Altered precipitation patterns and simulated nitrogen deposition effects on phenology of common plant species in a Tibetan Plateau alpine meadow

- Flowering dates were more synchronous under wetter conditions.
- Phenological shifts lag behind abiotic conditions.
- Augmenting fall and winter precipitation had dramatic effects on phenology.
- Precipitation effects were either diminished or reversed by N addition.
- Interannual microenvironment variation strongly influenced species phenology.

The interactive effects of five seasonal precipitation distribution patterns and two levels of N deposition (ambient and doubled) on phenological traits of six common plant species were studied in an alpine meadow of the Tibetan Plateau for two consecutive years. Seasonal precipitation patterns included ambient (control), reduced fall/winter/spring - increased summer, increased fall/winter - reduced spring, increased fall/winter - reduced spring/summer, and increased spring - reduced summer. Our results indicate that: 1) phenological trait variation was predominantly due to species differences (P < 0.0001), yet flowering dates for annual forbs and perennial graminoids were more synchronous under wetter conditions in the second season, which likely led to more intense competition for soil resources between these two groups; 2) the effect of treatments on species phenological traits appeared in the second growing season only, suggesting that phenological shifts in these species lag behind abiotic conditions and/or require cumulative exposure to these factors (≥2 years); 3) redistributing dormant season and spring precipitation to summer caused earlier flowering and longer flowering duration for perennial graminoids (P. pratensis, K. setchwanensis, respectively) and earlier and shorter duration of fruiting for the annual forb G. paludosa; yet N addition diminished and/or reversed these effects. Moreover, augmenting fall and winter precipitation at the expense of spring and summer precipitation decreased all phenological traits of P. anserina except for flowering date, but N addition reversed these effects, and; 4) interannual variation in species phenology was strongly influenced by environmental differences among the two growing seasons rather than different precipitation patterns and N addition. We discuss how seasonal shifts in precipitation and/or greater N deposition in the future may impact plant species fitness, species coexistence, and vegetation composition in alpine meadow ecosystems of the Tibetan Plateau.