Ontario is pledging to plant 50 million trees as part of an $80-million plan to fight climate change.

Premier Dalton McGuinty is set to make the announcement later Monday in Oakville, CBC News has learned.

The promise is part of the United Nations Environment Programme's Plant for the Planet: Billion Tree Campaign, which aims to encourage the planting of one billion trees in 2007.

Government sources say Ontario's effort will be the largest of all jurisdictions in the UN campaign to fight climate change.

Ontario, however, won't plant all the trees this year, instead promising to plant them over the next 13 years.

So far, 1.8 million trees have been planted by the province this year. The rest will be planted on public and privately owned lands by 2020.

The project should offer some relief to Toronto, where city councillors trimmed spending on tree planting and maintenance as part of a widespread $34 million in budget cuts.

McGuinty will also announce the development of a new Grade 11 course to focus on environmental issues.