More than 100 world leaders have promised to end and reverse deforestation by 2030, in the COP26 climate summit's first major deal.
Brazil - where stretches of the Amazon rainforest have been cut down - was among the signatories on Tuesday.
The pledge includes almost £14bn ($19.2bn) of public and private funds.
Experts welcomed the move, but warned a previous deal in 2014 had "failed to slow deforestation at all" and commitments needed to be delivered on.
Felling trees contributes to climate change because it depletes forests that absorb vast amounts of the warming gas CO2.
UK Prime Minister Boris Johnson, who is hosting the global meeting in Glasgow, said "more leaders than ever before" - a total of 110 - had made the "landmark" commitment.
"We have to stop the devastating loss of our forests," he said - and "end the role of humanity as nature's conqueror, and instead become nature's custodian".
The two-week summit in Glasgow is seen as crucial if climate change is to be brought under control. The countries who have signed the pledge - including Canada, Brazil, Russia, China, Indonesia, the Democratic Republic of the Congo, the US and the UK (the full list is here) - cover around 85% of the world's forests.
Some of the funding will go to developing countries to restore damaged land, tackle wildfires and support indigenous communities.
Governments of 28 countries also committed to remove deforestation from the global trade of food and other agricultural products such as palm oil, soya and cocoa.
These industries drive forest loss by cutting down trees to make space for animals to graze or crops to grow.
More than 30 of the world's biggest financial companies - including Aviva, Schroders and Axa - have also promised to end investment in activities linked to deforestation.
And a £1.1bn fund will be established to protect the world's second largest tropical rainforest - in the Congo Basin.
Media caption,A logger, an environmental policeman, a cattle rancher and an environmentalist share their views on Amazon's future
Prof Simon Lewis, an expert on climate and forests at University College London, said: "It is good news to have a political commitment to end deforestation from so many countries, and significant funding to move forward on that journey."
But he told the BBC the world "has been here before" with a declaration in 2014 in New York "which failed to slow deforestation at all".
Indonesia is the world's largest exporter of palm oil, a product found in everything from shampoo to biscuits. Production is driving tree destruction and territory loss for indigenous people.
Meanwhile, Russia's huge natural forests, with more than one fifth of the planet's trees, capture more than 1.5 billion tonnes of carbon annually.
In the planet's biggest rainforest, the Amazon, deforestation accelerated to a 12-year high in 2020 under Brazilian President Jair Bolsonaro.
Asked whether leaders like Brazil's Mr Bolsonaro could be trusted to abide by the pledge, the UK's Environment Secretary George Eustice said "we should be really positive when countries engage".
"Last time there was an attempt at getting such a commitment on forests [in 2014], Brazil didn't take part, neither did Russia, neither did China.
"Brazil, they've really engaged with us on this agenda. It's a big step for them."
But pressed on whether the agreement will be enforceable, Mr Eustice said: "It doesn't go as far into talking about enforcement mechanisms and so forth, that's not the nature of these agreements."
He said what was different about this pledge in particular is that there is "the finance to back [it] up".
US President Joe Biden said he was "confident" the global pledge could be met, telling world leaders: "All we need to do is summon the will and do what we know is right. We can do this."
He said the US would lead by example, and announced it would spend $9bn (£6.6bn) to conserve and restore forests.
Ana Yang, executive director at Chatham House Sustainability Accelerator, who co-wrote the report Rethinking the Brazilian Amazon, said: "This deal involves more countries, more players and more money. But the devil is in the detail which we still need to see."
She said it was a "big building block" in the mission to keep global temperature rises below 1.5C.
But many people living in the Amazon, including in its urban areas, depend on the forest for their livelihoods and they need support in finding new incomes, she added.
Tuntiak Katan, from the Coordination of Indigenous Communities of the Amazon Basin, welcomed the deal, saying that funds should be invested in supporting indigenous communities who are able to manage and protect forests.
Mr Katan, an indigenous Shuar from Ecuador, told the BBC indigenous communities globally protected 80% of the world's biodiversity but faced threats and violence.
"For years we have protected our way of life and that has protected ecosystems and forests. Without us, no money or policy can stop climate change," he said.
One of the biggest causes of forest loss in Brazil is to grow soy beans, much of which goes to China and Europe for animal feed for pigs and chickens, said Dr Sizer.
"We all end up consuming that, unless we're strict vegetarians and we don't eat soy. It's a very serious problem that we're all connected with."
Trees are one of our major defences in a warming world. They suck carbon dioxide out of the atmosphere, acting as so-called carbon sinks. They absorb around one third of global CO2 emitted each year.
Currently an area of forest the size of 27 football pitches is lost every minute.
Depleted forests can also start to release CO2. If too many trees are cut down, scientists are worried that the planet will reach a tipping point that will set off abrupt and unpredictable climatic change.
On the second day of the two-week climate summit, the US and EU are also launching an initiative that aims to drive global efforts to cut emissions of methane, a greenhouse gas which comes from sources including fossil fuel extraction and livestock farming.
Under present targets, the world is on track for warming of 2.7C by 2100 - which the UN says would result in "climate catastrophe".
