How is phenology of grassland species influenced by climate warming across a range of species richness?

We imposed two different global changes, climate warming and declining species richness, to synthesized grassland ecosystems, in order to determine both their single-factor effect as well as their possible interactions on plant phenology. Twelve climate-controlled chambers were therefore exposed to two temperature treatments (0 °C and 3 °C above ambient air temperature) and three species richness treatments (1, 3 and 9 species). Measurements consisted of visual phenological observations of the appearance of flower buds until flowering and of autumn senescence stages, as well as of readings of the leaf chlorophyll content. In spring, chlorophyll concentrations increased under warming (9.7%) and decreasing species richness (39.4%). Warming also induced an earlier spring start (4 days) and enhanced flower production, whereas decreasing species richness likewise enhanced flower production but delayed the start of spring (16 days). In autumn, leaf senescence started simultaneously at the two temperature treatments and three species richness levels, but the end of the senescence was delayed in the heated chambers, as well as in the monocultures. Although warming and species richness interacted with species identity, no warming×species richness interactions were found. In conclusion, although warming and species richness influenced the duration of the growing season, none of these effects were reflected in the above-ground biomass of the individual plants. Under the projected climatic changes, warming will therefore not necessarily enhance the productivity of the individual plants.