An environmental pledge will be missed unless 58 million trees are planted in Scotland in the next 15 months, Conservatives have warned.
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 completely 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.
"There have always been a lot of concerns across rural Scotland about where all these trees are going to be accommodated, and these can only increase given that the clock is running down on this 2015 target.
"Ministers also have to set out how much it is going to cost to plant 58 million more trees and where it is going to get the money from."
To meet the pledge, the Government will need to find a way to plant almost four million trees a month until 2015, the Conservatives pointed out.
At the time of the announcement, the Government urged the rest of the world to follow its lead in tackling climate change.
Stewart Stevenson, who was minister for climate change, said at the time: "Tackling climate change will require a huge international effort and this tree-planting initiative will make an important contribution to reducing global emissions.
''Greater forest cover will help cut emissions through either directly absorbing CO2 or by providing more sustainable materials for construction and renewable energy.
''Scotland is leading international action on climate change and the rest of the world must follow our lead.''
The Scottish Forestry Commission revealed the progress in a letter to the Conservatives, released under freedom of information rules.
It stated: "In 2010, Scottish ministers pledged to plant 100 million trees by 2015 as part of the Climate Group's States and Regions Alliance Billion Trees initiative.
"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 2,000 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.
"The figure is based on new planting and not restocking of felled areas. The figure is based at an average stocking density of 2,000 trees per hectare."
In response to the claims, a spokesman for Paul Wheelhouse, minister for environment and climate change, said: "The Tories are barking up the wrong tree yet again.
"In contrast to policies of the Tory Government, Scotland is shouldering the vast majority of tree planting in Britain with nearly two-and-a-half times more tree planting in Scotland compared to south of the border.
"The Scottish Government are working hard to achieve the 100 million trees target. Since 2011, Forestry Commission Scotland has been working closely with a wide range of land managers under the Woodland Expansion Advisory Group to ensure that future tree planting is well integrated with other land use interests. In June last year, this group agreed to planting 100,000 hectares of trees over the next 10 years.
"This 100,000 hectares figure is now the main emphasis of our planting targets. However, Scotland's offer to plant 100 million trees by 2015 represents 10% of the ambitious Climate Group States and the Regions one billion tree pledge and we are making a significant contribution towards this global aspiration."