The aim of this work was to analyse how the seasonal biomass growth and allocation in a boreal bioenergy crop (Phalaris arundinacea L., hereafter RCG) were affected by elevated temperature and CO2 under different levels of groundwater. For this purpose, plants in peat monoliths representing young and old cultivations were grown in auto-controlled environmental chambers over two growing seasons (April-September, 2009 and 2010) under elevated temperature (ambient + 3.5°C) and CO2 (700 μmol mol−1). Three levels of groundwater, ranging from high (0 cm below the soil surface), to normal (20 cm below the soil surface) and low (40 cm below the soil surface), were used.
Compared to growth under ambient conditions, elevated temperature (ET) enhanced leaf development and photosynthesis in the RCG plant. Consequently, ET enhanced biomass growth during early growing periods. It also reduced photosynthesis and caused earlier leaf senescence during later growing periods. ET therefore reduced total biomass growth across the entire growing season. Elevated CO2 (EC) significantly increased biomass growth throughout the growing period primarily because of increased leaf area and photosynthesis. Lower groundwater level (LW) decreased the growth of RCG, mainly because of lower leaf area and photosynthesis. Furthermore, LW accelerated the cessation of growth, thus making the growing season shorter compared with the effects of higher groundwater levels. The LW- induced reductions in biomass growth were exacerbated by ET and partially mitigated by EC. The combination of elevated temperature and CO2 (ETC) slightly increased plant growth. The age of cultivation did not affect the biomass growth among the three major organs (leaf, stem and root) and thus did not affect total biomass. Biomass allocation was clearly controlled by plant phenology.
Biomass growth was mainly allocated to leaves and stems in the early growing season, to stems in the middle of the growing season and to roots later in the growing season. Under EC, root growth contributed more to total biomass growth compared to the leaf and stem growth in biomass, regardless of groundwater levels. The opposite was observed under ET and ETC and well-watered conditions (which was opposite to what was observed under LW). Our results show that climatic treatments affected biomass growth and biomass allocation to each of the three plant organs, while the direction and extent of climate-related changes in biomass growth and 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 purposes and the concurrent sequestration of carbon in soils under changing climates in the mire sites used to cultivate RCG.