Experimental demography of rhizome populations of establishing clones of Solidago altissima

Abstract

1. Solidago altissima forms 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 yearly in situ from 1988 to 1992. Branching angles, lengths and survival of rhizomes, and production of daughter rhizomes, were calculated from digitized maps. 2. We hypothesized that genets of S. altissima would 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. 3. Most 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. 4. In 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. 5. Mowing strongly decreased rhizome density from 1988 to 1992, mainly due to reduced production of new rhizomes. 6. Rhizome cutting led to a slight decline in the rhizome population and to shorter lengths of rhizomes in their second year compared with controls. 7. The 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. 8. Yearly mowing could successfully prevent the further spread of S. altissima because it acts on rhizome and shoot dynamics as well as on seed output.