Increasing the total number of trees, or the proportion of earth's surface covered by forests, by 1.5% is a different proposition than increasing biomass... think about the difference between watching a giant oak grow 1.5% dry weight (start with one tree, end with one tree, but meet the carbon target) as opposed to planting an equivalent mass of new saplings (start with one tree, now have fifteen trees, but no growth occurred so carbon target is not met). The difference here is time. For your new saplings to actually put a dent in carbon emissions requires them to grow, which requires time.
Furthermore, we need to meet this target each and every year until our carbon emissions stabilize. 1.5% growth year over year is exponential and wouldn't take too long for us to literally run out of room to plant more forests. To be specific, that means doubling the biomass of all forests on earth every 45 years or so. The concerted effort required by humanity to do so would be a project on a scale never before seen in human history. If we planted two or three saplings for each and every tree already planted in the world, and passed an international resolution making it illegal to cut down any living tree, we would have a small chance at making our first 45 year target.
A related but interesting thing is carbon capture through algae. If I recall correctly, algae is responsible for a huge amount of carbon processing on this planet. Not only does algae grow faster than trees, it takes up a lot less space and the growth/cultivation of algae can be industrialized fairly easily. It may be far more reasonable for humanity to domesticate algae on a large scale (my first thoughts go to growing algae-based foods, producing algae-based paper products, and using algae to make combustible fuels) than to increase forest coverage.