Factors affecting the growth of Verbesina virginica

Verbesina virginica (frost weed) is a tall, perennial, herbaceous plant that has been observed to occur in Bexar County, Texas primarily under established tree canopies such as Quercus fusiformis (scrub live oak) and Ulmus crassifolia (cedar elm). The distribution of Verbesina virginica, and possible factors affecting its distribution are not well reported. To better understand these factors, a combination of field studies, greenhouse experiments, and leaf level Li-Cor gas exchange analysis were utilized to examine species distribution, determine the species' nutrient requirements, and the plants response to various light levels, respectively. Over five transects, extending from a canopy into an adjacent open grassland, 283 total plants were counted. Of those, 98.9% (280 plants) were found inside the canopy dripline. Mean soil depth along the transects was highly variable and not significantly different from one location to another. However, a light gradient was found ranging from low light beneath the canopy (207 +/- 53 mumol CO2 · m-2 · sec-1) and increasing into the open grassland (2126 +/- 71 mumol CO2 · m-2 · sec -1). Overall, dry plant mass and leaves per plant increased with increased nutrient additions however, the plants did tolerate the native low nutrient Patrick soil. Gas exchange measurements for Verbesina virginica over 19 light levels ranging from 0 to 2000 mumol · m-2 · sec-1 found its maximum rate of photosynthesis (Amax) to be 12.68 +/- 1.40 mumol CO2 · m-2 · sec-1, while its mean dark respiration (Rd) was 0.75 mumol CO2 · m-2 · sec -1. The mean light saturation point (Lsat) for V. virginica was 287.74 mumol · m-2 · sec-1, and mean light compensation point (Lcp) was 16.2 mumol · m-2 · sec-1. These results fall within established ranges reported for understory species, and together with observed field distributions suggest that Verbesina virginica is a shade species.