Phenology of grassland plants exposed to elevated temperature

Background/Question/Methods

Temperature often plays a major role in controlling phenology in plants. Elevated temperature can accelerate or delay flowering and alter the timing of emergence and senescence. We used a field warming experiment to examine the potential impact of global warming on the timing of emergence, flowering, and senescence of twelve grassland plant species, including grasses and forbs. For two years, established plots of grassland plants were warmed 2 and 4^oC over ambient with overhead infrared heat lamps. Individuals of the twelve species were tagged and their height and reproductive stage recorded weekly. For each species, the percentage of individuals flowering or seeding in each plot was also monitored weekly. Plot-level senescence was estimated by measuring canopy reflectance in the autumn of 2010. From these data, Normalized Difference Vegetation Index (NDVI) values were calculated for each plot at each date.

Results/Conclusions

For several legume species, warming significantly affected the date at which the first flowering individual was observed, but this differed between years. In 2009, flowering of Amorpha canescens and Dalea purpurea started 8-9 and 4-5 days earlier in the +4 and +2^oC plots, respectively, compared to ambient plots (p-values: 0.005 and 0.008). In 2010, D. purpurea started to flower 11 days earlier at +4^oC than at ambient temperature (p-value: 0.007). Lupinus perennis also showed a trend towards earlier flowering under warming in both years, but this trend was only significant in 2009 (p-value: 0.03). Grasses and non-leguminous forbs did not display a shift in flowering when warmed. Plots warmed 2 and 4^oC displayed 11% and 24%  higher NDVI values than ambient plots, indicating delayed senescence (p-value: 0.0001).           

The effect of warming on flowering was species-specific with some species flowering earlier under elevated temperatures. Altering the timing of flowering can affect trophic interactions and competition with neighboring plants. The delayed senescence suggests that the growing season will be lengthened in the future, impacting the carbon cycle.