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We hypothesized that in seasonally-arid systems the evolution of annuality is controlled by both moisture regime and substrate quality, with infertile soils either precluding annuality or selecting for improved nutrient acquisition in annuals. The grass flora of the Cape Floristic Region to test these ideas.

We compared intrinsic variation in life history and nutrient acquisition traits between populations of Ehrharta calycina J. E. Sm. (Poaceae) situated along an aridity gradient and on diverse substrates. We also evaluated the importance of moisture regime and substrate as predictors of life history across 79 Cape grass species.

In E. calycina, rhizome survivorship, plant growth and reproductive maturation rate were interrelated and closely tied to wet season duration. By contrast, life history variation was poorly correlated with soil nutrients, and there was little evidence of enhanced nutrient acquisition in annual populations. Across multiple species, however, substrate was identified as a significant co-predictor of life history.

Our identification of substrate as an important predictor of life history across multiple species but not within E. calycina is not paradoxical, since E. calycina consistently acts as an annual species in avoiding the ultra-oligotrophic, quartzitic sands that dominate the Cape mountains. Overall, our data support the joint influence of climate and substrate on the evolution of annuality and associated life history traits.