The Limited Effect of Reduced Typhoon Frequency on Stream Hydrochemistry in a Subtropical Forest Watershed

Tropical cyclones are often accompanied by large amount of precipitation potentially impacting stream hydrochemistry. Global warming is altering typhoon disturbance regime. Little is known about how cyclone changes, especially cyclone-frequency reduction may affect stream hydrochemistry. In this study, we compared water and nutrient input via precipitation and output via streamflow between a frequent-typhoon period (2013–2017), with 1.2 typhoon yr⁻¹, and a no-typhoon period (2018–2022) at a long-term monitoring site, the Fushan Experimental Forest of Taiwan. Precipitation and streamflow quantities were not different between the two periods because typhoons increased the fluctuation but not the mean of monthly precipitation in the major typhoon months (July–September). Inputs of Mg²⁺, NO₃⁻, and SO₄²⁻ via precipitation were greater in the frequent-typhoon period than the no-typhoon period while inputs of other ions were not different between the two periods. Only the output of Mg²⁺ was different between the two periods, greater in the frequent-typhoon period. Output/input ratio of NO₃⁻ was greater in the no-typhoon period than the frequent-typhoon period despite the greater input in the frequent-typhoon period, while no differences were found for others. Increases in mineralization rates due to warming is suggested to be the cause of the greater NO₃⁻ output/input ratio during the no-typhoon period. Relationships between stream discharge and ion export were similar between the two periods both with and without removing typhoon events. The limited variation in hydrochemistry between periods of contrasting cyclone activities suggests high resilience of the undisturbed subtropical forests to changes in cyclone frequency at the decadal scale.