The stress gradient hypothesis (SGH) proposes that the interactions
between species are negative (competitive) under conditions that are
favorable for mutual population growth but become increasingly
positive (facilitative) as environmental stress increases. We tested
this hypothesis in a greenhouse experiment that varied the initial
densities of two functionally similar mesic tallgrass species, Andropogon gerardii (big
bluestem) and Sorghastrum nutans (yellow indiangrass). Half of the
artificial communities received sufficient water applications to
prevent moisture-related stress throughout the length of a growing
season, while the other half were subjected to water deficiencies in the
second half of the growing season.
Results/Conclusions
Measurements of survivability, population growth and
biomass productivity did not provide evidence in support of the SGH, as both
species performed more poorly with increasing density of their
interspecific competitor, regardless of water treatment. These
findings suggest that the increasingly positive interactions cannot be
relied upon to maintain the stability of codominance between these two
important tallgrass species as the frequency of drought increases
under climate change. Moreover, other mechanisms of coexistence must
be responsible for their contemporary abundances. Other species pairs that share functional similarity may exhibit the same limitation.