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Timing of precipitation is equally important as amount for determining ecosystem function, especially aboveground net primary productivity (ANPP), in a number of ecosystems. In tallgrass prairie of the Central Plains of North America, grass flowering stalks of dominant C4 grasses, such as Andropogon gerardii, can account for more than 70 % of ANPP, or almost none of it, as the number of flowering stalks produced is highly variable. Although growing season precipitation amount is important for driving variation in flowering stalk production, it remains unknown whether there are critical periods within the growing season in which sufficient rainfall must occur to allow for flowering. The effect of timing of rainfall deficit (drought) on flowering of A. gerardii, was tested by excluding rainfall during three periods within the growing season (starting in mid-April, mid-May and mid-June). Mid-summer drought (starting in mid-June) strongly reduced the flowering rate (e.g., density and biomass) of A. gerardii (e.g., as high as 94 % compared to the control), suggesting flowering is highly sensitive to precipitation at this time. This effect appeared to be related to plant water status at the time of flowering stalk initiation, rather than an indirect consequence of reduced C assimilation. Our results suggest that increased frequency of growing season drought forecast with climate change could reduce sexual reproduction in this dominant grass species, particularly if it coincides with timing of flowering stalk initiation, with important implications for ecosystem functioning.

1. Random species loss has been shown experimentally to reduce ecosystem function, sometimes more than other anthropogenic environmental changes. Yet, controversy surrounds the importance of this finding for natural systems where species loss is non-random.2. We compiled data from 16 multi-year experiments located at a single native tallgrass prairie site. These experiments included responses to 11 anthropogenic environmental changes, as well as non-random biodiversity loss either the removal of uncommon/rare plant species or the most common (dominant) species.3. As predicted by the mass ratio hypothesis, loss of a dominant species had large impacts on productivity that were comparable to other anthropogenic drivers. In contrast, the loss of uncommon/rare species had small effects on productivity despite having the largest effects on species richness. 4. The anthropogenic drivers that had the largest effects on productivity nitrogen, irrigation, and fire experienced not only loss of species but also significant changes in the abundance and identity of dominant species. 5. Synthesis. These results suggest that mass ratio effects, rather than species loss per se, are an important determinant of ecosystem function with environmental change. K E Y W O R D S anthropogenic change, biodiversity, climate change, dominant species, ecosystem function and services, global change ecology, mass ratio hypothesis, non-random species loss 856 | Journal of Ecology SMITH eT al.

Summary Flower production can comprise up to 70% of above‐ground primary production in grasslands. Yet we know relatively little about how the environment and timing of rainfall determine flower productivity. Evidence suggests that deficits or additions of rainfall during phenlologically relevant periods (i.e. growth, storage, initiation of flowering and reproduction) can determine flower production in grasslands. We used long‐term data from the Konza Prairie LTER to test how fire, soil topography and precipitation amounts during four phenologically relevant periods of the growing season constrain three aspects of flowering in three dominant C4 grass species. Specifically, we examined the probability of flowering, flowering stalk density and individual flowering stalk biomass for Andropogon gerardii, Schizachyrium scoparium and Sorghastrum nutans. We found that each of the three species responded to the amount of precipitation during phenologically relevant periods in unique ways. All aspects of A. gerardii flowering were sensitive to precipitation during the flowering stalk elongation period (20 June – 3 August). The probability of S. nutans flowering was partly determined by precipitation during the rapid growth phase (21 April – 4 June), whereas flowering stalk density of this species depended on rainfall during flowering stalk elongation (20 June – 3 August). In contrast, all aspects of flowering of S. scoparium were relatively independent of rainfall during any period. Our results demonstrate that three functionally similar, co‐dominant C4 grass species respond differently to phenologically relevant precipitation periods. As a result, drought during any phenological window during the growing season can adversely impact biomass and flowering production of grasslands via species‐specific reductions in flowering stalk density and biomass. A http://onlinelibrary.wiley.com/doi/10.1111/1365-2435.12904/suppinfo is available for this article.

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