The East Asian summer monsoon (EASM) is a key component of the global monsoon system, with significant impacts on the climate and ecosystems in Taiwan. However, information about its impact on vegetation diversity during the Holocene in Taiwan and high resolution climate records are unclear and absent. The pollen, charcoal and diatom records from Tien pond are used to provide records with a multidecadal resolution of vegetation changes, fire frequency and hydrological conditions, reflecting the pattern of monsoon intensity and its relationship with subtropical forest ecosystems during the last 8,500 years cal bp. Past temperatures inferred from the percentage of upper montane forest (UMF) taxa and PC2 of principal component analysis (PCA) indicated records of the warm Holocene Thermal Maximum (HTM) during the period 7,000–2,860 cal bp. Past precipitation inferred from diatom concentration and PC1 of PCA showed reduced monsoon precipitation during the thermal maximum. This appears to be the opposite of the pattern in northern China, where the precipitation decreased toward the late Holocene. The positive correlation between the palynological richness index (PRI) and arboreal pollen percentages highlighted the importance of forest canopy cover in driving forest diversity. The high richness in the warm/dry HTM suggests that subtropical montane forests support rich biodiversity and can serve as refugia for taxa during climate fluctuations. These results have implications for conserving and managing subtropical forest ecosystems under future climate change scenarios.