Lichen hydration, moisture dynamics and climate change: A synthesis of established methods and potential new directions

The use of a functional trait approach has generally shown success in understanding how lichens are distributed, explaining their occurrence and abundance. Indeed, this success highlights the importance of understanding trait variability within and among lichen species, and at a community scale, especially where traits are related to hydration dynamics and subsequent photosynthetic activity. This review summarises what we know about lichen traits affecting hydration. We show that some lichen attributes – still the focus of attention today – were being described nearly a century ago and have since undergone refinement and redevelopment. Yet there remains a need for clear standardisation of trait measurements, and to help organise this progress we offer a distinction between core traits – whose effects are well categorised, and which have strong predictive control over measures of individual fitness and species or community response – and ancillary traits – that are worthy of investigation, but that currently have a less certain or a less clearly generalisable or transferable role in functional trait studies. Furthermore, we argue for recognition that lichens are not only poikilohydric (well cited) but also poikilothermic (less well cited), and that traits affecting hydration are closely coupled to traits (such as thallus colour) affecting their thermal properties. Thallus colour is easily quantified as a core trait and can be applied to crustose lichens so that – along with hydrophobicity – the transferability of functional traits is better achieved across lichen growth-forms, extending beyond macrolichens. Key future challenges include the scaling of lichen trait responses realised at microhabitat scales, to understand emergent effects at landscape and ecosystem scales, and we outline how new technologies are rapidly developing, to bridge this gap. Although not exhaustive, the review offers a precis of targeted background literature, helpful to lichenologists approaching trait-based ecology, or ecosystem ecologists approaching lichens.