Life history strategies in variable environments: Demography, delayed germination and bet-hedging in a desert annual Plantago
Temporal variability in the environment can affect population dynamics and life history strategies. Annual plants in desert environments are subject to large fluctuations in precipitation, both within and among years. I used a combination of field and laboratory studies to characterize variability in the populations dynamics and life history of a desert winter annual plant. I conducted demographic studies over four seasons in four populations of Plantago insularis Eastw. [Plantaginaceae] located along a precipitation gradient in the Sonoran Desert. I quantified the fates of germinated and non-germinated individuals by monitoring permanent quadrats and repeatedly sampling the soil seed bank. A fraction of seeds in all populations delayed germination and formed a persistent seed bank. Populations with more among-year variation in precipitation had greater variability in reproductive success for germinated seedlings, and the population in the most xeric environment was the most variable. Survival of non-germinated seeds was less variable than reproductive success of germinated seeds in all populations. Thus, non-germinating seeds reduced temporal variation in population dynamics by increasing population growth in dry years and decreasing growth in wet years. In this field study, populations in historically more xeric environments had lower mean germination fractions. Using geometric mean growth rate as an estimate of fitness, I demonstrate that germination fractions less than one were adaptive in three of four populations. Results of experimental studies with up to twelve populations of P. insularis, suggest that field germination responses emerged from a combination of two processes. Lower water availability during germination resulted in lower germination fractions. When water was amply available, all populations could express high germination fractions. A pattern of delayed germination consistent with bet-hedging as an adaptation to temporal variability in reproductive success arose in the field, in part, because frequencies of wet and dry germination conditions differed among populations. Phenotypic plasticity in germination response for seeds in the most xeric, environment conferred greater fitness than would have been possible with any one constant germination fraction.
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