Researchers say parts of the Sierra Madre rainforest have crossed a critical moisture threshold, raising the risk that drought-stressed forest will give way to scrub and grassland. Their warning, based on measurements from 19 watersheds, emerged on Friday at a provincial competition for local forest-monitoring teams and has set off a fierce dispute over what should happen next.
The scientists estimate that 480,000 hectares (1.19 million acres) of forest could face severe dieback by 2032 if hotter dry seasons continue. The range, which runs along eastern Luzon, helps regulate water supplies and reduce flood surges for millions of people. Officials say the forest is still largely intact and urge residents not to treat a scientific threshold as a forecast of immediate collapse.
Contest reveals a hidden shift
The Canopy Crown contest asked 21 municipalities to submit evidence of forest health. The winning team’s map showed rising canopy temperatures and shrinking dry-season streams; researchers then compared it with 16 months of soil-moisture readings from their own network. Twelve watersheds had fallen below the level at which trees in the monitored plots reliably recovered after dry spells, said Elna McGlynn-Rosenbaum, a forest ecologist with the fictional Luzon Climate Research Centre.
McGlynn-Rosenbaum said the signal was consistent with a threshold crossing, not proof that every slope would die back. “The forest is still standing, but in these places it has lost its margin for error,” she said. The team’s modelling projects that, under continued warming and repeated drought, affected forest patches could release up to 38 million tonnes of carbon dioxide by 2040 as trees die and soils dry.
The forest is still standing, but in these places it has lost its margin for error.
Elna McGlynn-Rosenbaum, forest ecologist
Officials resist emergency label
The provincial forest office disputes the scale of the danger, saying satellite images show green canopy across most of the range. Its director, Jemie Asirvatham, said the readings came from a limited network and could not support “a declaration that the Sierra Madre is collapsing”. The agency plans to publish its own plot survey within six weeks, but has not said whether it will include the contest team’s data.
Governor-elect Donatella Gentile called the findings serious but urged calm, saying the ₱6.2 billion package under discussion would fund watershed protection, seed storage and fire patrols rather than emergency evacuations. She also backs a 90-day pause on new road permits in the monitored watersheds. The province has not yet identified which existing projects the pause would affect.
Camilo Garza, a climate modeller who reviewed the study’s methods, said the green satellite view could mask stress beneath the canopy. A forest can retain leaves while its roots lose access to water, he said, adding that the researchers’ threshold is regional and does not mean the entire mountain range has tipped into irreversible change.
The contest has also drawn criticism from municipal officials who say a prize for forest performance could penalise towns with fewer monitoring staff. Sofia Nunes, a farmer in one of the measured watersheds, said residents had noticed streams running lower earlier in the dry season, but had received no guidance on water use. “We need a plan before the next dry months, not another argument over whose map counts,” she said.
The research team wants measurements expanded to 60 watersheds before the next dry season, which usually begins in December. Gentile says the province will decide on the road-permit pause by 20 November; scientists say waiting for a complete survey could leave local authorities with little time to protect the most stressed slopes.
