Phenology Effects on Carbon and Water Dynamics in a Pristine Boreal Fen A multi-decadal analysis from 2005-2007 & 2014-2022

Abstract

Boreal peatlands are major constituents of the global carbon (C) and water cycles and therefore important climate regulators. These cycle dynamics are strongly determined by plant phenology (e.g.,vascular plant photosynthesis and seasonality) and evapotranspiration (ET), next to other abiotic factors. While it is evident that climate change affects these networks of interactions, it is only poorly understood to what extent, as site-specific empirical evidence over multiple decades is lacking. This study investigates the role of phenology (leaf area index: LAI) for peatland C (net ecosystem exchange: NEE) and water fluxes (ET and water table: WT) by combining inter-annual and seasonal observations at the ecosystem level, in Siikaneva fen, Finland (2005-2007, 2014-2022). Interactions with temperature (T), vapor pressure deficit (VPD), photosynthetically active radiation (PAR) and rain were included and tested with various statistical methods (trend- and correlation-analysis, commonality analysis, time lag testing, stepwise multiple-regression, and structural equation modelling).Significant trends were found for LAI (increasing) and WT (decreasing), suggesting ongoing changes in the ecosystem (climate change signal). Within this system LAI provided substantial effects on NEE, less so on ET. These relationships differed seasonally: the greenup season showed the strongest effects of LAI on NEE and ET (and other interactions). However, e.g., PAR and VPD became more relevant for NEE and ET in the later seasons. A time lag analysis suggested that effects (e.g., LAI on NEE) can be delayed at the scale of weeks. Inconclusive evidence was found for the LAI effect on ET and WT (via ET). It is assumed that ET is a limited measure to differentiate between evaporation (E) and transpiration (T) by leaves or the ground, as they might cancel each other out. A shading of increasing LAI over the season might shade moss-ET (and photosynthesis).The applied methods provided complementary information with differing suitability to reveal certain parts of complex network dynamics. A consistent, while seasonally changing, importance of plant phenologyfor peatland NEE can be concluded. Phenology showed clear correlation with ET, however, the effect size could not be quantified conclusively. Future studies should partition E and T or incorporate the contribution of mosses to both ET and NEE for a more comprehensive understanding. Collectively, this study highlights the role of phenology for peatland C and water fluxes and provides evidence for climate change induced alterations of biotic-abiotic interactions at the ecosystem scale.