Why planting trees isn't always a good idea

This is a response to a story we covered earlier this year about planting trees for climate change. A study in the journal Science claimed that the Earth has space for an extra billion hectares of trees; and if they were planted, it would lock away enough carbon dioxide to limit global warming to 1.5 degrees Celsius by 2050. Since then, Science has published not one - not two - but five comments and rebuttals to the original paper. They criticise various aspects of the method and results; one in particular was authored by almost fifty scientists, and said that the available area for trees was overestimated by by five times. One of those co-authors, physical geographer Toby Pennington, told Phil Sansom why planting trees is more complicated than it seems...