Smouldering fire in a nutrient-limited wetland ecosystem: Long-lasting changes in water and soil chemistry facilitate shrub expansion into a drained burned fen

Wildfires are natural phenomena which regulate functioning and stability of fire-adapted ecosystems. However, their occurrence may impair the functioning of fire-susceptible ecosystems by disturbing nutrient cycling and biodiversity. This work aimed to identify environmental factors shaping post-fire patterns of shrub expansion in a drained, burned peatland. This research was conducted in a fire-susceptible drained rich fen, located in Biebrza National Park (Poland), which was subjected to a large-scale smouldering fire in 2002. In 2014, water and soil chemistry were studied alongside with foliar nitrogen (N), phosphorus (P) and potassium (K) contents of a native shrub (Salix cinerea) in four vegetation types present after the fire. Unburned areas were dominated by herbaceous plants. Willows present were sparse and low, with chloroses and necroses. Their foliar nutrient content indicated strong K limitation. Moderately burned areas were dominated either by willows or nitrophilous plants. Willows in moderately burned areas had high chlorophyll content in leaves and their foliar nutrient content indicated a lack of evident nutrient limitation. In the moderately burned areas, relatively high contents of phosphates (P-PO₄^3-) were recorded in soil and water. In areas with high fire severity, willows were withdrawing and their foliar nutrient content indicated N limitation. Decreased content of P-PO₄^3- and ammonium (N-NH₄⁺) in soil and water was also observed there. Thus, fire-induced changes in fen geochemistry were recorded twelve years after a disturbance which shaped the long-term dynamics of shrub expansion. The fire ceased K limitation in burned areas and increased P availability. Strong K limitation, which is typical in degraded fens, appeared to be critical for keeping unmanaged fen meadows with low shrub cover. The occurrence of strong K limitation in drained fen ecosystems may reduce the need for investment in conservation practices used to restrict shrub expansion (e.g. regular mowing or shrub removal).

Keywords: Macronutrients; Nutrient limitation; Potassium limitation; Shrub encroachment; Succession; Wildfire.