Short-term growth experiments – A tool for quantifying lichen fitness across different mineral settings

Lichens are frequently used as indicators of air-borne depositions, yet the differential effect of different mineral nutrients on lichen growth is still poorly understood. Here we quantify relative lichen growth rates (RGR) in representatives of three functional groups (chloro-, cephalo-, cyanolichens; Xanthoria parietina, Lobaria pulmonaria, Lobarina scrobiculata, respectively) placed in growth cabinets and subjected to eight fertilization regimes, i.e. four concentrations of NH4NO3 with and without the addition of other essential nutrients. After the experimental period of 14 days, we were able to document: (1) most N in the sprayed solutions that could fill the lichen's water holding capacity (WHC) was taken up, except for lower uptake in the cyanolichen at high N irrigation; (2) an increase in chlorophyll a (Chl a – a proxy of photosynthetic capacity) in the chlorolichen with increasing N irrigation; (3) a strong corresponding decline in Chl a in the cyanolichen; (4) unchanged Chl a in the cephalolichen; and (5) a decline in RGR in all species with increasing N irrigation. Addition of other essential nutrients significantly boosted RGR in all species, suggesting that N was not the most limiting element. We conclude that (1) short-term growth experiments can characterize lichens’ fitness in response to mineral nutrient deposition, and that (2) WHC contributes to lichens’ capacity to capture mineral nutrients from wet depositions.