Treatment effects on performance of N-fixing lichens in disturbed crusts of the Colorado Plateau

Biological soil crusts arrest soil erosion and supply nitrogen to arid ecosystems. To understand their recovery from disturbance, we studied performances of Collema spp. lichens relative to four experimental treatments plus microtopography of soil pedicels, oriented north-northwest to south-southeast in crusts. At sites in Needles (NDLS) and Island in the Sky (ISKY) districts of Canyonlands National Park, lichens were transplanted to NNW, SSE, ENE, WSW, and TOP pedicel faces and exposed to a full-factorial, randomized block experiment with four treatments: nutrient addition (P and K), soil stabilization with polyacrylamide resin (PAM), added cyanobacterial fiber, and biweekly watering.

After 14.5 mo (NDLS) and 24 mo (ISKY), both visual rankings of lichen condition and measures of chlorophyll fluorescence were generally higher at ISKY than on more fertile but less stable soils at NDLS. On ENE and NNW pedicel faces, both these values and nitrogenase activity (NDLS only) exceeded corresponding values on WSW and SSE faces. Treatment effects were site specific and largely negative at NDLS; both nutrient and cyanobacterial addition led to poorer lichen condition, and added nutrients led to reduced fluorescence. Responses to nutrients may have been mediated partly by disturbance of unstable soils and by competition with cyanobacteria.

In a separate experiment investigating recruitment responses to adding fungal spores or Nostoc cells, rates of Collema establishment responded significantly to the former but not the latter. Low establishment overall suggests that natural recruitment may occur mainly from iscidia or thallus fragments, not spores.

Measured simultaneously on artificial soil pedicels at NDLS and ISKY, both deposition and erosion declined at NDLS across the four pedicel microaspects as: WSW > SSE > NNW > ENE (or ENE > NNW), during fall and spring trials. Patterns were similar at ISKY, but WSW ≈ SSE for spring deposition, and deposition did not differ by microaspect in fall. Greater deposition at ISKY, despite higher abundance of cyanobacteria, may be explained by stronger wind velocities.

Together, microtopographic differences in erosion, microclimate, and nutrient regimes help explain variable lichen performance, but microtopography influenced lichen performance more consistently than did any treatment. Demonstrated effects of pedicel development in crust recovery concur with prior surveys showing greatest microbial biomass and/or cover on ENE and NNW exposures at various spatial scales.