Upland swamps are peat‐accumulating, groundwater‐dependent and fire‐prone wetland ecosystems. Drying caused by anthropogenic processes such as underground mining, ditching and climate change may disrupt surface and groundwater flows and result in bottom‐up controls on wetland expression. Fire is an endogenous, recurring disturbance that drives a top‐down consumptive force in many upland swamp systems. When compounded with drying, fire may facilitate permanent community transitions. A dearth of ecological data and temporal lags have hampered our ability to predict risks associated with multiple disturbances in wetland plant communities. We collected intact wetland mesocosms from valley floors and lower slopes of four undisturbed swamp sites. We transferred the mesocosms to a glasshouse and established three different soil moisture availability levels to simulate wetland drainage. After 20 months of the hydrological treatment, we simulated a fire event by sequentially applying biomass removal (clipping), heat and smoke to half of the mesocosms. We monitored species biomass, richness and composition over a 3.5‐year time frame. We found evidence of a temporal lag in biomass response to low water availability and synergistic hydrological and fire effects on species richness. In unburnt conditions, richness declined with low water availability but was maintained under high and medium water availability. After simulated fire under medium water availability, however, richness also declined and converged with depauperate low‐water mesocosm richness. Representation by many obligate swamp species declined in low compared with high water availability mesocosms over time, an effect that was amplified by the fire treatment. Synthesis. Our evidence of lagged effects of hydrological change on wetland vegetation and compounding effects of fire should be considered in impact assessments, monitoring programs and ecosystem management to avoid irreversible wetland change in drying environments.

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Upland swamps are peat-accumulating, groundwater-dependent and fire-prone wetland ecosystems. Drying caused by anthropogenic processes such as underground mining, ditching and climate change may disrupt surface and groundwater flows effecting a bottom-up control on wetland expression. Fire is an endogenous, recurring disturbance that drives a top-down consumptive force in many of these systems. When compounded with anthropogenic drying, fire may facilitate permanent community transitions. A dearth of ecological data and temporal lags have hampered our ability to predict risks associated with multiple disturbances in wetland plant communities. We collected intact wetland mesocosms from valley floors and lower slopes of four undisturbed swamp sites. We transferred the mesocosms to a glasshouse and established three different soil moisture availability levels to simulate wetland drainage. After 20 months of the hydrological treatment, we simulated a fire event by sequentially applying biomass removal (clipping), heat and smoke to half of the mesocosms. We monitored species biomass, richness and composition over a 3.5-year time frame. We found evidence of a temporal lag in biomass response to low water availability and synergistic hydrological and fire effects on species richness. In unburnt conditions, richness declined with low water availability but was maintained under high and medium water availability. After simulated fire in medium water availability, however, richness also declined and converged with depauperate low water mesocosm richness. Representation by many obligate swamp species declined in low compared with high water availability mesocosms over time, an effect that was amplified by the fire treatment. Synthesis: Our evidence of lagged effects of hydrological change on wetland vegetation and compounding effects of fire should be considered in impact assessments, monitoring programs and ecosystem management to avoid irreversible wetland change in drying environments.