Nitrogen enrichment drives accelerative effect of soil heterogeneity on the flowering phenology of a dominant grass

Plant phenology is affected by both abiotic conditions (ie. temperature,
nitrogen enrichment, drought) and biotic conditions (ie. species
diversity). The degree of spatial heterogeneity in soil resources is known
to influence community assembly and dynamics, but the relationship between
resource heterogeneity and phenology, or the potentially interactive
effects of soil resources on phenology, are less understood. We leveraged
a tallgrass prairie restoration experiment that has manipulated soil
nitrogen availability and soil depth over 20 years to test the effects of
environmental heterogeneity, nutrient enrichment, and potentially
interactive effects of global change drivers (nutrient enrichment and a
drought manipulation) on the phenology of a highly dominant prairie grass
(Andropogon gerardii). We recorded the timing of major developmental
stages of A. gerardii in plots containing four soil heterogeneity
treatments (control, soil depth heterogeneity, nutrient/depth
heterogeneity, and nutrient/precipitation heterogeneity). We found that
the boot, first spikelet, and emerged spikelet stages of A. gerardii
occurred earlier in treatments with greater heterogeneity of soil
nitrogen, and this effect was driven by the accelerative effect of
nitrogen enrichment on phenology. Reduced precipitation increased the
flowering length of A. gerardii, but did not otherwise affect
developmental phenology. There were no interactive effects among any soil
resource treatments on phenology. These results advance our understanding
of the relationship between plant phenology and global change drivers,
which is important for understanding and predicting the timing of plant
resource use and the provision of resources to higher trophic levels by
plants under varying levels of resource availability.