El Niño Drought Affects Tree Mortality in New Guinea Forest

Climate change is increasing drought frequency and intensity in many tropical forests. It remains uncertain how the changing patterns of drought may impact the forests of New Guinea where rugged terrain provides contrasting wetness conditions. We examined mortality responses of 21,102 trees (DBH ≥ 1 cm) across 147 quadrats in a 50-ha lowland forest plot during the 2015–2017 El Niño drought in Papua New Guinea, comparing mortality across three topographic positions (valley, slope, and ridge) during pre-drought (2009–2015), drought (2015–2017), and post-drought (2017–2019) periods. Small trees showed 2–4 times higher mortality than large trees, but both size classes responded proportionally similarly to drought (no size × drought interaction). Topographic effects varied significantly with the census period. Valley trees maintained the highest mortality during pre-drought and drought periods (7.80% and 8.81% per year respectively for 1-cm DBH trees) but recovered rapidly post-drought (mortality declining to 4.91% per year). Ridge trees showed contrasting patterns, with mortality increasing from 4.66% to 7.07% per year and remaining elevated post-drought. Critically, post-drought mortality exceeded pre-drought levels (p < 0.01) while drought-period mortality did not differ significantly from pre-drought (p = 0.081), indicating strongly lagged mortality responses. These results indicate that topographic position governs both immediate drought vulnerability and recovery trajectories, with ridge habitats showing persistent stress despite harboring drought-adapted species. Our results highlight the distinct topographic responses and vulnerabilities of New Guinea's rainforests to drought.