Why is the rainforest lichen Methuselah’s beard (Usnea longissima) so rare in British Columbia’s inland temperate rainforest?

Abstract

Coastal (CTR) and inland temperate rainforests (ITR) in western North America share a rich oceanic lichen flora. The distinctive Methuselah’s beard lichen (Usnea longissima) is an exception to this pattern of shared distributions, with very few ITR locations. Does this absence reflect dispersal limitations or climatic intolerance? To answer this question, we transplanted U. longissima thalli from the CTR to three ITR locations, assessing growth rates against reciprocal CTR transplants. Canopy microclimate measurements provided concurrent data on growth conditions. Growth rate responses (length, mass and area) were evaluated after summer and full-year transplants. Notwithstanding extended drought conditions during the summer period, annual transplants at two of the three ITR locations supported growth rates comparable to those at the CTR source U. longissima population, with summer dewfall and autumn rains being major ITR hydration sources. Thalli transplanted to a third ITR site (summer measurements only) in a location transitional to the drier interior plateau lost both mass and length. Based on these findings we suggest that the absence of U. longissima from much of the ITR reflects the combined influence of dispersal limitations and requirements for stands with long site continuity and topographically induced summer wetting of thalli by dewfall.

Influence of canopy structure and light on the three-dimensional distribution of the iconic lichen Usnea longissima

“…Both niche specialism and dispersal limitation (Williams and Ellis, 2018) contribute to its strong preference for humid old-growth boreal forests with long continuity of canopy cover. Despite high growth capacity (Gauslaa et al, 2007;Keon and Muir, 2002;Strother et al, 2022), the high degree of niche specialism and dispersal limitation suggest that the lichen has limited capacity to shift its vertical and horizontal distribution in response to changes in canopy structure and climate. The development of populations is driven by a small number of 'source trees' with large populations occurring at higher vertical positions, while 'sink trees' with small populations close to ground are less important.…”

“…They have low probability of being transported over longer distances than a few m's from the source trees, consistent with the clustering of host trees. Transplant studies suggest that dispersal limitations are important in determining the distribution of U. longissima (Keon and Muir, 2002;Strother et al, 2022) and other old-growth forest lichens (Sillett et al, 2000;Scheidegger and Werth, 2009).…”

“…This mechanism suggests that the lichen is most abundant in the vertical zone where the light environment is optimal for growth. The lichen has high growth-potential (Keon and Muir, 2002;Gauslaa et al, 2007;Jansson et al, 2009;Strother et al, 2022) including diffuse or intercalary growth along the main axis (Rolstad and Rolstad, 2008;Sanders and de los Ríos, 2012). The lichen occurred in the lower half of the partially shaded transition zone from dim light to bright light, which is characterized by high variability in light transmittance (Parker, 1997).…”

“…This lichen occurs in the northern hemisphere (Clerc, 2011) and may reach a length of several meters, constituting the longest lichen in the world. It is suitable for studying niche specialism in lichens for several reasons: (1) Its unique morphology and preference for the lower canopy in boreal forests enable quantification from the ground (Esseen et al, 1981), (2) Its distribution is strongly linked to climate at global, regional and local scales (Gauslaa et al, 2007;Smith et al, 2016;Eriksson et al, 2018;Strother et al, 2022), and (3) It is associated with old-growth spruce forests in Europe (Esseen et al, 1981;Gauslaa, 1997, Josefsson et al, 2005 and a flagship species for conservation of biodiversity in boreal forests (Esseen et al, 1992). Usnea longissima is extinct or endangered in European countries and red-listed as vulnerable as well as protected by law in Sweden (SLU, 2020).…”

Regionally Varying Habitat Relationships in Lichens: The Concept and Evidence with an Emphasis on North-Temperate Ecosystems

“…Furthermore, a wider habitat range observed (e.g., more phorophyte species occupied; Table 1 ) may partly result from local population size (for any reason) when habitats differ in their quality. Investigation of such patterns is expectably difficult in the case of delayed, hidden, and cumulative impacts of past events, such as recovery from disturbance, or a history of repeated exposure to contaminants [ 129 , 130 ]. An important study on epilithic lichens found that their colonization rate of habitat patches is related to both the local abundance and range size [ 131 ].…”