Effects of leaf herbivory and autumn seasonality on plant secondary
metabolites: a meta-analysis
Abstract
Plant secondary metabolites (PSMs) are produced by plants to overcome
environmental challenges, both biotic and abiotic. We were interested in
characterizing how autumn seasonality in temperate and subtropical
climates affects typical PSM production in comparison to herbivory.
Herbivory is commonly measured from spring to summer when plants have
high resource availability and are prioritizing growth and reproduction.
However, autumn seasonality also challenges plants as they cope with
limited resources and prepare survival for winter. This suggests a
potential gap in knowledge on how autumn seasonality affects PSM
production differently from herbivory. Using meta-analysis, we recorded
production of 22 different PSM subgroups from 58 published papers to
detect a typical response across all PSMs. We also compared production
of five phenolic subgroups – hydroxybenzoic acids, flavan-3-ols,
flavonols, hydrolysable tannins, and condensed tannins. We calculated
effect sizes from herbivory studies (absence to presence) and temperate
to subtropical seasonal studies (summer to autumn), while considering
other variables (e.g., plant type, increase in time since herbivory,
temperature, and precipitation). We did not detect a shared effect of
herbivory or season on PSM production across all subgroups. However, we
discovered herbivory having a positive effect on flavonol production and
autumn seasonality having a positive effect on flavan-3-ol and condensed
tannin production. We discuss how these responses might stem from three
factors: 1. some PSMs are constitutively produced by plants in autumn
whereas others are induced only following herbivory, 2. plants produce
metabolites with higher costs only during seasons when other resources
for growth and reproduction are less available, and 3. some PSM
subgroups serve more than one function for plants and such functions can
be season dependent. The outcome of our meta-analysis is that autumn
seasonality changes PSM production differently from herbivory, and we
see value in further investigating seasonality-herbivory interactions
with plant chemical defense.