Forest fires in Brazil and Indonesia contributed to a record loss in global tree cover in 2016, equivalent to the size of New Zealand, an independent forest monitoring network said on Monday.

Man-made global warming increasing the risks of wildfires by adding to extreme heat and droughts in some regions, it said. This year, California and Portugal have been among places suffering deadly blazes.

Worldwide, global tree cover losses rose 51 percent in 2016 to 29.7 million hectares (73.4 million acres) from 2105, according to data from the University of Maryland compiled by Global Forest Watch (GFW).

That was a record high for GFW records stretching back to 2000, and contrasts with some other satellite measurements that indicate a slowdown in the pace of forest clearances to make way for farms, cities and roads.

"We saw quite a dramatic spike in 2016," Mikaela Weisse, research analyst at the U.S. think-tank World Resources Institute which oversees GFW, told Reuters. "That seems to be related to forest fires in countries including Brazil, Indonesia and Portugal."

GFW measures loss of tree cover and does not estimate net changes in forests to take account of re-growth and new plantings. In a well-managed forest, for instance, growth of young new trees will make up for losses through harvesting.

By contrast, the U.N.'s Food and Agriculture Organization, using different methods, says the net global rate of deforestation has slowed by more than 50 percent in the last 25 years.

A U.N. assessment in 2015 said the world has lost forests totaling 129 million hectares since 1990, equivalent to the size of South Africa.

GFW said Brazil's Amazon region lost 3.7 million hectares of tree cover in calendar 2016, almost three times more than in 2015.

That contrasts with official Brazilian data showing that deforestation in the Amazon fell 16 percent in the year to July 2017 compared to the same period a year earlier. Brazil said it was the first decline in three years.

Weisse said GFW data often detect smaller-scale losses in tree cover, including in layers beneath the forest canopy, than the official data. The Brazilian data are better at recording clearances of large blocks of forest.

GFW also said Indonesia lost almost 1 million hectares of tree cover in 2016, probably the delayed result of a severe fire season in 2015.

"Wildfires are becoming more common," U.N. Secretary-General Antonio Guterres said in a speech in March about the increasing risks of climate change.

This month, 42 people have died in wildfires in California. In Portugal, more than 100 people have died in wildfires in the past four months.