Experimental demography of rhizome populations of establishing clones ofSolidago altissima

1Solidago altissimaforms large compact below‐ground rhizome systems. The rhizome systems of 30 genets in a population that had colonized an old‐field in 1984 were mapped yearlyin situfrom 1988 to 1992. Branching angles, lengths and survival of rhizomes, and production of daughter rhizomes, were calculated from digitized maps. 2We hypothesized that genets ofS. altissimawould prevent intraclonal crowding by producing longer but fewer rhizomes as they increased in size. In addition, we tested the effects of environmental factors on rhizome dynamics by (i) mowing in late summer, (ii) cutting of rhizome connections, and (iii) removal of close neighbour plants. 3Most rhizomes were initiated at an angle of 60°−65° relative to the parent rhizome. Subsequent growth tended towards the parent axis and towards more centrifugal growth. Rhizome angles were not influenced by any treatment. 4In control genets, rhizome density increased 1.3‐fold from March 1988 to March 1992. The number of newly produced rhizomes per genet decreased and their length increased from 1988 to 1992. Longer rhizomes had a higher survival probability than shorter rhizomes did. The number of newly produced rhizomes per parent rhizome was positively correlated with mean rhizome length per genet. 5Mowing strongly decreased rhizome density from 1988 to 1992, mainly due to reduced production of new rhizomes. 6Rhizome cutting led to a slight decline in the rhizome population and to shorter lengths of rhizomes in their second year compared with controls. 7The removal of close neighbour plants led to increased rhizome densities. These rhizomes grew longer than controls in unmown plots and shorter in mown plots and had higher probabilities of surviving into their second year. 8Yearly mowing could successfully prevent the further spread ofS. altissimabecause it acts on rhizome and shoot dynamics as well as on seed output.