Direct and indirect trade‐offs between resistance, growth and reproduction in the Japanese stinging nettle <i>Urtica thunbergiana</i>

Abstract 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. Read the free Plain Language Summary for this article on the Journal blog.