When you say that something is `sparse’ what you mean is that there is very little of it and what little there is, is unevenly distributed. For example, when you talk about sparse vegetation, you are saying that the vegetation is thin, not thick, and this thin vegetation is distributed over a relatively large area. There is vegetation in some places, but not in all.
*Thanks to the sparse traffic, we managed to cover the distance in two hours. *Why he wastes so much time on his sparse hair is beyond me.
`Scarce’, on the other hand, suggests that the shortage of the commodity is temporary. It is not permanent. Scarcity can be man made as well. For example, usually in our country, the day before the budget, petrol becomes scarce.
*I understand that during the Second World War, chocolates became scarce.