This would only lead to a constant shift in total CO2 emissions because there is only so much space you can utilize. When a plant dies it releases the CO2 back to the atmosphere. For the money, you can likely get higher CO2 abatement (i.e. close a coal fired power plant and replace it with renewables subsidy).

In the first case, you save the CO2 roughly equivalent to the total weight of the plants. In the pther case, you save x tonnes of CO2 every year.