Discrete ecological gradient in thermokarst ponds in a palsa mire in northern Norway.

Palsa mires constitute a zonal peatland type in the discontinuous permafrost region of the Northern Hemisphere. They typically consist of permafrost mounds and thermokarst ponds. Global warming has accelerated thawing of permafrost in palsa mounds and an increase in the area of thermokarst ponds in recent decades. Understanding long-term consequences of this process requires in-depth knowledge of the internal diversity of palsa mire vegetation types and their functions. Most studies so-far focused on the palsa mounds. Hereby, we focus on the thermokarst ponds, analysing their vegetation composition and habitat conditions from the top of a palsa plateau down to a fen without current palsa formation close to an adjacent river. We observed a distinct ecological gradient from Sphagnum-dominated ponds in the uppermost part of peat plateau to brown moss-dominated fen flarks at the riverside. This reflected well the poor - rich gradient typically recognised in mire vegetation, confirmed by our hydrochemical analyses. However, in contrast to the gradual shifts in species composition along typical mire zonation in temperate regions, palsa microtopography with mounds, rims, strings, and hollows, creates a sequence of mire basins forming a discrete gradient from base-poor to base-rich conditions, allowing different plant species to dominate these distinct locations.