The Scottish Government set a goal to plant 100 million by 2015 but figures show only 42% progress since the target was announced in 2010.
The aim now appears unachievable, according to Conservative rural affairs spokesman Alex Fergusson.
"The drive to plant more trees is an admirable enough aim, and the SNP certainly made the most of the announcement," he said.
"However, like so much the SNP comes up with, it appears to be completely unachievable.
"It has set an overly ambitious target and now has to explain how this will be met."
At the time of the announcement, the Government urged the rest of the world to follow its lead in tackling climate change.
The Scottish Forestry Commission revealed the progress in a letter to the Conservatives, released under freedom of information rules.
It stated: "This Scottish figure of 100 million trees was based on achieving 50,000 hectares of woodland creation over that period at an average stocking of 2000 trees per hectare.
"On this basis, planting over the past three years equates to new planting of nearly 42 million trees, achieving 42% of the 'pledge' to date."