Andrew McCornick said farmers were being told that trees were the answer, but there was no scientific agreement on whether this would warm or cool the planet.
For example, adventurer Ben Fogle visited Siberia for a recent Channel 5 documentary and was told by a climate scientist that although tree canopies were protecting the land surface, they were also heating the permafrost and creating warming.
See also: 9 reasons to plant trees on your land
“Trees have darker canopies than grassy vegetation, absorbing more sunlight and heating the land surface – a problem not yet included in the calculations of afforestation by advocates,” Mr McCornick told delegates at the union’s annual conference in Glasgow on Thursday (6 February).
In the case of Scotland, planting more trees on farmland would probably mean having to import food from another country that was de-foresting, he warned.
Exporting carbon footprint
For example, Europe currently imports 70% of its feed needs – the majority from Brazil, which is deforesting – therefore exporting our carbon footprint.
The Scottish government has a target for 24,710 acres (10,000ha) of new forest to be planted each year – increasing to 15,000ha by 2024 – as part of efforts to combat climate change.
Planting trees is viewed by many conservationists as widely beneficial as part of efforts to tackle the climate crisis. Mr McCornick agreed that trees have a role to play, saying “the right tree in the right place, and not blanket afforestation, will play a part in climate change and carbon storage”.
But he suggested that trees were being used as a distraction from the real issue: reduction of fossil fuel usage and transforming energy systems.
“The tree model will displace the potential to grow crops, livestock farming and rich flora and fauna that grazing systems deliver, with the associated employment and communities benefiting the national GDP,” Mr McCornick said.
Scottish Forestry is piloting two schemes in the Scottish Borders on better integration of woodland in farming.
Mr McCornick said the projects needed to be ambitious, but also honest about the biodiversity and climate change costs and benefits. He also said farmers needed to be given the right information and advice to be able to make the right changes.
Climate expert Mike Robinson, chief executive of the Royal Scottish Geographical Society (RSGS), told the conference that he believed agriculture probably could not function without methane emissions from ruminant livestock.
But he added: “The important point to mention is that agriculture has a net zero target by 2045. Carbon sequestration will play an important role in this.”
A major United Nations climate change summit, COP26, will take place in Glasgow in November.