Evolutionary ecology along invasion fronts of the annual grass Aegilops triuncialis
Over the last two decades, Aegilops triuncialis (barbed goatgrass) has rapidly spread into many annual grassland and serpentine soil sites within California, USA. The capacity of this species to invade edaphically stressful serpentine soil is especially unusual. It is unclear whether genetic differentiation, phenotypic plasticity, or both have allowed A. triuncialis to invade competitive (i.e. high productivity non-serpentine annual grassland) and edaphically stressful (i.e. low productivity serpentine) environments. We used a reciprocal transplant field experiment to examine the effects of plasticity and genetic variation on A. triuncialis phenology and demography along invasion fronts associated with interspecific competition and edaphic gradients. We reciprocally transplanted seeds collected behind invasion fronts (core subpopulations) and along invasion fronts (edge subpopulations). For both gradient types we measured higher reproduction and population growth at invasion front edges. This was true for both edge and core subpopulation seed sources, suggesting that phenotypic plasticity may facilitate invasive spread. Consistent planting site effects indicated that phenotypic plasticity is a primary contributor to A. triuncialis demographic responses along interspecific competition gradients. In contrast, significant seed source effects suggest genetic differentiation along invasion fronts in serpentine edaphic gradients. Although persistent maternal environmental effects cannot be ruled out entirely, seed source effects suggest genetic differences between serpentine subpopulations located behind and beyond the invasion fronts for plant survival, plant size, total seed production, and individual seed size. Rapid expansion of A. triuncialis in California may reflect an evolutionary capacity in this species for both phenotypic plasticity and genetic differentiation.
KeywordsAegilops triuncialis Invasion front Genetic differentiation Local adaptation Phenotypic plasticity Serpentine
The authors wish to thank the personnel at the University of California McLaughlin Reserve and the University of California Hopland Research and Extension Center for logistic support. Comments from S. Keller and two anonymous reviewers on earlier versions of the manuscript were very helpful. Special thanks to K. Lyons for generously sharing her expertise on goatgrass invasion ecology and evolution. This research was supported by U.S.D.A. grant ARS-X3 W-3402 to K. Rice and A. Dyer, U.S.D.A. NRI grant 2005–35320–15314 to J. McKay and K. Rice, and Packard Foundation Grant 2000–01607 to K. Rice.
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