Seasonal biomass allocation in a boreal perennial grass (Phalaris arundinacea L.) under elevated temperature and CO₂ with varying water regimes

The aim of this study was to analyze and model how biomass is allocated to the leaves, stems, and roots of perennial grass (reed canary grass, Phalaris arundinacea L., hereafter RCG) under elevated temperature (ET) (+approx. 3 °C) and CO₂ (approx. 700 μmol mol⁻¹) and with variable groundwater levels (high to low water levels) in a boreal environment. For this purpose, RCG plants were grown in environmentally controlled chambers over two growing seasons (April–September of 2009 and 2010), and the plant organ biomass (leaves, stems, and roots) was measured seven times over the entire growing season. The results showed that biomass production was mainly allocated to the leaves (LMF) and stems (SMF) early in the growing season, to the stems in the middle of the growing season, and to the roots (RMF) later in the growing season. Compared to ambient conditions, ET treatments increased LMF and SMF, and decreased RMF over the growing season under well-water conditions. Under low groundwater level, ET treatments decreased LMF and increased RMF throughout the growing season, and increased SMF in early periods and then decreased later in the growing season. CO₂ enrichment did not significantly affect the seasonal allocation pattern between plant organs. The effect of the groundwater level on biomass allocation was stronger than that of the climatic treatments. In conclusion, plant phenology controlled the seasonal course of biomass allocation in RCG and climatic treatments affected biomass allocation to each of the three plant organs, while the direction and extent of climate-related changes in biomass allocation depended on the availability of groundwater. The influence of groundwater level appeared to be crucial for the carbon gain regarding the production of RCG biomass for energy purpose and the concurrent sequestration of carbon in soils under changing climate in the mire sites used to cultivate RCG.