Despite global importance of wetlands, estimations of their production and biomass have received little attention (Campbell et al., 2000). This chapter concentrates on analysis of the composition and above-ground biomass of floodplain grasslands and fen vegetation in the Northern forest zone. Both vegetation types were extensively used for hay and/or grazing up to the middle of the 20th century, and abandoned later. Systematic biomass estimations were conducted in the 1970s–1980s (Estonian data from 1977–1980; most data from Canada from 1972–1978 (Campbell et al., 2000)) when they were feasible for agricultural use. Papers on vegetation production and above-ground biomass of wetlands are quite scarce nowadays. Biomass has sometimes been measured for developing community structure theories, e.g. Zobel & Liira (1997) included some wet grasslands into analysis of richness vs biomass relationship. Still, some thorough reviews can be found like an overview of biomass of rich fen types in South England and Wales by Wheeler & Shaw (1991). New interest in the subject has risen in the context of biomass use for bioenergy production (e.g. Rosch et al., 2009). Many plant species cannot survive without special accommodation to wetland conditions. The composition of wetland vegetation is mostly controlled by the wetland water level (WL) (Bootsma & Wassen, 1996; Hajkova et al., 2004; Barry et al., 2008). Wilcox & Nichols (2008) and Ilomets et al. (2010) found that the diversity and habitat value of plant communities depend on the wetland WL and the water level amplitude between dry and wet seasons (WLA). In fens with a constantly high WL rhizome-spreading graminoids and herbs dominate, while drainage and fluctuating WL support high tussock-forming graminoids. A specific feature of both floodplain grassland and fen vegetation is high patchiness due to variations in WL and WLA caused by microtopography (Liira et al., 2009). Tussocks, formed by herbaceous plants or tree stumps, locally increase the habitat variability even more (Liira et al., 2009; Ilomets et al., 2010). Total biomass of wetland vegetation is significantly affected by three main factors: the N:P ratio, total nutrient supply and morphological and physiological traits of plants (Gusewell, 2005). Biomass variations are higher on moister sites such as wet floodplain grasslands (Truus & Puusild, 2009) and fens (Ilomets et al., 2010). The height and coverage of tussocks increases with denser or deeper drainage. About 52% of the vascular plant species variance occurs due to four environmental variables: amplitude of WL (between spring flooding and midsummer dry period), midsummer WL, mire water pH and electrical conductivity (Ilomets et al., 2010).