Host tree species mediate corticolous lichen responses to elevated CO2 and O3 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 (Befula papyrifera) and trembling aspen (Populus tremuloides) after 10 years exposure (1998-2007) to elevated CO2 (eCO(2)) and O-3 (eO(3)) 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(2), eO(3), and the combination of eCO(2), eO(3), each replicated once in each of three blocks. Over the course of the experiment, ambient CO2 increased from 343 to 386 ppm while eCO(2) averaged similar to 530 ppm CO2. Ambient ozone concentrations averaged similar to 37 ppb and similar to 49 ppb for e0 3 although exposures decreased with time. Tree growth and leaf area index were negatively affected by eO(3) and stimulated by eCO(2), resulting in higher photosynthetically active radiation (PAR) in eO(3) and lower in eCO(2). 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(3) and lowest in eCO(2). 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(2) and eO(3) 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(3) and eCO(2) on the host trees. (C) 2020 Elsevier B.V. All rights reserved.

Automated summary

Lichens play a significant role in the biodiversity and functioning of ecosystems. Research on corticolous lichens exposed to elevated CO2 and O3 levels found that their responses were influenced by factors such as host tree species and photosynthetically active radiation.