Life history characteristics of two Gulf Coast Helianthus species restricted to soil types

Background/Question/Methods

Helianthus annuus and H. argophyllus are the most common species of sunflowers in Nueces County, Texas.  Previous studies have found H. argophyllus was restricted to sandy soil sites, while H. annuus was nearly restricted to clay soil sites and that the germination phase of development does not appear to play a role in this distribution.  In our area H. argophyllus senesces in October but it is not uncommon to see H. annuus growing and flowering every month of the year.  To better understand the life history characteristics of these species and to involve large numbers of students in a research project the Islander SEEDS chapter at TAMUCC developed a project to follow these species through the growing season.  We hoped to determine how long individual H. annuus plants survive and to quantify differences between these species which may help to understand their distribution.  Ultimately, once we understand the life span of H. annuus individuals we hope to determine if the growing season of this species could be extended in other areas that may experience warmer winter conditions in the future.  Two 2x2 m plots were established for each species in July 2010.  Each plant in the plots was tagged with a plastic tag.  Height, number of inflorescences and seed heads were recorded for each plant at regular intervals until October. 

Results/Conclusions

As expected, H. argophyllus senesced in October.  Although H. annuus was growing and flowering at other sites, all H. annuus plants in our plots and the area around our plots were dead by early November.  These results indicate that there are different cohorts of H. annuus are active throughout the year.  Our study did not allow us to investigate this issue more carefully.  We hope to design an experiment to address this issue in the future.  An ANVOA indicates there are significant differences for maximum height and reproductive output as measured by total number of inflorescences between species (P < 0.0001).  These results will help guide our next study in which we hope to understand the length of time individual H. annuus plants are active and to better quantify lifetime reproductive output in each species.