Up in the Tree – The Overlooked Richness of Bryophytes and Lichens in Tree Crowns

Fischer M (2013) Up in the Tree - The Overlooked Richness of Bryophytes and Lichens in Tree Crowns. PLoS ONE 8(12): e84913. doi:10.1371/journal.pone.0084913 Up in the Tree - The Overlooked Richness of Bryophytes and Lichens in Tree Crowns Steffen Boch 0 Jrg Mller 0 Daniel Prati 0 Stefan Blaser 0 Markus Fischer 0 Hans Henrik Bruun, University Copenhagen, Denmark 0 1 Institute of Plant Sciences and Botanical Garden, University of Bern , Bern , Switzerland , 2 Institute of Biochemistry and Biology, University of Potsdam , Potsdam , Germany Assessing diversity is among the major tasks in ecology and conservation science. In ecological and conservation studies, epiphytic cryptogams are usually sampled up to accessible heights in forests. Thus, their diversity, especially of canopy specialists, likely is underestimated. If the proportion of those species differs among forest types, plotbased diversity assessments are biased and may result in misleading conservation recommendations. We sampled bryophytes and lichens in 30 forest plots of 20 m 20 m in three German regions, considering all substrates, and including epiphytic litter fall. First, the sampling of epiphytic species was restricted to the lower 2 m of trees and shrubs. Then, on one representative tree per plot, we additionally recorded epiphytic species in the crown, using tree climbing techniques. Per tree, on average 54% of lichen and 20% of bryophyte species were overlooked if the crown was not been included. After sampling all substrates per plot, including the bark of all shrubs and trees, still 38% of the lichen and 4% of the bryophyte species were overlooked if the tree crown of the sampled tree was not included. The number of overlooked lichen species varied strongly among regions. Furthermore, the number of overlooked bryophyte and lichen species per plot was higher in European beech than in coniferous stands and increased with increasing diameter at breast height of the sampled tree. Thus, our results indicate a bias of comparative studies which might have led to misleading conservation recommendations of plot-based diversity assessments. - Funding: The DFG (German Research Foundation; http://www.dfg.de/en/index.jsp) funded the study in the framework of the Biodiversity Exploratories SSP 1374 "Infrastructure-Biodiversity-Exploratories"(FI 1246/6-1, FI 1246/9-1). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing interests: The authors have declared that no competing interests exist. Assessing diversity is a major task in ecology and conservation science. Understanding patterns of diversity and guiding conservation decision-making both rely on an unbiased assessment of diversity, for instance of species richness per given area and given sampling effort [1,2]. In plants, diversity assessments are usually plot-based and all species up to accessible heights are sampled (e.g. [3,4]). However, this approach may lead to underestimating the diversity of species not occurring on the ground (i.e. epiphytes). Furthermore, if the proportion of those underestimated species differs among habitat types or varies systematically with some environmental drivers, plot-based diversity assessments will be biased and may result in misleading conservation recommendations. Bryophytes and lichens are two very diverse groups of cryptogams, which occur on a wide range of substrates, inhabit essentially all terrestrial and some aquatic habitats and many of them live as epiphytes [5,6]. In forests, the species richness and composition of cryptogam communities varies among geographical regions, forest management types, main tree species, and stand ages [7-10]. On individual trees, the species composition of epiphytic bryophyte and lichen communities changes with tree age, because of changing bark structure as well as quality, and with tree height as abiotic and biotic conditions change from trunk bases to the uppermost twigs [11-15]. Ecological studies, which monitored bryophytes and lichens or compared their species richness in forests among different regions, management types or tree species composition usually sampled all species of a defined area [7,8,15] or only epiphytes on trunks of individual trees [16-18]. However, in these surveys epiphytic species were only sampled up to accessible heights (e.g. 2 m above ground). Thus, epiphytic bryophytes and lichens in tree crowns are rarely investigated, especially for forests in temperate Europe. All previous studies describing the vertical distribution, composition, richness or biomass of epiphytic bryophytes and lichens have been Table 1. Characteristics of the three Biodiversity Exploratories. conducted outside continental Europe and mainly investigated single or few fallen or cut trees (e.g. [15,19-35]). Only few studies quantified the number of epiphytic bryophyte and lichen species, which are exclusively growing in tree crowns and compared this number to the one of the trunk of the same tree below 2 m [30,35]. However, none of these studies compared the species richness of epiphytic bryophytes and lichens in tree crowns with the total species richness of ground plots, which would have allowed them to assess the number and proportion of overlooked species during regular surveys. Some epiphytes may be recorded on the ground because of epiphytic litter fall and of species growing on fallen branches and twigs, but the true number and proportion of crown specialists remains unknown without surveying tree crowns [36]. Thus, it also remains unclear how many epiphytic bryophyte and lichen species are on average overlooked in regular cryptogam surveys. Furthermore, whether the number or proportion of these overlooked species is consistent among geographical regions, forests types, or stand ages has never been comprehensively analyzed in a comparative study. Therefore, we sampled the species richness of bryophytes and lichens in European beech (Fagus sylvatica L.) and coniferous forests of different stand ages, in three German regions, distinguishing species recorded in a regular survey of all substrates on a plot, epiphytic species occurring only at the base of trees, and epiphytic species occurring only in the crown of trees. Our main questions were: How many species per tree and per plot are on average overlooked without sampling bryophytes and lichens in tree crowns? Does the number and proportion of overlooked species vary among regions, tree species, and tree age? Materials and Methods Study system This study was conducted as part of the Biodiversity Exploratories project ([37]; www.biodiversity-exploratories.de) in three regions of Germany: (1) the UNESCO Biosphere area Schwbische Alb (Swabian Jura), situated in the low mountain ranges of South-western Germany, (2) the National Park Hainich and its surrounding areas, situated in the hilly lands of Central Germany, and (3) the UNESCO (...truncated)