Host tree species mediate corticolous lichen responses to elevated CO2 and O3 after 10 years exposure in the Aspen-FACE system

Highlights: • Corticolous lichens were surveyed after free-air exposure to elevated O₃ and CO₂. • Lichen richness was greater on aspen than birch trunks. • Lichen cover, richness and abundance were not directly affected by fumigants. • On aspen, lichens responded positively to higher light in elevated O₃ treatment. • No treatment response noted for lichens on birch; Indirect responses most important Lichens contribute significantly to the biodiversity and functioning of many ecosystems. Although lichens are useful air pollution bioindicators and may respond in significant ways to global change, they are studied infrequently under field conditions in chamberless exposure systems. We surveyed corticolous lichens on paper birch (Betula papyrifera) and trembling aspen (Populus tremuloides) after 10 years exposure (1998–2007) to elevated CO₂ (eCO₂) and O₃ (eO₃) in the Aspen-FACE experiment in Rhinelander, WI, USA. This experiment utilized chamberless exposure rings, 30 m in diameter, with both host trees planted together in one quadrant. Four treatments were allocated among 12 rings: ambient, eCO₂, eO₃, and the combination of eCO₂ + eO₃, each replicated once in each of three blocks. Over the course of the experiment, ambient CO₂ increased from 343 to 386 ppm while eCO₂ averaged ~530 ppm CO₂. Ambient ozone concentrations averaged ~37 ppb and ~49 ppb for eO₃ although exposures decreased with time. Tree growth and leaf area index were negatively affected by eO₃ and stimulated by eCO₂, resulting in higher photosynthetically active radiation (PAR) in eO₃ and lower in eCO₂. We assessed lichen richness and cover on five host trees per ring on the north-facing side of the trunks, which were higher on birch than on aspen. Neither of the lichen measures on birch responded to the exposure treatments, while on aspen lichen cover was highest in eO₃ and lowest in eCO₂. On aspen, lichen cover was positively related to PAR and dominated by Caloplaca. No relationship was found for birch, although Lecanora exhibited a negative relationship with PAR. These lichens were insensitive to direct effects of eCO₂ and eO₃ at the levels applied. Instead, they responded to indirect effects, such as host tree species, and changes in understory PAR, resulting from direct effects of eO₃ and eCO₂ on the host trees.