High Frequency Minirhizotron Observations Reveal Diel Patterns of Root Growth Under Water Stress

Growth and death of vegetation control both the amount of carbon held in living biomass and much of the flow between the atmosphere and soils, the largest terrestrial carbon reservoir. However, long-term field datasets on belowground growth dynamics are rare. Knowledge is lacking on how root growth patterns relate to the environment, fluctuating carbon and water availability, and internal plant physiological control, at both seasonal and diel scales. We imaged roots in a temperate grassland using automated minirhizotrons and interpreting root length density (RLD) automatically twice a day for nine months. From winter to autumn, we always observed some root growth in individual images, but with decreasing net RLD (indicative of net turnover) becoming predominant later in the season. Roots grew day-round except during a period of about one month in spring when root growth was only in daytime. This was a period without rain, during which soil moisture depleted rapidly, but not at its driest point. We discuss potential causes for both overall and short-term growth patterns, particularly the change in drivers of growth from internal storage to new photosynthates, or limits imposed by water availability - both soil moisture and rainfall - under shifting environmental conditions.