That region certainly has dried up over the millennia and a major factor to this has been the effects of human cultivation, as well as mining on the land. The last few decades have been most severe because the timber industry has clear-cut most of the country. However, Afghanistan is also quite mountainous and due to the drastic climate shift between summer and winter what happens is these mountains are eroded to bare rock of any good top soil that could produce vegetation. This results in all that fertile soil getting washed down into small river valleys where most of the greenery occurs in concentrated pockets.
I imagine back in Bactrian times the terrain would have been fairly similar, though with more conifer trees along the base of mountain ranges and more fertile valley pockets.
Iran is a similar mountainous terrain but with a large plateau in the middle that depends on rainfall. If it's a good rainy season this large expanse of relatively flat land can be much like a semi-fertile plainland good for pasturing. Of course if there is not so much rainfall, as has been the case in recent times, the plateau becomes arid and semi-desert. The border region where Iran and Afghanistan meet is a large desert because no mountain ranges are near enough to drain rain and snow water into it. Interestingly in the far north of Iran, between the Alborz mountain range and Caspian Sea, a region in ancient times known as Hyrcania, is a tropical forest. This is were the Caspian tiger used to live until hunted to extinction abut a hundred years ago.