The comparative biology of the two closely-related species Penstemon tenuiflorus Pennell and P. hirsutus (L.) Willd. (Scrophulariaceae, section Graciles): IV. Effects of shade, drought, and soil type on survival and growth

Effects of shade, drought, and soil type on survival and growth of the two closely-related species Penstemon tenuiflorus and P. hirsutus were compared in a greenhouse study. Eighty to 100% of 15 replicate plants of P. tenuiflorus and 60% to 93% of 15 plants of P. hirsutus survived in the three light levels under which they were grown, i.e., no shade, intermediate shade, and heavy shade. Plant dry mass (root, shoot, total) decreased with increase in shading: no shade > intermediate shade ≫ heavy shade. For a given level of shading, there were no significant differences between the two species for any of 12 plant growth variables measured. In a 30-day drought experiment during which soil moisture decreased to the permanent wilting percentage on day nine, four and three of five replicate plants of P. tenuiflorus per treatment, and four and one of those of P. hirsutus, survived the 18- and 21-day drought, respectively; no plants of either species survived the 24-, 27-, or 30-day droughts. Of the five soil types tested, plants of both species survived and grew best in greenhouse soil (high P) and worst in black shale soil (very low pH). Plants of P. hirsutus grown in greenhouse soil and in sandstone soil produced significantly more dry mass than did those of P. tenuiflorus in these soil types. Amount of growth was relatively low for plants of both P. tenuiflorus and P. hirsutus grown in native cedar glade and limestone-ledge soils (both with very low P), respectively. This study suggests that P. tenuiflorus is a bit more tolerant to drought than is P. hirsutus, whereas P. hirsutus has the ability to grow better on a greater variety of substrate types than does P. tenuiflorus. The major findings of Parts I-IV of this comparative study of the biology of P. tenuiflorus and P. hirsutus are summarized.