Responses of Erodium to Varying Microsites: The Role of Germination Cueing

Abstract

I examined the role of germination cueing in the dynamics of Erodium botrys and E. brachycarpum populations exposed to localized, spatial variation within Californian annual grasslands. In laboratory trials, germination rates of both species increased significantly in seeds previously exposed to fluctuating temperatures (25—45°C) during dry storage. Temperature fluctuations per se were more important than temperature maxima for increasing germination rates. Light conditions during germination had no significant effect on germination rates. Field trials were also conducted. Summer soil temperature regimes were characterized for three germination microsites of Erodium: (1) under the dense litter of annual grasses, (2) in bare soil, and (3) under mounds created by burrowing pocket gophers. Diurnal fluctuations in soil surface temperature were 17°—51°C on bare soil, 20°—41° under litter, and 18°—35° under gopher mounds. Summer field manipulations examined the rates of release from seed dormancy for buried seeds; both species showed a strong positive relationship between the germination rates and the mean diurnal range of temperatures for each microsite. To examine whether seed germination rates were accurate predictors of the suitability of a site for Erodium colonization, germinated seeds of both species were introduced into each microsite type at the start of the growing season. Emergence rates and survival were significantly reduced in mound microsites; seed output per plant was lower in litter microsites. There was a strong positive relationship between Erodium colonization success, as measured by net reproductive rate, and the mean summer diurnal range of soil temperatures. The accuracy of the cueing response in predicting site suitability was reduced with drought or heavy grazing. The results overall suggest significant adaptive value for germination cueing in both Erodium species.