Detecting Subtle Change in Species and Trait Composition and Quantifying Its Uncertainty in a Boreal Peatland

ABSTRACT Aims Climate change‐induced changes in vegetation may be at first subtle and occur only locally within the ecosystem, complicating their reliable detection. We aimed to quantify short‐term changes in species and trait composition in a moss‐dominated ecosystem and to examine the associated uncertainty at the local and study area scales. Location Pristine boreal fen (Siikaneva) in Central Finland. Methods We applied the recently developed pre‐emptive joint species distribution model ( pJSDM ) to quantify the change in plant species and functional trait composition over 12 years. pJSDM allows spatially continuous prediction of change and the associated uncertainty from pointwise observations to the whole ecosystem. It includes the pre‐emptive competition for space within mosses that are an important component in many high‐latitude ecosystems. To address the mechanisms shaping the plant community, pJSDM was extended to predict the change in trait distribution parameters. Results Within the studied peatland, we detected changes in species and trait composition that were small in magnitude but occurred with high probability. Some of the changes occurred only locally, others over the whole study area, and some of the local changes occurred in opposing directions. The species originally found in the drier locations increased in abundance. Also, an increase in Sphagnum capitulum size was detected, indicating adaptation to drier conditions. The cover of wet‐adapted species decreased at the study area scale but displayed local increases. Conclusions The studied peatland showed nonuniform change in species and trait composition. The observed short‐term changes are in line with earlier descriptions of multidecadal drying and ombrotrophication of peatland vegetation and suggest increasing contrasts within the vegetation. The applied approach, pJSDM paired with trait distribution parameters, showed potential in revealing ongoing subtle changes in moss‐dominated vegetation.