Crustose lichen

Crustose lichens form a crust that strongly adheres to the substrate (soil, rock, tree bark, etc.), making separation from the substrate impossible without destruction. The basic structure of crustose lichens consists of a cortex layer, an algal layer, and a medulla. The upper cortex layer is differentiated and is usually pigmented. The algal layer lies beneath the cortex. The medulla fastens the lichen to the substrate and is made up of fungal hyphae. The surface of crustose lichens is characterized by branching cracks that periodically close in response to climatic variations such as alternate wetting and drying regimes.

Lepraria incana, a powdery lichen

Lecidea atrobrunnea, an endolithic lichen

Acarospora fuscata, an epilithic lichen

Amandinea punctata, an endophloedic lichen

Psora decipiens, a squamulose lichen

Peltula euploca, a peltate lichen

Acarospora socialis, an effigurate lichen

Lecanora dispersa, a lobate lichen

Crustose lichen forms a thin crust adhering closely to the substratum. In some cases, this crust may be thick and lumpy, and may be detached, in part, or submerged below its surface. The thallus of a crustose lichen is usually only discernible because of the discolouration of the substrate. Some crustose lichens have thalli consisting of scattered or loosely grouped granules. Crustose lichens differ from the leprose lichen by having an upper cortex and algal cells that are located directly beneath the cortex. The thallus of a crustose lichen has a patchwork or crazy-paving appearance. The patches, or areolae, can be as large as 1 cm in diameter or very small and raised, giving them the appearance of a wart. The surface of the thallus is generally smooth, however it is sometimes broken up by “rimose” cracks. These cracks are a by-product of thallus surface shrinkage, which is caused by alternate wetting and drying. An underlayer of fungal hyphae, the hypothallus, is present on some species of crustose lichens. A dark rim on the areolae may form in areas where the hypothallus is exposed. This may also be present on the thallus itself. These fungal hyphae are usually what attach the thallus firmly to the substrate.

In general, lichens do not grow very quickly. Annual growth rates vary among different growth forms. Crustose lichens have the lowest rates of growth. The diameter and area of the thallus exponentially increase, provided that organic substances are distributed uniformly in all parts of the lichens. However, as the thallus increases in size, the circumference also increases, which corresponds to a larger increase in volume. As a result, movement and uniform distribution of organic substances become more difficult. The growth of crustose lichens is dependent on several factors, including moisture levels, sunlight, and temperature. High rates of precipitation and high moisture levels promote the growth of crustose lichens. Crustose lichens are more prevalent in areas with higher precipitation. A similar trend is observed when aridity is taken into account. Crustose lichens prefer sites of lower aridity. The amount of sunlight that lichens receive determines the rate at which photosynthesis occurs. Moreover, surface area also influences photosynthetic rates. In high sunlight conditions, foliose lichens with broad lobes are prevalent. In comparison, crustose lichens have less surface area than foliose lichens and will tend to have slower photosynthetic rates. Generally, higher levels of sunlight promote growth of the crustose lichens. Extreme temperatures are unfavorable for the growth of crustose lichens. Temperatures below 0 °C can result in cessation of growth and thalli freezing. Annual growth rates for the Rhizocarpon subgenus show a correlation with annual and winter mean temperatures, but not with mean summer temperatures. Unfortunately, little faith can be put in these correlations because they use unvalidated measures of unknown accuracy and precision and measurement of growth was done along a single diameter. Since thallus growth along any radius might not match growth along any other radius it is unclear if these correlations are meaningful. Various publications can be consulted to see that there is tremendous within thallus variation in lateral growth (e.g., In addition, it should be noted that the scientific basis of lichenometric dating and the reliability of lichen growth rate measurements in general have recently been questioned and critically reviewed in a paper by Osborn et al. (2015) Those valid criticisms of lichenometric dating have yet to be answered.

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