Biomass allocation in Ceiba pentandra (Malvaceae) under water stress and high CO2 concentration

Climate models predict prolonged droughts in parts of Amazonia; however, it is unclear how trees of the region will respond to these changes. The aim of this study was to evaluate how elevated CO2 and water stress affect biomass allocation to plant organs and photosynthesis of Ceiba pentandra at the juvenile stage. The treatments were two CO2 levels (400 and 800 ppm) and two water regimes (soil at 50 and 100% field capacity) applied for 138 days. We measured the amount of biomass allocated to organs of the plant, photosynthesis and plant water use efficiency during the experimental period. Light saturated photosynthesis expressed on a leaf mass basis decreased 34.6% at elevated CO2 and 24.7% under water stress. Total biomass gain per plant was unresponsive to elevated CO2, but declined 43% under water stress (p < 0.001). However, plant water use efficiency increased 50% at elevated CO2 and also under water stress (p < 0.001), but in contrast, the shoot/root ratio was unaltered by water stress and elevated CO2. These results improve our understanding of the effect of elevated CO2 and water stress in Ceiba pentandra, a multipurpose tree with wide occurrence in the Amazonian region.