Moss and peat hydraulic properties are optimized to maximize peatland water use efficiency

Peatland ecosystems are globally important carbon and terrestrial surface water stores that have formed over millennia. These ecosystems have likely optimized their ecohydrological function over the long-term development of their soil hydraulic properties. The optimization of peat hydraulic properties is examined to determine which of the following conditions peatland ecosystems target during this development: (i) maximize carbon accumulation, (ii) maximize water storage, or (iii) balance carbon profit across hydrological disturbances. To identify this control, the short-term hydrological response of a 0.5-m-deep peat profile was simulated during a 50-day rain-free period. A total of 5000 Monte-Carlo model realizations were conducted, with peat hydraulic properties differing between each realization (values derived from known probability distributions). Saturated hydraulic conductivity (K_s) and empirical van Genuchten water retention parameter α were shown to provide a first order control on simulated water tensions. For hypothetical combinations of K_s and α, the probability that water tension exceeds the ecologically important threshold of 100 mb within 24h showed a bimodal distribution. A peak at high probabilities was associated with profiles of high K_s and low α. Such a profile is optimized for water storage. A peak at low probabilities is associated with low K_s, high α, and is optimized for carbon accumulation. Actual hydraulic properties from five northern peatlands fall between this binominal distribution, balancing the competing demands of carbon accumulation and water storage. We argue that peat hydraulic properties are thus optimized to maximize water use efficiency and that this optimization occurs over a centennial to millennial timescale.