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

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 CO (eCO) and O (eO) 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, eCO, eO, and the combination of eCO + eO, each replicated once in each of three blocks. Over the course of the experiment, ambient CO increased from 343 to 386 ppm while eCO averaged ~530 ppm CO. Ambient ozone concentrations averaged ~37 ppb and ~49 ppb for eO although exposures decreased with time. Tree growth and leaf area index were negatively affected by eO and stimulated by eCO, resulting in higher photosynthetically active radiation (PAR) in eO and lower in eCO. 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 eO and lowest in eCO. 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 eCO and eO 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 eO and eCO on the host trees.