Long-term effects of water level drawdown on plant-mediated nutrient cycling in sedge-pine fens

The water level, directly or indirectly, regulates all ecosystem processes in peatlands. The responses of vegetation to altered water levels may in longer run affect the functions of peatland ecosystems in yet largely unexplored ways. This paper deals with the effects of a persistent water level drawdown on plant-mediated cycling and retention of nutrients in oligotrophic sedge-pine fens. Six sites forming a drainage succession chronosequence were chosen for measurements: two undrained sites, and sites drained 8, 22, 30, and 55 years before the beginning of the study The total nutrient pools found in plant biomass consistently increased following water level drawdown. This was caused by increases in shrub and tree nutrient pools both above- and below-ground. Moss nutrient pools remained at about pre-drainage levels, or even increased, although there was a temporary decrease at 2-3 decades after water level drawdown. The relatively small pre-drainage nutrient pools in graminoid and herbaceous biomass clearly decreased. The annual litterfall fluxes of most nutrients first temporarily peaked during the first decade, and then dropped again to about the pre-drainage levels. Later on, these fluxes again steadily increased and had reached or surpassed pre-drainage levels when 5 decades had passed, except for K. There was a general increase in the amount of nutrients taken up and allocated in above-ground plant parts, with the exception of K. The amounts of nutrients allocated to below-ground plant parts either decreased at least temporarily, or remained rather unchanged. The decline of sedges following water level drawdown causes drastic changes in nutrient cycling, even though their biomass proportion in a typical sedge-pine fen is only about 20% of the total plant biomass. The nutrient cycles shift to dominance by arboreal vegetation in two decades. Mosses may remain an important component in nutrient cycling even after water level drawdown, however.