Relative humidity predominantly determines long-term biocrust-forming lichen survival under climate change

Abstract

15 1. Manipulative experiments show a decrease in dryland biological soil crust cover and al-16 tered species composition under climate change. However, the underlying mechanisms are 17 not fully understood, and long-term interacting effects of different drivers are largely un-18 known due to the short-term nature of the studies conducted so far. 192. We addressed this gap and successfully parameterized a process-based model for the bi-20 ocrust-forming lichen Diploschistes diacapsis as a common and globally distributed rep-21 resentative of biocrust communities to quantify how changing atmospheric CO2, tempera-22 ture, rainfall amount and relative humidity affect its photosynthetic activity and cover. We 23 also mimicked a long-term manipulative climate change experiment to understand the 24 mechanisms underlying observed patterns in the field. 25 3. The model reproduced observed experimental findings: warming reduced lichen cover 26 whereas less rainfall had no effect. This warming effect was caused by the associated de-27 crease in relative humidity and non-rainfall water inputs as major water sources for li-28 chens. Warming alone, however, increased cover because higher temperatures promoted 29 photosynthesis during the cool morning hours with high lichen activity. When combined, 30 climate variables showed non-additive effects on lichen cover, and fertilization effects of 31 CO2 leveled off with decreasing levels of relative humidity. 32 4. Synthesis. Our results show that a decrease in relative humidity, rather than an increase in 33 temperature may be the key factor for the survival of dryland lichens under climate 34 change and that CO2 fertilization effects might be offset by a reduction in non-rainfall 35 water inputs in the future. Because of a global trend towards warmer and thus drier air, 36 3 this will affect lichen-dominated dryland biocrust communities and their role in regulat-37 ing ecosystem functions, worldwide. 38 39 40

“The effect of water potential on the water balance of lichens and mosses – distribution patterns of internal and external water”

“…In addition to these physiological considerations, the distribution of water content into different pools and its effect on turgor loss is a relevant factor in modelling water uptake and loss by lichens and mosses. The process-based non-vascular vegetation model LiBry, which simulates a large number of different physiological strategies at the phenotype level (Porada et al, 2013;2019), has so far assumed that ψ is zero as long as external water is stored and becomes negative at the onset of evaporation of the internal water (Baldauf et al, 2020). However, a negative potential could already occur at external water evaporation due to capillary forces.…”

Bark Water Storage Plays Key Role for Growth of Mediterranean Epiphytic Lichens

“…The process-based Lichen and Bryophyte model (LiBry) which is applied in this study was developed to estimate carbon uptake by non-vascular vegetation at the global scale, based on climatic conditions, in order to quantify the contribution of the organisms to the global carbon balance, and, furthermore, to other global biogeochemical cycles (Porada et al, 2013(Porada et al, , 2014(Porada et al, , 2017. The model version used here builds on the latest developments, published in Porada et al (2018), Porada et al (2019), and Baldauf et al (2020).…”

“…p kB was chosen to correspond roughly to the saturated hydraulic conductivity of loam (Carsel and Parrish, 1988), since appropriate values for bark could not be found in the literature. In the current version of the model (Baldauf et al, 2020), the water potential of non-vascular vegetation is computed by the following equation:…”

“…The maximum value of p SatX = 1.0 means that the water potential of the simulated strategy becomes negative as soon as water is extracted from the tissue, due to evaporation. At a saturation of around 23%, the water potential reaches its prescribed minimum value of -50 MPa [see Baldauf et al (2020) for details]. Thus, a value of S NV = 1.0 means that the strategy has a relatively dense tissue structure and a high ability to take up water from the bark reservoir.…”