I increased the AA only because the tree leaves looked quite noisy.
This is for those who don't quite understand how anti-aliasing/pixel sampling affects your render (Disclaimer: I am not 100% sure that Terragen works in the same way as other 3D renderers, but I am assuming so, as AA is a pretty common process and it's goal in various renderers are usually the same).
Say if you have 3 geometric faces that occupy the area of two pixels. A very low AA setting will sample less points and return a value for your pixel that may not look exactly right. A higher setting will sample more points in the pixel, returning a better average of what colour the pixel should contain. Below, on the left the pixel is only sampled once, resulting in quite different values for adjacent pixels. When these pixels are viewed at 1:1 on your screen it will look quite noisy. In the example on the right, the pixels are sampled more times, returning more accurate values, and reducing the contrast between the pixels, thus reducing the noise.
This mainly applies to fine detailed geometry. Having noise in your atmosphere, for example probably won't be entirely fixed by increasing AA, though it may help.
