Two adaptive strategies to face variable environments have been proposed for annual plants in deserts: delayed and plastic (predictive) germination. All theories and models have been formulated based on studies on winter annuals, although half of the world deserts have summer annuals. We conducted an experiment to quantify the response of summer annual communities located along an altitudinal gradient (proxy for aridity gradient) to simulated precipitation levels. We expected to find increasingly lower germination fractions (more conservative) as we moved from the highest (less arid) area to the lowest (more arid) area, and also higher germination thresholds in the more arid end of the gradient. The experiment was implemented with two variables: altitude (2900, 3150, 3400 and 3600 m a.s.l.) and simulated precipitation (20, 40, 60, and 80 mm). The full generalized linear model showed that the number of seedlings was related to precipitation, altitude, and their interaction. The number of seedlings was positively related to the precipitation: more seedlings germinated in the less arid sites, and their slopes were steeper, suggesting stronger effects of precipitation in less arid areas. These results supported our first prediction: seed banks from the drier area tend to have more conservative germination responses. We also found a plastic behavior in all four altitudes. Our results show that summer-annuals may utilize both survival strategies as a way to cope with environmental uncertainty.
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