Contrasting demographies and persistence of rare annual plants in highly variable environments

Spatial and temporal variation in critical life history traits differ markedly between two co-occurring, rare and endangered annual plants, Gilia tenuiflora arenaria and Chorizanthe pungens pungens, that have restricted, but overlapping geographic distributions in coastal habitats of central California, USA. Environmental variation was extreme during the 6-year study (1997–2002), with the greatest differences between an intense wet, warm El Nino year (1998), followed by a very dry, cool La Nina (1999). Both species have similar increases in adult survival and seed set with wetter, warmer weather in spring, but they differed in other traits: more Gilia germinated in years with high rainfall and temperature, while more Chorizanthe germinated in dry, cooler years; Gilia abundance increased with the number of years since the previous large El Nino event, while Chorizanthe abundance declined; and Gilia abundance was independent of the previous year’s seed set, while Chorizanthe density was directly related to the previous year’s seed set. The strong negative associations of Chorizanthe abundance with warm, wet weather and with time since the previous El Nino probably reflect the particular sequence of annual weather patterns from 1997 through 2002, especially the extremes in 1998 and 1999. Since it germinates readily in most years under a wide range of winter conditions and does not develop a long-lived seed bank, Chorizanthe seedling abundance reflects recent additions and depletions of its seed bank, rather than prevailing weather, per se. In contrast, Gilia seeds may remain in the seed bank for many years, until relatively rare winter conditions trigger significant germination. These species-specific demographies enhance persistence and coexistence of these species, but the mechanisms differ from each other and from those described for annuals in other highly variable environments.