Clone structure in four Solidago altissima (Asteraceae) populations: rhizome connections within genotypes

We studied clone structure and degree ofgenotypic mixing of Solidago altissima L. (Asteraceae) clones in four old fields near Ithaca, New York. The fields differed in time from agricultural abandonment and were approximately 1, 5, 20, and 35 years old. In the three older fields, three 0.75 m2 plots were excavated intact and rhizome connections among ramets were mapped. In the youngest field 30 ramets were dug up singly. The genotype of all ramets was determined using electrophoresis of four polymorphic enzyme systems. Fields differed in the number and dispersion of genotypes within plots, and the degree of connection among ramets in the same clone. The one-year-old field was composed of single ramet genotypes which had probably established from seed the previous year. The five-year-old field contained many small contiguous clones of S. altissima with highly interconnected ramets. In the oldest two fields clones were highly intermixed and ramets of the same genotype were not extensively interconnected. These results demonstrate that clones of S. altissima display considerable phenotypic variability between fields and pattems of clone development may differ. The causes of this variability remain to be identified. We suggest that either selection for different genotypes or changing habitat conditions during succession may lead to changes in clone form. ECOLOGISTS HAVE BEGUN to focus increasing attention on the biology of clonal species (Cook, 1983; Jackson, Buss, and Cook, 1985; Harper, Rosen, and White, 1986). Many species of plants form clones through the underground growth of structures such as rhizomes and root buds that render the study of clonal biology in the field difficult and time-consuming. In addition, the physical connections between parent and daughter ramets often decay, thereby obscuring the genetic relatedness of ramets within a clone. Consequently we know relatively little about the physical and genetic structure of most natural populations of clonal plant species. Three elements of structure may affect the functioning and interaction of clones in the field. First, the relative position of ramets will Received for publication 3 November 1987; revision accepted 13 July 1988. We thank Nurit Fischler, Ed Gooding, Art Jankowski, Lucille McCook, and Heather Robertson for assistance in the field. Special thanks to Bemie May and the Comell Laboratory of Ecological and Evolutionary Genetics (CLEEG) for help with electrophoresis and to Monica Howland for drawing the figures. Warren Abrahamson, Michael Cain, Dave Gill, Peter Marks, and several anonymous reviewers made helpful comments on the manuscript. This work was supported by NSF Grant BSR8213831 to Robert E. Cook, NSF Grant DEB-812-0053 to Richard B. Root, and an NSF predoctoral fellowship to Peter Wimberger. influence the outcome of interactions with neighboring plants (Hartnett and Bazzaz, 1983; Goldberg and Werner, 1983; Bulow-Olsen, Sackville Hamilton, and Hutchings, 1984; Schmidt, 1986). Second, the degree of physiological connection among ramets will determine how much resource sharing between ramets can potentially occur. Such sharing may minimize differences in the performance of individual ramets and cause the clone to function as a single physiological individual (Pitelka and Ashmun, 1985). Finally, if plant genotype affects competitive ability, insect and disease resistance, or the probability of outcrossing, then the genetic identity of ramets will determine how neighboring clones interact with each other and other species (e.g., Kinsman, 1982). Although studies of the overall genetic diversity of clonal plant populations have been conducted (Silander, 1985; Ellstrand and Roose, 1987), they usually do not examine the genotypic structure on a scale small enough to assess the potential for interactions among genotypes. For many species we do not know whether clones form discrete spatial units or whether ramets from different clones grow intermixed. Although a number of studies have measured the overall size of natural clones (Cook, 1983, for references), very few studies have mapped genotypes at a scale that permits definition of potential ecological interactions (Edwards, 1984; Reinartz and Popp, 1987).