It was already explained somewhere although I don't think in skyrim, but in another game possibly or in a book in one of the games, that the sun had actually gone out for a full year.
The plants however continued to grow and life went on.
It seems the sun does not provide anything but light in this world.
Oh and the magic of the sun harms vampires
Aside from Meridia saying otherwise in her Skyrim quest, consider the following:
1. "many scholars reject the proposal that the Sun's Death event of http://www.uesp.net/wiki/Lore:First_Era#1E_668 catastrophically affected plant life"
Many. Not most. Many. And this tells us that there is or was a proposal that it
did catastrophically affect plant life. The counter proposal could be supported by showing that there is no evidence of all the plants dying or mass famine occurring. The original proposal must have been based on something or it wouldn't have been made and refuted by anyone.
2. Red Mountain erupted around the time the Dwemer wiped themselves out and Azura turned Chimer grey. Some crazy [censored] was going on then. Oh. And four mortals tapped into the heart of Lorkhan and turned themselves into gods.
You've just given yourself godly powers which you intend rule over your people with. An ash cloud is going to wipe out plant life across Tamriel, starving everyone. What do you do?
Plants do require sunlight, but they were saved by a divine miracle. If you don't like "a wizard did it", tell it to the Tribunal.