ASSESSING LIMITATIONS ON THE GROWTH OF ENDANGERED PLANT-POPULATIONS .1. EXPERIMENTAL DEMOGRAPHY OF ERYSIMUM-CAPITATUM SSP ANGUSTATUM AND OENOTHERA-DELTOIDES SSP HOWELLII

This study was designed to identify factors that limit the growth of populations of two federally-listed endangered plants-Erysimum capitatum ssp. angustatum (= ECA) and Oenothera deltoides ssp. howellii (= ODH)-both endemic to Antioch Dunes National Wildlife Refuge (Contra Costa County, California, USA). Herein we report on the relevant aspects of experimental seedling demography in situ. Seeds were precisely sown into areas that differed with respect to substrate (dune, clay and clay that had been mechanically disked) and exposure (shaded and exposed dune areas). ECA was very dependent on sandy dune substrate for high germination and survivorship, as well as the vigorous growth of established plants. Mortality was linked to exposure and substrate drought, rather than herbivory or soil nutrient deficiency. Disking the clay substrate promoted germination but not survivorship or plant growth and could not, therefore, be used to improve the availability of high-quality habitat for ECA at the refuge. In contrast, substrate characteristics and exposure were not as critical to ODH because individuals became established and reproductive on disked clay as well as on exposed dune sand. Mortality of ODH was linked to insect herbivory on the dune sand and moisture or nutrient availability on the disked clay and clay substrates. Therefore, in order to increase the size and areal extent of these populations at Antioch Dunes, management should include (1) mechanical disking of clay substrate to improve habitat availability for ODH; (2) removal of non-native vegetation to provide more unstabilized, shaded dune substrate for ECA and to reduce insect herbivory on ODH seedlings; (3) creation of new sand-dune habitat for both taxa by importing riverine sand; and (4) re-establishment of ODH and ECA populations in habitat restored by disking or weeding or into habitat created by sand importation.