Between invaders and a risky place: Exotic grasses alter demographic tradeoffs of native forb germination timing

Abstract

AbstractPriority effects are hypothesized to play an important role in community assembly and may promote suppression of native by exotic species. Work in a range of grassland systems has proved valuable for testing these effects, demonstrating that earlier germination by some exotic annual grasses contributes to their competitive dominance over natives. Yet few studies have measured native forb germination phenology under field conditions, and the demographic consequences of emergence timing for competitive interactions and native fitness are not well understood. We focused on three native annual species in a southern California grassland dominated by exotic Bromus spp. over three years, measuring (1) seedling emergence rates, for both early (October) and later (November and December) germinators; (2) effects of exotic grasses on native survival and reproduction, through a grass removal experiment; and (3) interactions between emergence timing and grass competitive effects on native mortality, survival, and flowering. We quantified tradeoffs of emergence timing, by estimating mortality experienced by early germinants until the late cohort emerged (early survival), and then for both cohorts from the time of late emergence to flowering (spring survival). The two most common focal natives, Amsinckia intermedia and Phacelia distans, varied substantially in germination phenology but primarily emerged early. The less abundant Clarkia purpurea germinated late. Late emergence reduced spring survival in control plots but not those where exotic grasses were reduced experimentally, supporting the importance of priority effects and benefits of early germination in competition with grasses. However, early emergence entailed a high cost of initial mortality risk in some years. We found no effect of emergence timing on size at flowering. Estimates of net survivorship to flowering suggest that late emergence consistently was associated with the highest survival when exotic grasses were reduced experimentally. Early emergence was more favored in control than in exotic grass reduction plots, but the survival tradeoffs differed substantially between years. These results suggest that priority effects contribute to suppression of native forbs, but may not consistently promote higher fitness for earlier germinators. Instead, exotic invasion may increase yearly variation in the fitness consequences of native germination phenology, with potential implications for bet hedging strategies.