The comparative 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

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 non-heated 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.