Climate-driven degradation of aged poplar and pine timber in historic buildings under simulated heat-rainfall exposure

Climate-driven degradation of historic timber structures is increasingly intensified by extreme heat and rainfall events, yet the behavior of naturally aged timber at different degradation stages remains unclear. This study experimentally investigates aged poplar and pine timber from historic buildings in northern China under simulated heat, rainfall, and coupled exposures. Surface color change, mass loss, and compressive behavior were evaluated. Results show that aged poplar exhibits strong sensitivity to moisture-related exposure, characterized by pronounced discoloration, holocellulose degradation, and sustained mass loss, with mechanical degradation mainly reflected in post-yield behavior. In contrast, aged pine shows weaker and more dispersed responses, governed primarily by pre-existing degradation and material heterogeneity. Specimen size does not control degradation magnitude but influences statistical detectability. These findings highlight that degradation is jointly governed by species characteristics, degradation stage, and specimen scale, emphasizing the need for multi-indicator assessment in heritage timber conservation.