Does exogenous carbon extend the realized niche of canopy lichens? Evidence from sub‐boreal forests in British Columbia

Abstract

Foliose lichens with cyanobacterial bionts (bipartite and tripartite) form a distinct assemblage of epiphytes strongly associated with humid microclimatic conditions in inland British Columbia. Previous research showed that these cyano- and cephalolichen communities are disproportionately abundant and species-rich on conifer saplings beneath Populus compared to beneath other tree species. More revealing, lichens with cyanobacterial bionts were observed beneath Populus even in stands that did not otherwise support them. We experimentally test the hypothesis that this association is due to the interception of glucose-rich nectar that is exuded from Populus extra-floral nectaries (EFN). Using CO2 flux measurements and phospholipid fatty acid (PLFA) analysis with experimental applications of 13C6-labeled glucose, we demonstrate that cyano- and cephalolichens have a strong respiratory response to glucose. Lichens treated with glucose had lower net photosynthesis and higher establishment rates than control thalli. Furthermore, lichens with cyanobacterial bionts rapidly incorporate exogenous 13C into lichen fatty acid tissues. A large proportion of the 13C taken up by the lichens was incorporated into fungal biomarkers, suggesting that the mycobiont absorbed and assimilated the majority of applied 13C6 glucose. Our observations suggest that both cyanolichens and cephalolichens may utilize an exogenous source of glucose, made available by poplar EFNs. The exogenous C may enable these lichens to become established by providing a source of C for fungal respiration despite drought-induced inactivity of the cyanobacterial partner. As such, the mycobiont may adopt an alternative nutritional strategy, using available exogenous carbon to extend its realized niche.

The evolution of fungal substrate specificity in a widespread group of crustose lichens

“…A more commonly invoked reason for substrate specificity is preference for specific chemical environments, often expressed in terms of cation ratios and pH [ 53 , 54 , 62 ], potentially also extending to carbohydrate chemistry [ 63 ]. In some cases, species responses to these factors suggest, if anything, a broadening of the niche by facilitating species' occurrence on multiple substrate types, e.g.…”

Non‐parametric methods reveal non‐linear functional trait variation of lichens along environmental and fire age gradients

“…It is also possible that shrubs, many of which are in the Salicaceae, chemically enhance cyanolichens with sugar‐rich leaf exudate (Campbell et al. ), an alternative carbon source that might offset the lichen's need to photosynthesize. However, scatterplots between cyanolichen and Salix species cover show no strong relationship, at least when all plots are included in the comparison.…”

Sunscreening fungal pigments influence the vertical gradient of pendulous lichens in boreal forest canopies

“…Coxson and Coyle (2003) supported this hypothesis and predicted higher photosynthetic rates for Alectoria sarmentosa in the lower canopy where it occurs, but concluded that growth responses alone did not explain the niche partitioning in pendulous lichens. Recently, phosphorus availability (McCune and Caldwell 2009) and exogenous carbohydrates (Campbell et al 2013) containing contrasting pigments are taxonomically more related to each other than to the genera (Alectoria and Usnea) containing usnic acid (Thell and Moberg 2011). Both melanins (e.g., Gauslaa and Solhaug 2001) and usnic acid (McEvoy et al 2007) screen light, and thus protect underlying photobionts.…”