Effects of source and exposure environment on vegetative and
reproductive traits
Vegetative and reproductive traits responded differently to source and
exposure environments in the greenhouse, as expected based on the
fundamental relationship of each type of trait with overall fitness.
Reproductive traits corrected for biomass showed stronger source effects
than vegetative traits, and less variability across environmental
treatments. According to evolutionary theory, traits with the strongest
impact on fitness should show evolutionary conservation (Scheiner 1993,
Stearns & Kawecki 1994, Sih 2004). In parallel, the demographic
buffering theory predicts that the most influential processes in species
life cycles should be maintained relatively constant around local
optimal values, to reduce variation in population growth rates (Pfister
1998, Burns et al . 2010, Hilde et al . 2020). For
short-lived plants like P. lanceolata , reproduction has been
identified as the most influential fitness component (Silvertownet al . 1996, GarcĂa et al . 2008, Shefferson and Roach
2012), which may explain the smaller role of plasticity and the higher
consistency in genetic differentiation found for biomass corrected
reproductive traits.
Stronger genetic differentiation in reproductive investment seemed to be
facilitated by a higher plasticity in vegetative traits, buffering
short-term environmental perturbations (Scheiner 1993, Alpert & Simms
2002, Sih 2004). This phenomenon, known as fitness homeostasis, has been
highlighted before as a mechanism for maintaining high individual
performance across a range of environments (Sultan 1995, Richardset al . 2006). The adjustment of vegetative traits to
environmental conditions was manifest in our greenhouse experiment in
several ways, and is best exemplified by SLA patterns. SLA increased in
the shade treatment to optimize light capture and decreased in dry
conditions to reduce water loss through leaf surface, common plastic
responses in herbaceous plants (Poorter et al . 2009, Dwyeret al . 2014). Remarkably, some effects of exposure treatments on
SLA were opposed by source environment effects suggesting
countergradient variation (sensu Conover & Schultz 1995), such as the
positive effect of source Aridity combined with the negative effect of
the dry treatment. This apparent contradiction possibly arises because
water scarcity in populations from dry sites is compensated through
selection for higher RSR and/or stomatal function.
The complex interplay between plasticity and genetic differentiation,
and the trait-specific nature of environmental effects found in our
study highlight the variety of strategies for plant response to local
conditions (see also Albert et al . 2010b, Le Bagousse-Pinguetet al . 2015, Roybal & Butterfield 2019), but also the difficulty
of assessing the mechanisms and drivers of trait variation. The trait
patterns found in P. lanceolata , including countergradient
variation, could be partly explained by the influence of additional
drivers not considered in the analyses, such as nutrient availability or
biotic interactions (Chevin & Lande 2015). Additionally, further
research could be undertaken to disentangle genetic differentiation from
unaccounted maternal environment effects.