Direct and indirect trade-offs between resistance, growth, and reproduction in the Japanese stinging nettle Urtica thunbergiana

Most studies of trade-offs between defence, growth, and reproduction have
examined pairwise correlations between these processes, used ratio-based
measures for defence allocation such as allelochemical concentration and
trichome density, and estimated resource allocation to growth and
reproduction in terms of biomass. However, for statistical and biological
reasons, it may be preferable to analyse these processes holistically, to
use absolute amounts of resistant traits and leaf mass/area, and to
measure growth and reproduction in terms of nodes with or without
flowers.  We aimed to identify how leaf stinging hairs as
resistant structures and leaf area as a functional trait affected growth
and reproduction in the Japanese stinging nettle Urtica
thunbergiana.  We conducted a greenhouse experiment with nettles
derived from a population that has been historically exposed to heavy
browsing by Sika deer (Cervus nippon) in Nara Park, Japan. We analysed
causal relationships between stinging hair number, leaf area, growth rate,
growth performance, and reproductive output using structural equation
modelling (SEM). In this analysis we adopted newly developed indices for a
plant’s stinging hair number and leaf area, and measures of growth and
reproductive traits in terms of nodes instead of biomass.  There
was a significant covariation between stinging hair number and leaf area.
Stinging hair number had direct negative effects on growth rate and
performance, while leaf area had positive direct effects on growth rate,
growth performance and reproductive output. The growth rate had a
significant direct positive effect on reproductive output, resulting in a
significant indirect negative effect of stinging hair number on
reproductive output. This indicates that there is not only a trade-off
between resistance and growth, but also an indirect trade-off between
resistance and reproduction through reduced growth rate and suggests
that U. thunbergiana sacrifices growth rather than reproduction
to increase resistance. Our study provides future work on trade-offs
between defence, growth, and reproduction with a new methodological
framework that can assess indirect as well as direct trade-offs, together
with the effects of leaf area as a functional trait on these processes.