comparitive biology of the two closely-related species Penstemon tenuiflorus pennell and P. hirsutus (L.) willd. (Scrophulariaceae, Section Graciles): III. Ecological life cycle, growth characteristics, and flowering requirements, The

Castanea, Jun 2002 by Clements, Richard K, Baskin, Jerry M, Baskin, Carol C

ABSTRACT

Ecological life cycles, growth characteristics, and flowering requirements of the two closely-related species Penstemon tenuiflorus and P hirsutus were compared via a combination of field, greenhouse, and growth chamber studies. In addition, field observations were made on plant-animal interactions involving these two species. Both species are hemicryptophytes that reproduce by seeds and by vegetative offshoots, and their life cycle stages, phenology (where their ranges overlap in northcentral Tennessee), growth characteristics, and flowering requirements are the same. Plants flower in spring, and seeds are mature by late summer; seed dispersal lasts from late summer until early spring. Seeds germinate primarily in early spring, and plants form a rosette their first year; neither species forms a persistent seed bank. In a nonheated greenhouse, plants bolted and flowered in their second year, whereas in the field bolting and flowering are delayed until at least the fourth year. Neither net assimilation rate, relative growth rate, nor eight of 10 other parameters of growth differed between plants of the two species grown under greenhouse conditions. Plants of both species require exposure to several hundred hours of vernalizing temperatures to flower; however, they are indifferent to photoperiod with regard to flowering (i.e., day-neutral). Leaves of P tenuiflorus are browsed by cottontail rabbits and inflorescences by whitetail deer, and seeds of both species are predated by lepidopteran larvae. Thus, the ecological life cycle, growth characteristics, physiological requirements for development, and plant-animal interactions are very similar in these two taxonomically-distinct species. It seems unlikely that any of these aspects of their autecology accounts for differences between them either in geographical distribution or habitat ecology.

INTRODUCTION

An objective of the discipline of ecology is to understand the complexity and diversity in morphology, physiology, life history, and other attributes of plants that determine their ecology (Clapham 1956, Grime 1984). To attain this goal, comparison, which is a method of study, has to be used (Bradshaw 1987). Bradshaw (1987) stated that, ". . . we can have no idea about the significance of a particular species' distribution unless we have one for a second with which to compare it." We have used the comparative approach to investigate the ecology of Penstemon tenuiflorus Pennell and P hirsutus (L.) Willd. (Scrophulariaceae), two taxonomically closely-- related, interfertile members of Section Graciles in eastern North America (Pennell 1935, Koelling 1964, Crosswhite 1965, Clements et al. 1998). Although there have been quite a few reports on some aspects of the life cycle biology of Penstemon species (e.g., seed germination, see RESULTS AND DISCUSSION), our study on P tenuiflorus and P hirsutus is the first to include all major stages of the life cycle. It also is the first comprehensive study on the comparative biology of Penstemon species, rare or otherwise.

The geographic range of Penstemon tenuiflorus extends from western Kentucky to the Black Belt of Alabama and Mississippi and that of P hirsutus from western Virginia and northcentral Tennessee to Wisconsin, Ontario, Quebec, and Maine (Clements et al. 1998). Penstemon tenuiflorus recently was reported, apparently for the first time, from the Ridge and Valley Physiographic Province, growing on the Ketona dolomite outcrops in Bibb County, Alabama (Allison and Stevens 2001). Penstemon tenuiflorus grows in open rocky woods, cedar glades, chalk prairies, xeric limestone prairies (prairie barrens), and (rarely) on river cliffs, usually on calcareous substrate (Clements 1995, Morris et al. 1993, Webb et al. 1997, Allison and Stevens 2001). The habitat of P hirsutus includes dry sandy or rocky open woods, rocky bluffs, rocky shores of lakes, alvars, rocky barrens, gravel hill prairies, and sand prairies (Gleason 1910, Core 1948, Post et al. 1985, Bartgis 1993, Clements 1995, Catling and Brownell 1999). Within their geographic range of overlap in northcentral Tennessee and southcentral Kentucky, the two taxa occupy different habitats, primarily limestone cedar glades for P tenuiflorus and limestone cliffs for P hirsutus.

This is the third in a series of papers on the comparative biology of Penstemon tenuiflorus and P hirsutus. In the first paper, Clements et al. (1998) demonstrated that differences in leaf pubescence and flower color, by which these two taxa easily are distinguished in the field, were maintained under uniform growth conditions for 4 years (until study ended). On the other hand, an SEM study showed that they did not differ in seed coat or pollen morphology (Clements et al. 1998). In the second paper, Clements et al. (1999) showed that flowering pattern for flowers and for inflorescences, flowering phenology, and mating system of these two species are the same. Further, progeny resulting from selfing in P tenuiflorus did not exhibit inbreeding depression as measured by seed weight, seed germination, and various growth parameters (P hirsutus progeny not tested for inbreeding depression). However, in both P tenuiflorus and P hirsutus significantly fewer seeds were produced by outcrossed than by selfed flowers. As a further contribution to the comparative biology of P tenuiflorus and P hirsutus, the present study investigated their ecological life cycles, growth characteristics, and physiological requirements for flowering. In addition, plants of the two species were observed in the field for disease, herbivory, and seed predation.