Experimental demography of the old‐field perennial<i>Solidago altissima</i>: the dynamics of the shoot population

Abstract

1 Control strategies are needed for the clonal species Solidago altissima , which is an aggressive invader in Europe that propagates vegetatively by persistent rhizomes that produce annual shoots. 2 The shoot dynamics of a population of S. altissima that had invaded an old‐field site in 1984 were studied from 1987 to 1992. Shoots from 120 genets of the same cohort were followed in relation to their parent genets. We assessed the effect of yearly mowing, yearly cutting of rhizomes and repeated removal of close neighbour plants on growth, survival, probability of reproduction and flux of these shoots. 3 In unmown plots, both shoot survival until the end of the growing season and the proportion of shoots producing seeds decreased (59% to 36% and 67% to 32%, respectively, from 1988 to 1992). The proportion of rhizomes producing shoots, however, rose (from c. 50% to more than 90%) so that annual shoot density increased almost fivefold by the end of the study, when it had almost reached equilibrium. 4 In contrast, shoot density in mown plots remained roughly constant from 1988 to 1992, after an increase from 1987 to 1988. Shoots were much smaller and a smaller proportion of them reproduced than in unmown plots. 5 Rhizome cutting (reduced clonal integration) produced a small but consistent decrease in height and in the percentage of sexually reproducing shoots. 6 The removal of close neighbour plants increased shoot density, and a higher percentage of these shoots survived and reproduced. 7 Experimental demographic studies thus show that (i) continued regular mowing reduces the production of both seeds and shoots, and so would prevent further spread of an invading S. altissima population; (ii) clonal integration may contribute to the species’ success as an invader because disconnected rhizomes produce smaller shoots with a lower chance of sexual reproduction; and (iii) competition decreases shoot density, mainly as a consequence of reduced shoot survival, and can therefore control S. altissima growth.