Controls on vertical root distribution dynamics in a temperate grassland across daily and seasonal scales

Background and aims “Hydromatching” is a phenomenon consisting of the daily-scale promotion of root growth in a newly wetted soil layer and/or a decline in root growth in drier layers. This phenomenon was previously observed on individual plants under controlled conditions. The aim of this study was to determine if hydromatching occurs also under natural settings at a grassland community scale. Our goal was also to assess what environmental parameter was driving root growth in the grassland community and to analyze seasonal shifts in root distribution. Methods We installed twelve minirhizotron tubes in a natural grassland. We imaged the tubes from May 2022 until August 2023 and we carried out image analyses to extrapolate root length and growth rates. We determined the major environmental driver of root growth through regression analyses and monitored the root growth dynamics after major rain events and during the spring-summer transition. Results Soil moisture was the strongest predictor of root growth. Following rain events, root growth shifted from deeper layers to shallow layers within 1-3 days, indicating the occurrence of hydromatching. During the spring-summer transition, we observed significant promotion of growth in deeper soil layers and decline in root length in shallower layers. Conclusions Root distribution responded to seasonality with drastic shifts at the whole root system level and to precipitation with smaller but significant rapid shifts. Both types of response could adhere to an optimization strategy, consisting of the promotion of root growth in resourceful areas while discarding roots where resources are less accessible.