Effects of drought stress on photosynthesis, rhizosphere respiration, and fine‐root characteristics of beech saplings: A rhizotron field study

Soil drought influences the C turnover as well as the fine-root system of tree saplings. Particularly during the period of establishment, the susceptibility to drought stress of saplings is increased because of incompletely developed root systems and reduced access to soil water. Here, we subjected beech saplings (Fagus sylvatica L.) to different levels of drought stress. Beech saplings were planted in rhizotrons, which were installed in the soil of a Norway spruce forest before bud burst. Soil moisture was manipulated in the following year during May to September. We measured photosynthetic net CO2 uptake, volume production of fine roots, and rhizosphere respiration during the growing season. Biometric parameters of the fine-root system, biomass, and nonstructural carbohydrates were analyzed upon harvest in October. Photosynthesis and rhizosphere respiration decreased with increasing drought-stress dose (cumulated soil water potential), and cumulative rhizosphere respiration was significantly negatively correlated with drought-stress dose. Fine-root length and volume production were highest at moderate soil drought, but decreased at severe soil drought. The proportion of fine-roots diameter < 0.2 mm and the root-to-shoot ratio increased whereas the live-to-dead ratio of fine roots decreased with increasing drought-stress dose. We conclude that the belowground C allocation as well as the relative water-uptake efficiency of beech saplings is increased under drought.