Timing Matters: Viticultural Land Use Determines Responses in Structure and Function of Fungal Stream Communities Across One Growing Season.

Fungal communities are critical for leaf decomposition, a central ecosystem function in streams. A wide range of anthropogenic stressors can alter their structure and function (i.e., leaf decomposition). Additionally, fungal communities are subject to seasonal turnover due to natural processes. Despite this, seasonality in interaction with varying stressor exposure has rarely been studied in the context of leaf decomposition. We investigated fungal community composition and leaf decomposition over one agricultural growing season by deploying leaf bags at least impacted forest and viticultural sites of 10 streams. Additionally, we transplanted leaf bags that had been colonised at the forest sites to viticultural sites to investigate how changes in stressor exposure affect the structure and function of fungal communities. Leaf decomposition was repeatedly lower in the viticultural treatment than in the forest treatment, which was partly explained by the environmental variables. The decomposition of the transplanted leaves varied across the time points and was overall more similar to that of the forest treatment. The fungal communities in April were similar across treatments, whereas all exhibited different seasonal community turnover. At later time points (June, August and September), the fungal communities from the forest and transplant treatment remained similar, likely triggered by the priority effects of the location of colonisation (forest). The viticultural treatment, however, deviated at these time points, which coincided with the timing of fungicide application. Overall, we show that both community composition and function of leaf decomposition exhibit seasonal and stressor-related variability. Thus, our study demonstrates that seasonality and the actual stressor regime need to be considered and well described when investigating land use effects on leaf decomposition and associated fungal communities.