Flexible or fortified? How lichens balance defence strategies across climatic harshness gradients.

Lichens play important roles in habitat formation and community succession in polar and alpine ecosystems. Despite their significance, the ecological effects of lichen traits remain poorly researched. We propose a trait trade-off for managing light exposure based on climatic harshness. In the harshest cold environments, where abiotic stress predominates over biotic pressures, lichens should rely on photostable, recalcitrant and immobile substances such as allomelanin and hydrophobic compounds. These compounds provide durable protection without the need for continual synthesis. In milder conditions where biotic interactions - for example, competition and pathogen presence - become increasingly pronounced, lichens should retain flexibility and produce simple protective secondary compounds that, in addition to functioning as light screens, can leach out to influence their direct environment. Preliminary empirical findings for Antarctic lichen species distribution are consistent with this hypothesised trade-off, in that lichens producing soluble compounds dominate in milder regions and are less represented at higher southern latitudes, where species producing insoluble compounds with a melanised thallus dominate. As climate change progresses, increasing temperatures and precipitation could make the currently coldest and driest areas more hospitable, allowing the ranges of lichens producing soluble compounds to expand, with cascading effects on rock weathering, nutrient cycling and other ecosystem processes.