Phragmites australis at an extreme altitude: Rhizome architecture and its modelling

In central Europe Phragmites australis is a lowland plant, occurring rarely up to the tree line. In the Velka Kotlina cirque (Jeseniky mountains, NE C zech Republic), where it reaches its maximum altitude at about 1350 m a.s.l ., its culms are 0.5-0.7 m high and the plants flower only in some years. D uring the last decade no germinable seeds have been observed. The architect ure of Phragmites rhizomes from this site was studied on seven randomly sel ected clonal fragments. They consisted of 3 to 10 partial tussocks (clumps) and 4 to 17 green shoots. The total length of the rhizomes was 9.7 to 50 m per plant. The number of nodes per plant was 96 to 431 and the longest int ernodes were 83 mm long. The number of side branches was 31 to 105 per plan t. The branching angle depended on the type of branched rhizome. The mean a ngles of horizontal rhizomes, which connect individual tussocks, were relat ively wide (modus 45 degrees, arithmetic mean 37 degrees), whereas within a tussock much sharper angles of branching prevailed (modal value 5 degrees, arithmetic mean 15 degrees). The mean internode-to-internode angle on cont inuing rhizomes was about 8 degrees, with a wide variation. An architectura l, spatially-explicit model of Phragmites rhizome growth has been developed , showing that the Phragmites population in the studied locality can be mai ntained by vegetative multiplication, and seedling recruitment is not neede d for its long-term persistence.