headland, near the riverbanks and back a little bit. Make sure they all face the camera, too. This technique is called "billboarding" and is a common trick in this field.
You can see an example of this in my Gallery
here. In this image, only the trees immediately outside the window are actual 3D models. The darker ones in the background are billboarded. Can you see the difference?
For the far distance, you have two options:
1) Post-processing with your paint app; or
2) Rendering the distant part of the landscape with a surface texture that makes it look like it's covered with trees.
I most often use post-processing as it's faster, I have more control over the final appearance, and it's less work. To post-process, render your scene with the trees and billboards in your 3D app and load the pic into your paint app. Use a Lasso Select to mask in the distant area of the landscape and exclude the foreground, rocky peak and sky. Feather the edges of the selection so it blends nicely with the rest - about 4-6 pixels feathering will do. Draw a tree in black on white in a separate pic about 48x48 pixels in size. Assign this pic as a Custom Brush, then return to your scene. In your Brush Options, set Scattering, Size Jitter, Opacity Jitter, and Hue/Saturation Jitter, choose a nice dark greenish colour from the palette, and sweep the brush over the masked area with broad strokes. Do the top (most distant) first, and work down to the bottom (closest), so the scattered trees overlie each other correctly. Increase your brush size as you work down so the trees appear to scale with distance. Experiment with the Jitter settings to get the effect you want. If you want extra realism, put the new trees on a separate layer and assign a Drop Shadow style to the layer in the direction of the lighting, so the trees appear to cast shadows.
You can see an example of this in my Gallery
here. This pic of Melenkurion Skyweir was done in Terragen and the trees were post-processed in just as I've described above.
If you realize that objects in the distance don't need to be as detailed as foreground objects, and thus break your project into several stages with different techniques like this, you'll take out so much back-breaking work and you won't flood your computer's RAM and CPU. Hope this helps, and I look forward to seeing the finished pic!

The only difference between light and dark is the ability to tell the difference.