Interactions between Hygrophila polysperma and Ludwigia repens grown in saturated soil and shallow water under ex situ conditions

Abstract

Ludwigia repens, a native, and Hygrophila polysperma, an invasive, are morphologically similar amphibious plants occurring in spring runs of Central Texas and other aquatic habitat in Florida. Hygrophila polysperma is a dominant species in static and slow flowing waters < 0.05 m s(-2) while L. repens occurs sporadically in similar conditions in the San Marcos River. Planting efforts to restore L. repens in flowing water > 20 cm were unsuccessful where H. polysperma was manually removed. In this interaction study, Ludwigia repens and Hygrophila polysperma were subjected to different planting ratios. L. repens exhibited greater growth in shallow static water at a depth of 15 cm when grown in equal or higher ratios compared to H. polysperma. No difference in growth rates was detected between the species grown in saturated soils. Photosynthetic rates were greater for L. repens in saturated soils and shallow water indicating this species is more tolerant of sunlight than H. polysperma. Hygrophila polysperma allocated greater biomass to its roots indicating this species may be more adaptable to scouring from floods and extended droughts by greater regrowth from roots. Moreover, Hygrophila polysperma exhibited lower light saturation rates indicating this species is more tolerant of lower light levels. The results of this study indicate that L. repens will be competitive in restoration efforts if planted in shallow water with no or minimal flows in areas where H. polysperma has been removed.