Hidden crown jewels: the role of tree crowns for bryophyte and lichen species richness in sycamore maple wooded pastures

Abstract

Tree crowns typically cover the vast majority of the surface area of trees, but they are rarely considered in diversity surveys of epiphytic bryophytes and lichens, especially in temperate Europe. Usually only stems are sampled. We assessed the number of bryophyte and lichen species on stems and in crowns of 80 solitary sycamore maple trees (Acer pseudoplatanus) at six sites in wooded pastures in the northern Alps. The total number of species detected per tree ranged from 13 to 60 for bryophytes, from 25 to 67 for lichens, and from 42 to 104 for bryophytes and lichens considered together. At the tree level, 29 % of bryophyte and 61 % of lichen species were recorded only in the crown. Considering all sampled trees together, only 4 % of bryophyte, compared to 34 % of lichen species, were never recorded on the stem. Five out of 10 red-listed bryophyte species and 29 out of 39 red-listed lichen species were more frequent in crowns. The species richness detected per tree was unexpectedly high, whereas the proportion of exclusive crown species was similar to studies from forest trees. For bryophytes, in contrast to lichens, sampling several stems can give a good estimation of the species present at a site. However, frequency estimates may be highly biased for lichens and bryophytes if crowns are not considered. Our study demonstrates that tree crowns need to be considered in research on these taxa, especially in biodiversity surveys and in conservation tasks involving lichens and to a lesser degree also bryophytes.Keywords Acer pseudoplatanus Á Alps Á Biodiversity Á Epiphytes Á Red-listed species Á Tayloria rudolphiana Communicated by Pradeep Kumar Divakar.

Vertical variation in epiphytic cryptogam species richness and composition in a primeval Fagus sylvatica forest

“…Kiebacher et al. () state that many red‐listed lichen species are crown specialists and a potential overestimation of the extinction risk of these species may result from excluding the crowns in biodiversity surveys and Red List assessments.…”

“…In this context, it should also be mentioned that not only diversity, but also species frequencies may be underestimated when only the stem base is sampled. Kiebacher et al (2016) state that many red-listed lichen species are crown specialists and a potential overestimation of the extinction risk of these species may result from excluding the crowns in biodiversity surveys and Red List assessments.…”

“…In southern boreal coniferous forests of Estonia, as much as two-thirds of the trees' total lichen species would have remained unrecorded if only the lower 2 m on the stem base had been sampled (Marmor et al 2013). Surveys of the epiphyte species richness on solitary standing broad-leaved trees in the northern Alps (Kiebacher et al, 2016) and coniferous trees in east and southwest Ireland (Coote et al, 2008) also showed similar patterns, even though the studied trees were exposed to environmental conditions along the vertical gradient that deviate from those of the forest interior, thus corroborating the robustness of our conclusion across different ecosystem types and environmental conditions.…”

“…Such studies have, for example, been conducted in temperate rain forests of Tasmania (Jarman & Kantvilas, ), the Pacific northwest of North America (McCune et al., ; Sillett & Rambo, ), and in tropical rain forests (Goda‐Sporn, Bos, Kessler, & Gradstein, ). Studies on vertical gradients in the epiphytic cryptogam vegetation in Europe have so far all concerned managed forests and their focus has primarily been on differences in species richness between the stem base (0–2 m above the ground) and the rest of the stem (above 2 m) (Boch et al., ; Fritz, ; Kiebacher, Keller, Scheidegger, & Bergamini, ; Marmor, Tõrra, Saag, Leppik, & Randlane, ). To our knowledge, primeval temperate European forests have so far not been studied with respect to the vertical distribution of cryptogamic epiphytes.…”

“…Studies on vertical gradients in the epiphytic cryptogam vegetation in Europe have so far all concerned managed forests and their focus has primarily been on differences in species richness between the stem base (0-2 m above the ground) and the rest of the stem (above 2 m) (Boch et al, 2013;Fritz, 2009;Kiebacher, Keller, Scheidegger, & Bergamini, 2016;Marmor, Tõrra, Saag, Leppik, & Randlane, 2013). To our knowledge, primeval temperate European forests have so far not been studied with respect to the vertical distribution of cryptogamic epiphytes.…”