Southeast Asia contains about half of all tropical mountain forests, which are rich in biodiversity and carbon stocks, yet there is debate as to whether regional mountain forest cover has increased or decreased in recent decades. Here, our analysis of high-resolution satellite datasets reveals increasing mountain forest loss across Southeast Asia. Total mean annual forest loss was 3.22 Mha yr⁻¹ during 2001–2019, with 31% occurring on the mountains. In the 2010s, the frontier of forest loss moved to higher elevations (15.1 ± 3.8 m yr⁻¹ during 2011–2019, P < 0.01) and steeper slopes (0.22 ± 0.05° yr⁻¹ during 2009–2019, P < 0.01) that have high forest carbon density relative to the lowlands. These shifts led to unprecedented annual forest carbon loss of 424 Tg C yr⁻¹, accelerating at a rate of 18 ± 4 Tg C yr⁻² (P < 0.01) from 2001 to 2019. Our results underscore the immediate threat of carbon stock losses associated with accelerating forest clearance in Southeast Asian mountains, which jeopardizes international climate agreements and biodiversity conservation.

东南亚约有一半的热带山地森林拥有丰富的生物多样性和碳储量，但关于近几十年来区域山地森林覆盖率是增加还是减少仍存在争议。在这里，我们对高分辨率卫星数据集的分析揭示了整个东南亚山地森林的减少。2001年至2019 年期间，森林年平均损失总量为 3.22 Mha yr ^-1，其中 31% 发生在山区。在 2010 年代，森林损失的前沿向更高的海拔（2011-2019年期间为15.1 ± 3.8 m yr ^-1，P  < 0.01）和更陡峭的斜坡（2009-2019 期间为0.22 ± 0.05° yr ^-1，P < 0.01) 森林碳密度比低地高。这些转变导致了前所未有的年度森林碳损失 424 Tg C yr ^-1， 从 2001 年到 2019 年以18 ± 4 Tg C yr ^-2 ( P < 0.01)的速度加速。我们的结果强调了碳库损失的直接威胁与加速东南亚山区森林砍伐有关，这危及国际气候协议和生物多样性保护。