Living on the edge: Ecology of an incipient Betula-fungal community growing on brick walls

Walls provide extreme habitats where plants are subjected to various environmental hazards, including water deficit, shortage or excess of irradiance, and nutrient unbalance, and soil communities are in initial stages of development. Here we analyzed the soil substrate, mycorrhizal status, leaf structure and chemistry, CO₂ exchange and water relations of birch trees growing on brick walls of a 19th century building. The substrate was characterized by high pH, Ca, S, Zn, Cl, and Na concentrations and low concentrations of P, K, Mg, and Fe. From 45 to 70% of live fine roots were colonized by ectomycorrhizal (ECM) species: Hebeloma mesophaeum, Hebeloma helodes, Lactarius pubescens, Tomentella lilacinogrisea. Birch on the wall had typical leaf N concentrations, but stomatal conductance was extremely low, and as a consequence, so was net CO₂ assimilation rate. This research showed how a rudimentary soil, plant, and microbial community could develop even in a vertical and chemically foreign environment. Birch plants were able to maintain balanced foliar stoichiometry in a highly unbalanced substrate and acquire enough nutrients to drive sufficient C gain to provide energy for themselves and fungal symbionts.