Positive effects of soil nitrogen pulses on individuals can have negative consequences for population growth during drought in a herbaceous desert perennial

1. Resource pulses generally result in a burst of biological activity at multiple scales. For plants, the increased activity is generally considered positive due to an overall up-regulation of physiological activity during the pulse. Longer-term effects remain an understudied aspect of resource pulses. 2. We monitored the short- and long-term effects of nitrogen (N) pulse to the long-lived desert perennial, Cryptantha flava. One group of plants were treated with a one-time application of N in the spring of 1999, a second group received two N pulses (one in the spring of 1999 and one in the spring of 2000), and a third group received ambient N (controls). 3. In the short-term, N-pulse treated plants rapidly increased leaf N concentrations, which in turn increased physiological activity and growth. But these responses were mediated by the availability of precipitation. 4. In a year with above-average precipitation, all plants increased in size, but N-treated plants grew more and had higher reproductive outputs than control plants. However, when the N pulse was followed by below-average precipitation in the next year, plants with the highest growth rates due to N pulses experienced greater reduction in size and reproduction coupled with increased mortality rates relative to controls. 5. At the population level a matrix model showed higher population growth rates in wetter years for N-treated plants compared to controls, but in drier years, N-treated plants showed lower growth rates. Size hierarchies were restructured as a result of the combination of variability in precipitation and N pulses creating more even size distributions. 6. Synthesis. The high degree of spatial heterogeneity of N pulses offers opportunities for enhanced growth and reproduction to individuals of C. flava within the larger population. Small plants with access to high soil N were able to maintain high survival, growth rates and reproduction as long as precipitation was adequate. However, increased N inputs probably resulted in a trade-off between reproduction and survival in small plants when precipitation was limiting. The high degree of unpredictability of resources in time and space ultimately contributes to the size hierarchies in this population and the variability in population growth rates.