PS 7-87 - Asynchronous intra-annual population dynamics facilitate co-dominance of two functionally similar prairie grasses

Monday, August 8, 2016
ESA Exhibit Hall, Ft Lauderdale Convention Center


Jesse E. Gray, Colorado State University; Melinda D. Smith, Colorado State University


In mesic regions of the Great Plains, two grass species, Andropogon gerardii and Sorghastrum nutans, account for the majority of plant individuals and determine ecosystem structure and function. Despite functional similarities, fluctuating population sizes, and shared resources, seldom is either species locally extirpated. This co-existence is thought to be mediated by differential timing of tiller emergence of each species. Under current climate conditions, A. gerardii tiller emergence completes during a relatively brief window early in the growing season. In contrast, although the majority of S. nutans tillers also emerge early in the growing season, tiller emergence can occur throughout the growing season. A. gerardii’s up-front tiller investment may provide an early-season advantage, but only in years when water availability is sufficient to sustain growth. Delayed tiller emergence might give S. nutans the advantage with delayed water availability. Thus, with climate changes, such as increased variability in precipitation regimes, the ability of these species to coexist may be altered. To examine processes contributing to mutual persistence of these ecologically important species and whether these are modified by climate change, we assessed population dynamics of these species in a 9-yr experiment that heated and increased the timing between rainfall events without changing total rainfall amount.  Late- and early-season tiller densities were measured and intra- and interannual density dynamics were analyzed in plots receiving ambient and altered growing season precipitation.

Nine-year average tiller densities in control plots confirm observations of asynchronous tillering patterns. In the altered-rainfall treatment, long-term average asynchrony was also observed, though with reduced average tiller densities across both species. However, in later years, this asynchrony deteriorated. From 2009-2013, 2/7 observed seasonal population shifts were synchronous across species in ambient plots, while 4/7 and 7/7 were synchronous in altered-rainfall, and altered-rainfall + heat treatments, respectively.  In years with low rainfall from March-May (2006, 2009), S. nutans late-season stem densities were significantly higher than A. gerardii’s, and in altered-rainfall + heated plots only, S. nutans maintained higher tiller density from 2009-2013. Across treatments, both species’ populations declined from 2005-2013, but more so in altered-rainfall treatments. These results suggest that intra-annual asynchrony in stem density exists between the two competing species, and this asynchrony may be a stabilizing mechanism for longer-term persistence of co-dominance between these species. Because the success of these strategies may depend on current precipitation variability, that mutual persistence may be at risk as variability increases.