A wind tunnel study of the resilience of three fungal crusts to particle abrasion during aeolian sediment transport

This study is a comparison of the strength, stability and resistance to abr asion of monospecific sand crusts formed by three species of free-living fu ngi: Aureobasidium pullulans, Trichoderma harzianum, and Absidia corymbifer a. Compared to three photoautotrophs examined in a previous study, the fung al crusts were thicker and stronger in flexure by a factor of two or more. In addition to the morphological properties of these fungi, the organic sub strate required for growth contributes to the total crust strength. Wind tu nnel tests, in which the crusts were subjected to particle impact from an u pwind sediment source (nominally 0.014 kg m(-1) s(-1)) and varied levels of freestream velocity (u(infinity)), showed that crusts comprised of Trichod erma and Absidia were stable at wind velocities under 10 m s(-1). Crusts of Aureobasidium generally ruptured at velocities well below this level and t o a much greater extent. The distinctive morphologies and metabolic rates o f the three fungi appear to explain these differences. As compared to the w ind tunnel experiments, independent tests of crust strength in flexure, whi ch were intended to simulate crust flaking, were found to be less sensitive indicators of relative crust stability in aeolian settings, Analysis of th e time rate of expansion of the perimeter of crust loss indicates that this relation is approximately exponential, with the exception of low wind velo cities near the threshold for grain ejection where the momentum of the salt ators at impact appears to be of prime importance. (C) 1999 Elsevier Scienc e B.V. All rights reserved.