Drought changes nutrient sources, content and stoichiometry in the bryophyte Hypnum cupressiforme Hedw. growing in a Mediterranean forest

We conducted a 6 year field experiment in an evergreen Quercus ilex forest where we simulated the increased drought projected by Global Circulation Models (GCM) and ecophysiological models for the immediate decades. We tested the hypothesis that enhanced drought will change C, N, P, K, Ca, Fe, Mg, Mo and S concentrations of the widespread moss Hypnum cupressiforme Hedw. and its capacity to absorb nutrients of soil-borne or airborne origin. During the period of study, from 1999 to 2005, the soil moisture in the drought treatment was on average 9% lower than the soil moisture in the control plots. Drought increased the K concentration by 10% and the C concentration by 6%, and decreased the Fe and Mo concentrations by 33% and 18%, respectively, and the N/P content ratio by 15%. A principal component analysis showed that 69% of the variation in moss elemental concentrations is explained by the drought treatment. Drought increased the enrichment factors of several elements, mainly of P, K, Ca, Mg, S and Mo, relative to bedrock extracts, thus showing that the proportion of these elements absorbed from the atmosphere had been increased by drought. The results show that drought increased the concentration of elements linked to drought resistance such as C and K, and decreased the contents of others important for plant productivity such as Mo and Fe. Drought thereby changed moss stoichiometry, and this could also affect the palatability and quite probably, the moss–herbivore relationships and decomposition rates.