In recent years, our ecological knowledge of tropical dry forests has increased dramatically. However, the functional contributions of whole ecosystem components, such as lichens, remain mostly unknown. In these forests, the abundance of epiphyte crustose lichens is responsible for the characteristic white bark on most woody plants, conspicuous during the dry season, but the amount of resources that the lichen component represents remains unexplored. We estimated lichen biomass in a Mexican tropical dry forest using the bark area of trees, the dry mass of lichens per unit area and the percentage of bark covered by lichens, together with previously known tree densities. The lowest 2.5 m of the forests main trunks contained 188 kg/ha of lichen biomass, with lichens covering 85% of the available bark for trees <12 cm DBH and 38% for trees >12 cm. Total epiphytic lichen biomass was 1.34–1.99 Mg/ha. Lichen biomass represented 61% of the foliar biomass in the forest. To our knowledge, this is the first time that a lichen biomass estimate is provided for an ecosystem in which crustose lichens are the dominant lichen growth form. Crustose lichens are typically considered to contribute little to the total lichen biomass and to be difficult to include in ecological analyses. The high lichen biomass in this ecosystem implies a significant ecological role which so far is unexplored. We suggest the crustose lichen component should not be underestimated a priori in ecological studies, especially in ecosystems with abundant lichen cover.

For Table S1:

Lichen tissue on top of a 2 cm² area of bark was carefully scraped from each of 20 previously collected pieces of bark, representing 20 common lichen species in the forest. Collection technique for these bark pieces consisted of using a knife to remove, from a living tree, a small and thin piece of the outer bark with the lichen of interest on it. Each cut was made carefully to avoid reaching into the cambium layer of the tree. Loss of material was avoided by continuously dampening each bark sample and scraping it with a single razor blade under the stereoscope. Special care was taken to avoid removing the bark layer which has a different texture and color than the wet lichens. The collected tissue was heated for 24 h at 60°C and weighed immediately to the nearest 0.1 mg with an Ohaus Analytical Plus balance. All measurements were transformed to g/m².

For Table S2:

We measured the bark surface area of 62 randomly selected trees that did not branch below 2.5 m. Trees were haphazardly selected a few meters to the side of a sinuous transect of 2.5 km that followed three trails inside the Reserve. The selected trees were a typical representation of the species present in the Reserve and included different DBH sizes and percentages of lichen cover. For each tree, we measured the bark surface area on the lowest 2.5 m of the main trunk using the formula for the lateral surface area of a truncated cone with diameter measurements at 0.3 m and 2.5 m high. The lichen cover of the measured area was visually estimated as a percentage by dividing the area in at least 8 smaller zones that included the lower and upper parts of the lowest 2.5 m of the main trunk at each cardinal point. The lichen biomass in the lowest 2.5 m of each tree was calculated as the product of the bark surface area in m², the lichen cover percentage per tree, and the mean lichen dry mass in g/m².