Climate change affects peatlands directly through increased air temperatures and indirectly through changes in water-table level (WL). The interactions of these two still remain poorly known. We determined experimentally the separate and interactive effects of temperature and WL regime on factors of relevance for the inputs to the carbon cycle: plant community composition, phenology, biomass production, and shoot:root allocation in two wet boreal sedge-dominated fens, "southern" at 62°N and "northern" at 68°Ν. Warming (1.5°C higher average daily air temperature) was induced with open-top chambers and WL drawdown (WLD; 3-7 cm on average) by shallow ditches. Total biomass production varied from 250 to 520 g/m , with belowground production comprising 25%-63%. Warming was associated with minor effects on phenology and negligible effects on community composition, biomass production, and allocation. WLD clearly affected the contribution of different plant functional types (PFTs) in the community and the biomass they produced: shrubs benefited while forbs and mosses suffered. These responses did not depend on the warming treatment. Following WLD, aboveground biomass production decreased mainly due to reduced growth of mosses in the southern fen. Aboveground vascular plant biomass production remained unchanged but the contribution of different PFTs changed. The observed changes were also reflected in plant phenology, with different PFTs showing different responses. Belowground production increased following WLD in the northern fen only, but an increase in the contributions of shrubs and forbs was observed in both sites, while sedge contribution decreased. Moderate warming alone seems not able to drive significant changes in plant productivity or community composition in these wet ecosystems. However, if warming is accompanied by even modest WL drawdown, changes should be expected in the relative contribution of PFTs, which could lead to profound changes in the function of fens. Consequently, hydrological scenarios are of utmost importance when estimating their future function.

Pristine peatlands are carbon (C)-accumulating wetland ecosystems sustained by a high water table (WT) and consequent anoxia that slows down decomposition. Persistent WT drawdown as a response to climate and/or land-use change affects decomposition either directly through environmental factors such as increased oxygenation, or indirectly through changes in plant community composition. This study attempts to disentangle the direct and indirect effects of WT drawdown by measuring the relative importance of environmental parameters (WT depth, temperature, soil chemistry) and litter type and/or litter chemical quality on the 2-year decomposition rates of above-and belowground litter (altogether 39 litter types). Consequences for organic matter accumulation were estimated based on the annual litter production. The study sites were chosen to form a three-stage chronosequence from pristine (undrained) to short-term (years) and long-term (decades) WT drawdown conditions at three nutrient regimes. The direct effects of WT drawdown were overruled by the indirect effects through changes in litter type composition and production. Short-term responses to WT drawdown were small. In long-term, dramatically increased litter inputs resulted in large accumulation of organic matter in spite of increased decomposition rates. Furthermore, the quality of the accumulated matter greatly changed from that accumulated in pristine conditions. Our results show that the shift in vegetation composition as a response to climate and/or land-use change is the main factor affecting peatland ecosystem C cycle, and thus dynamic vegetation is a necessity in any model applied for estimating responses of C fluxes to changing environment. We provide possible grouping of litter types into plant functional types that the models could utilize. Furthermore, our results clearly show a drop in soil summer temperature as a response to WT drawdown when an initially open peatland converts into a forest ecosystem, which has not yet been considered in the existing models.

Boreal forests are ecosystems with low nitrogen (N) availability that store globally significant amounts of carbon (C), mainly in plant biomass and soil organic matter (SOM). Although crucial for future climate change predictions, the mechanisms controlling boreal C and N pools are not well understood. Here, using a three-year field experiment, we compare SOM decomposition and stabilization in the presence of roots, with exclusion of roots but presence of fungal hyphae and with exclusion of both roots and fungal hyphae. Roots accelerate SOM decomposition compared to the root exclusion treatments, but also promote a different soil N economy with higher concentrations of organic soil N compared to inorganic soil N accompanied with the build-up of stable SOM-N. In contrast, root exclusion leads to an inorganic soil N economy (i.e., high level of inorganic N) with reduced stable SOM-N build-up. Based on our findings, we provide a framework on how plant roots affect SOM decomposition and stabilization.