The effects of soil moisture, soil oxygen, and soil salinity on the growth of Helianthus paradoxus
The largest population of Helianthus paradoxus (the Pecos sunflower = puzzle sunflower), a federally threatened annual species, is found along Leon Creek in the Diamond-Y Springs Preserve near Fort Stockton, Texas. Because of its limited distribution, its potential for extinction is high. This species occurs in bands parallel to an intermittent creek or drainage, immediately adjacent to the wettest part of the marsh. The dominant vegetation, surface soil salinity and soil water change as one moves from the drainage upland. It has been noted that during dry years the population of H. paradoxus colonizes closer to the drainage and during wet years the population of H. paradoxus colonizes away from the drainage. Associated with the changes in soil moisture, soil oxygen also change. This study evaluated the relationship between soil moisture, soil salinity, soil oxygen and the growth of Helianthus paradoxus. The study consisted of large growth boxes tilted at an angle. The boxes were watered from the lower end to achieve a gradient similar to that found in the marsh—with the lower end saturated with water and the upper end dry. There were two watering treatments, either de-ionized or Diamond-Y Springs Preserve creek water with a salinity of 8 g/kg. Helianthus paradoxus seedlings were grown in five rows, extending from the lower-wet end to the upper-dry end of the box. There were five plants per row. At the end of the experiment, plants were harvested. Stem length, basal diameter, aboveground dry mass, soil moisture, soil salinity, and soil oxygen were measured at each plant location. Maximum growth (stem length, basal diameter, or aboveground dry mass) occurred in the middle row of the boxes. Data suggest that soil moisture and soil salinity can be used to predict the growth of H. paradoxus . Quadratic transformations of both soil surface and subsurface water best explain the changes in growth, suggesting that this species is limited at reduced and excessive levels of soil moisture. Waterlogged soils may reduce the growth by restricting the diffusion of oxygen. In addition, higher salinity levels reduced growth. Data from the Diamond-Y Springs Preserve paralleled the findings of the controlled growth boxes. This suggest that abiotic factors, such as soil moisture, soil salinity, and oxygen; rather than biotic factors, such as competition from other species, define this species distribution at the Diamond-Y Springs Preserve, and probably other locations where it is found. The results from this study will be beneficial for the management of this threatened species at the Diamond-Y Springs Preserve, and perhaps help in determining other potential locations for establishment.
Bush, Janis Kathleen, "The effects of soil moisture, soil oxygen, and soil salinity on the growth of Helianthus paradoxus" (2002). ETD Collection for University of Texas, El Paso. AAI3049702.