Water availability dictates how plant traits predict demographic rates

A major goal in ecology is to make generalizable predictions of organism
responses to environmental variation based on their traits. However,
straightforward relationships between traits and fitness are rare and
likely vary with environmental context. Characterizing how traits mediate
demographic responses to the environment may enhance predictions of
organism responses to global change. We synthesized 15 years of
demographic data and species-level traits in a shortgrass steppe to
determine whether the effects of leaf and root traits on growth and
survival depend on seasonal water availability. We predicted that (1)
species with drought-tolerant traits, such as lower leaf turgor loss point
(TLP) and higher leaf and root dry matter content (LDMC and RDMC), would
be more likely to survive and grow in drier years due to higher wilting
resistance, (2) these traits would not predict fitness in wetter years,
and (3) traits that more directly measure physiological mechanisms of
water use such as TLP would best predict demographic responses. We found
that graminoids with more negative TLP and higher LDMC and RDMC had higher
survival rates in drier years. Forbs demonstrated similar yet more
variable responses. Graminoids grew larger in wetter years, regardless of
traits. However, in both wet and dry years, graminoids with more negative
TLP and higher LDMC and RDMC grew larger than less negative TLP and low
LDMC and RDMC species. Traits significantly mediated the impact of drought
on survival, but not growth, suggesting survival could be a stronger
driver of species’ drought response in this system. TLP predicted survival
in drier years, but easier-to-measure LDMC and RDMC were equal or better
predictors. These results advance our understanding of the mechanisms by
which drought drives population dynamics, and show that abiotic context
determines how traits drive fitness.