Diverse responses of phenology to global changes in a grassland ecosystem

Abstract

Shifting plant phenology (i.e., timing of flowering and other developmental events) in recent decades establishes that species and ecosystems are already responding to global environmental change. Earlier flowering and an extended period of active plant growth across much of the northern hemisphere have been interpreted as responses to warming. However, several kinds of environmental change have the potential to influence the phenology of flowering and primary production. Here, we report shifts in phenology of flowering and canopy greenness (Normalized Difference Vegetation Index) in response to four experimentally simulated global changes: warming, elevated CO(2), nitrogen (N) deposition, and increased precipitation. Consistent with previous observations, warming accelerated both flowering and greening of the canopy, but phenological responses to the other global change treatments were diverse. Elevated CO(2) and N addition delayed flowering in grasses, but slightly accelerated flowering in forbs. The opposing responses of these two important functional groups decreased their phenological complementarity and potentially increased competition for limiting soil resources. At the ecosystem level, timing of canopy greenness mirrored the flowering phenology of the grasses, which dominate primary production in this system. Elevated CO(2) delayed greening, whereas N addition dampened the acceleration of greening caused by warming. Increased precipitation had no consistent impacts on phenology. This diversity of phenological changes, between plant functional groups and in response to multiple environmental changes, helps explain the diversity in large-scale observations and indicates that changing temperature is only one of several factors reshaping the seasonality of ecosystem processes.

Fig. 1.

Shift in date of flowering onset caused by the four simulated global changes, for nine common species observed in the JRGCE. The effect shown for each species is the difference in mean first flowering date for all plots where each aspect of environmental change is elevated (n = 64) versus ambient (n = 64), over 3 years of observation.

Fig. 2.

Flowering onset data. (A) Mean day of first flowering in relation to proportional abundance for nine common species under ambient conditions. Mean dates of flowering onset averaged across the grass and forb species for all treatment combinations at ambient temperatures (B) and with warming (C). Treatment abbreviations are as follows: C, elevated CO₂; N, nitrogen deposition; R, increased rainfall; T, increased temperature. n = 8 plots per treatment, averaged across 3 years of observation. Error bars denote 1 SE of the mean.

Fig. 3.

Mean day of peak canopy greenness (NDVI) for all treatment combinations at ambient temperatures (A) and with warming (B). Treatment abbreviations are as follows: C, elevated CO₂; N, nitrogen deposition; R, increased rainfall; T, increased temperature. n = 8 plots per treatment, averaged across 4 years of observation. Error bars denote 1 SE of the mean.