Shortgrass steppe and northern mixedgrass prairie plant species traits

Despite progress in trait-based ecology, there is limited understanding of
the plant traits that structure semiarid grasslands. In
particular, it remains unclear how traits that enable plants to
cope with water limitation are related to traits that influence other key
functions such as herbivore defense and growth. The hypothesis that
drought and herbivory exert convergent selection pressures is supported
for morphological traits, but largely untested for struct­ural,
physiological, and phenological traits. Drought and economic traits can
also covary, but where and to what degree remains uncertain. Here we
address these uncertainties in semiarid shortgrass steppe and mixedgrass
prairie, the largest remaining grasslands in North America. Using a broad
selection of traits for 37 of the most common plant species in each
ecosystem, we ask whether traits that confer drought tolerance, avoidance
and escape covary with herbivore resistance traits and economic traits.
Results reveal that both drought tolerance and escape are coordinated with
other functions, but in opposite fashion. Drought tolerant species (low
leaf osmotic potential and high leaf dry matter content, LDMC) were also
herbivore resistant (high leaf toughness and cellulose) and at the ‘slow’
end of the economic spectrum (low leaf nitrogen, leaf phosphorus, and high
stem density). Conversely, drought escape via early senescence was
associated with lower drought tolerance, lower herbivore resistance, and
‘fast’ economic traits. Drought avoidance, as indicated by thick leaves,
may also be associated with lower drought tolerance (LDMC). Senescence
date and LDMC appear to be key traits in these semiarid grasslands,
differentiating species along multiple axes of function. Synthesis –
Covariation between drought, herbivory and economic traits means that, of
the many potential trait combinations, few actually exist within these
grasslands. Consequently, changes in land management and climate should
have predictable effects on drought resistance, forage quality and
productivity in the western Great Plains.