Impact of long‐term water level drawdown on functional plant trait composition of northern peatlands

Understanding how plant communities respond to increased evaporation and consequent water level drawdown (WLD) is critical for predicting the functioning of northern peatlands under climate change. Functional traits provide a quantitative link between vegetation and ecosystem functions and, therefore, constitute a useful concept for predicting responses to climate change. We studied the impact of long‐term experimental WLD on vascular plant and moss traits at a rich fen, poor fen and bog. Vegetation change was followed over a 15‐year period. In the final study year, the traits of the most common plant species were measured from control and WLD areas at each peatland type. We found equally high interspecific trait variation for Sphagnum mosses and vascular plants while the intraspecific variation was greater in the mosses. Community‐weighted mean (CWM) traits varied between sites; WLD had the strongest impact on those traits that the dominant plant group originally had high values for, and in most cases, WLD further increased these values. In the vascular‐plant‐dominated rich fen, WLD led to taller plants with a greater specific leaf area, features that under the prevailing water table were also greatest at that site. In the bog, characterized by dense Sphagnum moss stands with small individuals, WLD further enhanced these properties that increase the ability of a moss stand to remain moist under drier conditions. The poor fen was transitional between the two extremes, both in its vegetation composition and in its trait responses. Structural equation models (SEM) showed that WLD in the fens, indirectly via other traits, increased photosynthetic capacity while the impact in the bog site was the opposite. In the poor fen and bog, WLD directly increased vascular plant respiration while the increase in the rich fen was through other traits. WLD directly increased and decreased Sphagnum respiration in the poor fen and bog, respectively. Overall, the traits of the vascular plant and Sphagnum communities in the bog were more dependent on each other than they were in the fens. Based on these findings, it is evident that fens and bogs respond differently to WLD. This should be considered when predicting the effects of climate change on peatland carbon cycling. A free Plain Language Summary can be found within the Supporting Information of this article.