SUMMARY: 1) Understanding how plants respond and adapt to varying environmental conditions has attracted the attention of plant ecologists for decades. To study this process, altitudinal gradients are used because of their inherent variation in environmental conditions. In the current scenario of global warming, altitudinal gradients may also represent a valuable resource to deepen our understanding about plant adaptive responses to predicted changes in environmental conditions. 2) Nowadays, the study of adaptive variation demands equal contributions from ecology and genetics. We need to assess the effects of selective pressures on phenotypic variation but also the genetic and molecular basis of phenotypic traits. The annual plant Arabidopsis thaliana represents a useful model system for investigating adaptive variation. Here, I characterized patterns of life cycle variation in natural A. thaliana populations along an altitudinal gradient to unravel how the species copes with different environmental conditions. 3) I periodically monitored thousands of plants from eight populations in NE Spain over five years (2007–2011) to estimate survival and fecundity schedules for autumn- and spring-germinated plants. Data were used to estimate net reproductive rate for each life cycle type. Data were regressed against altitude to detect altitudinal climatic clines for life cycle variation. 4) Survival of spring-germinated A. thaliana plants was significantly higher than that of winter-germinated plants. Plants from both cohorts exhibited similar fecundity values. The net reproductive rate of spring-germinated plants was four-fold higher than that of autumn-germinated plants. The proportion of spring-germinated plants increased significantly with altitude. 5) Synthesis. Arabidopsis thaliana can behave as a winter or spring annual plant. Nevertheless, the spring annual life cycle is clearly relevant to maintain A. thaliana populations, particularly at high-elevation locations. It is suggested that germination timing represents one of the most relevant traits to focus our efforts to understand adaptive variation in A. thaliana. The results illustrate the potential of annual plants to adjust their life cycles to varying environmental conditions encountered along a climatic gradient, which could mitigate the effects of global warming on annual plant populations.