The abiotic and biotic factors that influence the germination and growth of Heliotropium tenellum in central Texas cedar glades
Cedar glades consist of open areas with shallow soils where limestone or dolomite rock is at or near the surface in which natural, long-persisting plant communities are dominated by herbaceous plants occur. Within cedar glades, surface light levels are high and soil moisture is frequently low. Cedar glades within Juniperus woodlands are well documented in eastern North America but they are also present in other regions of North America and throughout central Texas, particularly in the Edwards Plateau Region. The vegetation within the central Texas cedar glades is predominately herbaceous, with low plant cover, relative to the surrounding Juniperus woodlands. A summer annual that occurs within these cedar glades is Heliotropium tenellum. The biology and ecology of this species is largely unknown.
The purpose of the current study was to examine abiotic and biotic factors that may influence the germination and growth of Heliotropium tenellum in central Texas cedar glades. The study was conducted in the southeastern region of the Edwards Plateau in Bandera County, Texas (29" 41' 41.36" N, 99" 01' 33.41" W). Seed dimensions, percent germination, gas exchange rates, xylem water potential, and soil water potential of H. tenellum were measured. In addition, abiotic and biotic factors within the surrounding woodland and within the cedar glade open gaps, including light intensity, soil depth, density of H. tenellum, cover of H. tenellum, rock, leaf litter, other herbaceous plants, woody plants and bare soil were measured.
Seeds of H. tenellum were small and exhibit physiological dormancy, which required cold/wet stratification to break the dormancy. Acid scarification, when coupled with cold/wet stratification, increased germination significantly. Gas exchange rates, as well as light response curve variables, were all high which indicated that H. tenellum is a sun plant. Xylem water potential of H. tenellum at midday was not very low (-1.79 MPa) which indicated that it was not under a considerable amount of water stress, in spite of high light levels, high gas exchange rates, and low soil moisture. The distribution of H. tenellum within cedar glades appears to be most influenced by light levels, however, the presence of leaf litter, other herbaceous plants, and bedrock also appear to be important.