Using rainout shelters to evaluate climate change effects on the demography of Cryptantha flava

1. Precipitation in arid regions is temporally variable with much of it arriving in discrete, unpredictable pulses. Climate change models predict an increase in the variation of precipitation, with longer droughts and larger rainfall events, in addition to increased temperatures. 2. A life table response experiment (LTRE) was conducted with the herbaceous arid-land perennial Cryptantha flava (Boraginaceae) from 1997 to 2000, in order to determine how variation in precipitation affects asymptotic population growth (lambda) and vital rates. Variation in precipitation took two forms, through rainout shelters erected just before and during the 1999 spring growing season, and through naturally occurring variation over the 4 years. 3. An unexpected effect of rainout shelters on l was observed that could not be attributed to drought. The lambda value decreased greatly in control census plots in the 1998-99 transition but did not decrease in sheltered of plots. The 1999 spring, when shelters were in place, was unusually cold, resulting in frost damage to unsheltered plants. Plants under shelters experienced elevated nighttime temperatures and escaped frost damage. The greater value of lambda in the sheltered plots is attributable almost entirely to greater contributions of survival, growth and stasis. 4. There were residual effects of the 1999 drought treatment in the 1999-2000 transition as lambda decreased in the sheltered populations while l increased in control populations. Compared to controls, there were large negative effects of survival, growth, and stasis and a large positive effect of retrogression in the droughted plots. 5. Natural variation in fecundity contributed considerably to inter-annual variation in lambda in control plots. Annual variation in fecundity strongly paralleled annual variation in seedling establishment. The relative contributions of other vital rates varied greatly among the annual transitions. 6. Synthesis. The study illustrates the sensitivity of this arid-land species to early season frost events and to variation in precipitation. It suggests the importance of considering the seasonal timing of precipitation events when projecting the population-level consequences of global climate change, with particular attention given to precipitation that triggers germination and seedling establishment. Researchers using shelters to create drought treatments should consider their nighttime warming effects.