Genetic structure in a fragmented Northern Hemisphere rainforest: large effective sizes and high connectivity among populations of the epiphytic lichen Lobaria pulmonaria.

An extraordinary diversity of epiphytic lichens is found in the boreal rainforest of central Norway, the highest-latitude rainforest in the world. These rainforest relicts are located in ravine systems, and clear cutting has increased the distance between remaining patches. We hypothesized that the relatively small lichen populations in the remaining forest stands have suffered a depletion of genetic diversity through bottlenecks and founder events. To test this hypothesis, we assessed genetic diversity and structure in the populations of the tripartite lichen Lobaria pulmonaria using eight SSR loci. We sampled thalli growing on Picea abies branches and propagules deposited in snow at three localities. Contrary to expectations, we found high genetic diversity in lichen and snow samples, and high effective sizes of the studied populations. Also, limited genetic differentiation between populations, high historical migration rates, and a high proportion of first generation immigrants were estimated, implying high connectivity across distances <30km. Almost all genetic variation was attributed to variation within sites; spatial genetic structures within populations were absent or appeared on small scales (5-10m). The high genetic diversity in the remaining old boreal rainforests shows that even relict forest patches might be suitable for conservation of genetic diversity.

Size-dependent growth of two old-growth associated macrolichen species.

Relationships between thallus size and growth variables were analysed for the foliose Lobaria pulmonaria and the pendulous Usnea longissima with the aim of elucidating their morphogenesis and the factors determining thallus area (A) versus biomass (dry weight (DW) gain. Size and growth data originated from a factorial transplantation experiment that included three boreal climate zones (Atlantic, suboceanic and continental), each with three successional forest stands (clear-cut, young and old). When A was replaced by the estimated photobiont layer area in an area-DW scatterplot including all thalli (n = 1080), the two separate species clusters merged into one, suggesting similar allocation patterns between photobionts and mycobionts across growth forms. During transplantation, stand-specific water availability boosted area gain in foliose transplants, consistent with a positive role of water in fungal expansion. In pendulous lichens, A gain greatly exceeded DW gain, particularly in small transplants. The A gain in U. longissima increased with increasing DW:A ratio, consistent with a reallocation of carbon, presumably mobilized from the dense central chord. Pendulous lichens with cylindrical photobiont layers harvest light from all sides. Rapid and flexible three-dimensional A gain allows the colonization of spaces between canopy branches to utilize temporary windows of light in a growing canopy. Foliose lichens with a two-dimensional photobiont layer have more coupled A and DW gains.