# Thermal Plasticity of Multiple Traits Varies More Within Than Between Populations of Plantago lanceolata at Its Northern Range Edge

## TL;DR

This study finds that variation in how Plantago lanceolata plants respond to temperature changes occurs more within populations than between them at the species' northern range edge.

## Contribution

The study demonstrates that thermal plasticity variation is primarily within populations, challenging assumptions about population-level differences at range edges.

## Key findings

- Thermal plasticity was observed in leaf size, flowering traits, and pigmentation.
- Most variation in thermal plasticity was found within populations rather than between them.
- Higher pathogen symptoms and infection rates were detected under different temperature regimes.

## Abstract

Temperature plays a pivotal role in defining the distribution of species and the fitness of individuals within species' ranges. Phenotypic plasticity can allow individuals to cope with varying environmental conditions, including rapid climate change. Populations at range edges experience more variable conditions than core populations and thus are hypothesized to exhibit higher thermal plasticity. However, as the strength of plasticity often varies between individuals, it can also differ among local populations at range edges. We studied the extent of and variation in thermal plasticity for several traits within and between populations of the perennial herb
Plantago lanceolata
L. (Plantaginaceae) at its northern range edge. We sampled seeds from nine sites within a 50 × 50 km region and grew them under three temperature regimes in a greenhouse. We measured traits related to size, flowering, pathogen responses, and inflorescence pigmentation. We expected to find higher plasticity in traits less strongly connected to fitness and that differences between individuals would outweigh differences between populations in underpinning this variation in plasticity. Our results show thermal plasticity in leaf size and abundance, flowering probability and abundance, and pigmentation. Notably, we also found increased pathogen symptoms and higher infection rates of one of two viruses screened, highlighting the potential for changes in pathogen sensitivity and exposure under climate change. Importantly, in all traits but flower abundance, more variation in plasticity was attributable to differences within populations than between populations. Although this contribution was small in magnitude compared to thermal effects on traits, the higher intra‐ versus interpopulation variation in plasticity suggests that differences between individuals provide most of the variation in thermal plasticity, which may be driven by small‐scale variations in habitat conditions, highlighting the need for conservation strategies that consider microhabitat variation to support short‐term adaptive responses to thermal variability.

In this study, we investigate thermal phenotypic plasticity across multiple traits in
Plantago lanceolata
populations located at the species' northern range edge in the Åland Islands, Finland. Using a controlled greenhouse experiment with three temperature regimes, we assessed variation in vegetative, reproductive, pigmentation, and pathogen‐related traits across nine populations. Our findings reveal that while thermal plasticity is evident across several traits, the majority of variation in plasticity occurs within rather than between populations.