Nurse Plants, Mycorrhizae, and Plant Establishment in a Disturbed Area of the Sonoran Desert

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Abstract
Arbuscular‐mycorrhizal (AM) fungi stabilize the soil and enhance plant growth by alleviating nutrient and drought stress. Their contributions to agriculture are well known, but their role in desert ecosystems has received less attention. The AM status of perennial plants in disturbed and undisturbed plots were investigated in the Sonoran Desert near La Paz, Baja California Sur, Mexico to determine if AM fungi contribute to resource‐island stability and plant establishment. All perennial plants (46 species) in the study plots were AM, but root colonization varied widely ( 70%). Roots of plants that established in greatest numbers in plant‐free zones (colonizers) of disturbed areas were highly AM. Plants with trace (20 yr) nurse‐legumesP. articulataandO. tesotasupported the largest number of under‐story plants. Younger plants had only occasional associates. AM propagule densities in plant‐free areas were lower than under plant canopies (40 vs. 280 propagules/kg soil). Occurrence of soil mounds (islands) under plants owing to soil deposition was related to the nature of the canopies and to the AM status of the roots. Island soils were enmeshed with AM‐fungal hyphae, especially in the upper layer (approximately 10 cm). Seedlings ofP. pringlei, growing in a screenhouse for six months in soil collected underP. articulata, had a biomass ten times greater than plants growing in bare‐area soil. The results are consistent with the proposition that AM fungi contributed to the plant‐soil system of our study area by: (1) helping to stabilize windborne soil that settles under dense plant canopies; (2) enhancing the establishment of colonizer plants in bare soils of disturbed areas; and (3) influencing plant associations through differences in the mycotrophic status of the associates.