Belowground clonal expansion and biomass allocation of Phragmites australis along a sand burial gradient in mobile dunes

In May 2021, newly emerged ramets were identified from naturally established wild populations within the study area on the northeastern margin of the Ulan Buh Desert. For each ramet, the attached rhizome tip was carefully excavated to preserve belowground integrity. To compare rhizome expansion under contrasting field burial conditions, a total of 18 intact rhizomes were identified and classified into three burial depth categories, with six rhizomes in each category: shallow (0–20 cm), intermediate (20–40 cm), and deep (40–60 cm). The horizontal extension length of each rhizome was measured at 15-day intervals over a four-month period. Rhizome tips were marked during initial excavation to track subsequent growth direction and rate. In April 2021, a representative active sand dune on the northeastern margin of the Ulan Buh Desert was selected, and nine linear transects were established perpendicular to the prevailing wind direction, spaced 3 m apart. Six permanent quadrats (1 m × 1 m) were positioned along each transect to represent three microhabitats: interdune lowland, transition zone, and windward slope. Field sampling was conducted at three stages: early growing season (April 10), mid-season (June 10), and late season (September 10). During each sampling period, three previously unsampled transects were randomly selected for destructive harvest, and all P. australis individuals within each quadrat were harvested for trait and biomass measurements.