Poplars are considered to be among the best trees in terms of carbon sequestration according to studies done thus far. In addition, if they're converted to paper and other wood products, they continue to sequester the carbon for the life of those products.
I would regard it as prima facia deeply unlikely that processing wood into paper is carbon-negative, regardless of how long you then store the paper. It takes power to turn trees into paper.
It's easy enough to calculate: Do you need more than an equivalent amount of paper burned into energy, to make that paper.
The answer is no, paper mills power themself from trimmings from paper making. They use much less than the mass of paper they produce.
Making paper does not take very much energy. The energy used in cutting and preparing wood is not that much when compared to the amount of wood produced.
So yes, turning wood into paper is carbon negative (in a way, technically it's the growing of the tree).
Here's a layman's article on the "best trees" for carbon sequestration- http://www.slate.com/articles/health_and_science/the_green_l...