COS 112-5
The effect of drought timing on flowering of a dominant C4 grass in tallgrass prairie

Thursday, August 14, 2014: 2:50 PM
Regency Blrm A, Hyatt Regency Hotel
John D. Dietrich, Graduate Degree Program in Ecology, Colorado State University, Fort Collins, CO
Elsie M. Denton, Graduate Degree Program in Ecology, Colorado State University, Fort Collins, CO
A.K. Knapp, Graduate Degree Program in Ecology, Colorado State University, Fort Collins, CO
Melinda D. Smith, Graduate Degree Program in Ecology, Colorado State University, Ft. Collins, CO
Background/Question/Methods

The C4 grass, Andropogon gerardii, produces flowers at a highly variable rate; because of its dominance in tallgrass prairie, and the size of its flowering stalks (2-m+), flowering by this species can have a large impact on total aboveground productivity.  Little is known about what factors control flowering of A. gerardii, beyond a relationship with frequency of fire, and observations of very low flowering during drought years.  Precipitation falling between April 15 and July 14 has been shown to be particularly important for aboveground production; this is likely related to its impact on the initiation of flowering.  In order to evaluate the importance of precipitation timing within this broad interval, a drought timing experiment was conducted during the 2013 growing season at the Konza Prairie Biological Station, in northeastern Kansas.  Study plots were located in upland site burned in the spring.  Six precipitation treatments were imposed: an ambient precipitation treatment; three precipitation exclusions beginning at different times (April 20, May 22 and June 22) each excluding approximately 150 mm or 25% of the long term average growing season precipitation; and two water addition treatments of different magnitudes – one matching the long-term average (LTA) and the other 30% above the LTA.

Results/Conclusions

The highest density of flowering stems, and largest ratio of reproductive to vegetative biomass occurred in the long term average water addition, with similar levels in the ambient and plus 30% treatments.  The middle and late droughts were significantly below the LTA treatment; with reductions of 80 and 66% for density of flowering stems and 80 and 77% for the ratio of reproductive to vegetative biomass. This reduction in the ratio of reproductive to vegetative biomass was driven by a large and significant reduction in reproductive biomass rather than vegetative biomass. .  These results suggest that timing of drought plays an important role in flower production for A. gerardii.  It is not clear, however, how much of this effect is due to the level of reduction in soil moisture, which was different between the three drought treatments, as potential evapotranspiration is much higher in June and July than in April and May, vs. how much of this effect is due strictly to timing.  Mechanisms controlling flowering may respond directly to external cues, either dependent or independent of timing, or they may be mediated by more cumulative effects, such as the accumulation of biomass, carbohydrates or other nutrients.