Diagnostic Characteristics
Appears to have affinities with the imported Mediterranean P. arabica, but can be differentiated by the erect, scarious, bilobed apices of its sepals (Ertter 1985). Its small size is also relatively unique within the genus. Paronychia franciscana, the only other species of Paronychia in California (introduced from Chile) is a coastal, mat-forming perennial to 4 dm across, with herbaceous sepals lacking scarious margins; it is unlikely to be confused with P. ahartii (Ertter 1985).
Habitat
Well-drained rocky outcrops and rocky soils within volcanic uplands; often on vernal pool edges, higher ground around vernal pools, nearly barren clay of vernal swales, or other vernally moist sites with thin soils. Prefers the most stony microsites within its habitat, where vegetation is sparse and the density of competing annual plants is low. Sites are within valley and foothill grassland and cismontane (foothill) woodland plant communities. This species is restricted to the poorest, most sterile, rocky terrace soils bordering the Central Valley. Terraces on which it grows include 1) alluvial terraces deposited as glacial outwash during the Pleistocene, largely composed of gravel and cobbles, and 2) volcanic mudflow deposits of the Tuscan Formation with shallow soils. Soils are very red (clay rich), rocky/gravelly, and shallow; soils known to support populations include Tuscan and Corning soils. Sometimes associated with mima-mound topography. Associated species tend to be annual grasses and forbs and include common catchfly (Silene gallica), white-headed navarretia (Navarretia leucocephala), smooth catear (Hypochaeris glabra), Narrow-leafed owl's clover (Castilleja attenuata), California plantain (Plantago erecta), and silver hairgrass (Aira caryophyllea). 0 - 500 m.
Ecology
This very rare plant only produces about 10 seeds per individual. Grows in very poor soils where the density of competing annual plants is less than 25%. Associates include; Aira caryophyllea, Allium bolanderi, Allocarya austiniae, A. greenei, Blennosperma nanum, Brodiaea minor, Calycadenia fremontii, Chlorogalum angustifolium, Chorizanthe polygonoides, Cicendia quadrangularis, Dichelostemma pulchellum, Dodecatheon clevelandii, Erodium botrys, E. cicutarium, E. moschatum, Escholtzii lobbii, Evax acaulis, Hypochoeris radicata, Juncus bufonius, Lasthenia californica, L. platycarpha, Layia fremontii, Lepidium nitidum, Lessingia nemaclada, Lomatium utriculatum, Lupinus bicolor, Micropus californica, Microseris acuminatus, M. douglasii, Minuartia californica, Navarretia bakeri, Odontostoma hartwegii, Orthocarpus erianthus, Parvisedum pumilum, Plagiobothrys austinae, P. greenei, Plantago erecta, Pogogyne serpylloides, P. zizyphoroides, Polygonum parryi, Rigiopappus leptocladus, Scleranthus annuus, Scribneria bolanderi, Trifolium depauperatum, Triteleia hyacintha, Vulpia microstachys.