Transplant success for the endangered herb Arenaria paludicola at Golden Gate National Recreation Area: Effect of site, propagation type, and competition

Acierto, Krystal; Hendricks, Rebecca; Bontrager, Megan; Parker,, Ingrid

We established experimental populations of Arenaria paludicola in two main sites and four additional microsites at the Golden Gate National Recreation Area, within the plant’s historical range. We compared establishment success of nursery-grown vs. greenhouse-grown transplants, and we tested the effect of initial clearing around the plants. Planting was completed December 10, 2011, and plots were censused in January, February, April, and at the end of July, 2012. Of the two sites, the Rodeo Beach site was more successful: plants survived better, were more robust, and were substantially larger in July. At the time of site selection, the Rodeo Beach site had less standing water and greater cover of Oenanthe sarmentosa, the species considered to be an indicator for A. paludicola habitat based on previous work. Interestingly, the biggest jump in mortality occurred from February to April, rather than from April to the end of July, suggesting that moisture stress was not the main cause of mortality over this time period. In fact, locations with higher gravimetric soil moisture content experienced higher, not lower, plant mortality for greenhouse-propagated plants. We expected competition from surrounding plants to have a negative effect on A. paludicola, and we tested this by experimentally clearing vegetation from a 20cm-diameter circle around half of the plants. We found that clearing around plants increased survival at Rodeo Beach but decreased survival and health at the wetter Miwok site. The magnitude of the effect was small at both sites and disappeared by July. Thus clearing is probably not worth the extra time and effort in A. paludicola outplants. Finally, we expected that propagation methods would strongly influence plant survival, with plants relocated from an outdoor nursery having greater tolerance to natural environmental conditions than plants transplanted directly from greenhouse conetainers. Surprisingly, plants from conetainers had significantly better early survival than nursery transplants, and greenhouse plants were consistently healthier and larger throughout the study at Miwok. However, at the Rodeo site, the nursery plants gained an advantage in later survival, health, and size. It is unclear at this time whether propagation type will have a consistent long-term effect on success. Experimental transplants can provide important information for endangered species management; our results suggest that site selection is the most important factor in the early success of A. paludicola reintroductions.