It depends on both the program's capabilities and also how much detail the modeler wants to put into the model. The more detail, the more work. That's because 3D models are made of a mesh of flat triangles or squares. The more detailed the model, the more triangles/squares (polygons) there are in the model. That means more memory has to be used to load the model, store it in active memory, and compute color, transparency, shininess, and bumpiness for every single polygon. So you see, it's much easier to make, load, display, and render a forest of trees that are made out of a few planes than it is to do the same for a forest where each tree is made out of thousands of polygons.
There are various tricks to ease your computer's workload when displaying or rendering a forest. You can use "billboards" which are photographs of real trees and put these in the distance where their two dimensional nature won't be noticed. However, because they're flat, they won't cast realistic shadows like a three dimensional model. You can put high detail trees in the foreground and lower detail trees in the background.
I don't use Blender, but I understand it can do very sophisticated things. Sketchup, however, is designed more for simple shapes. I've never seen a very realistic tree made with Sketchup. Other programs like Bryce 7, Vue, Carrara, Poser, and other more high end programs can make and/or import very highly detailed tree models.
Here are some examples. These could be used in Bryce, Vue, Poser, DAZ Studio, Carrara, and a few others.
Pred Pack - Fir Trees
Lisa's Botanicals - Mighty Oak