Why is Solidago shortii narrowly endemic and S. altissima geographically widespread? A comprehensive comparative study of biological traits

Aim and location: Solidago shortii T. and G. occurs in a small area of north-central Kentucky (USA), whereas S. altissima L. is geographically widespread in NorthAmerica and is naturalized in Europe, Asia and Australia. Solidago shortii grows in a variety of habitats, but primarily in rocky/shallow soil areas, and S. altissima occurs in adjacent old fields/pastures. The purpose of this study was to identify differences in the biology of these two species that might contribute to an explanation for differences in their geographical distribution and habitat. Methods: Results of an extensive literature review and of original research by the authors were used in a detailed comparison of the two species. The most ecologically relevant aspects of the biology investigated were morphology, genetics, geography, life cycle, life history, reproductive biology, physiology, biotic ihteractions and response to disturbance. Results: Although S. shortii and S. altissima are quite similar in many aspects of their biology, there are some important differences. Solidago altissima isa better competitor than S. shortii via its greater height, larger leaf area and more extensive clonal growth. On the other hand, S. shortii is a better drought-stress tolerator than S. altissima via allocation of a higher percentage of biomass to roots, higher root/shoot ratio and (seemingly) greater capacity to maintain leaf turgor (i.e. not wilt as readily) under xeric conditions. Moreover, higher seed production in S. altissima and its capacity to form a larger- andlonger-lived soil seed bank than S. shortii probably contribute to differences in colonization abilities. Landscape disturbances and metapopulation dynamics are important factors in maintenance of the species population of S. shortii, and in prevention of its extinction. Main conclusions: Solidago shortii probably is a relatively young species that as had insufficient time to expand its range. Furthermore, it is not well suited for migration to, or colonization of, habitats outside its rafige due to poor competitive and colonization abilities.