Effect of Nurse Plants on the Microhabitat and Growth of Cacti

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Abstract
Seedlings of the cactus Carnegiea gigantea at two sites in the Sonoran Desert [USA] were found only in sheltered microhabitats, 89% occurring under the canopy of Ambrosia deltoidea and Cercidium microphyllum. In contrast, 29% of the seedlings of Ferocactus acanthodes, which tolerates higher temperatures than C. gigantea, occurred in unsheltered microhabitats, where maximum soil surface temperatures reached 71.degree.C. Most (70%) of the sheltered seedlings of F. acanthodes occurred under the canopy of the perennial bunchgrass Hilaria rigida. Shading by nurse plants reduced the total daily photosynthetically active radiation (PAR) availabe for their associated seedlings. Near an equinox, a seedling of C. gigantea located at the centre of an A. deltoidea shrub received 77% less total daily PAR, which reduces its predicted net CO2 uptake by 90% compared with an unshaded seedling. Similarly, a seedling of F. acanthodes located at the centre of an H. rigida plant received 64% less total daily PAR and could fix 65% less CO2 than could an unshaded seedling. H. rigida and F. acanthodes have overlapping shallow root systems (mean root depth of 0.08 m for H. rigida and 0.05 m for seedlings of F. acanthodes), which accentuates competition for water. A water uptake model, which closely predicted the soil water potential in the root zone of H. rigida, indicated that a seedling of F. acanthodes located at the centre of the bunchgrass took up 32% less water than did an exposed seedling. Assuming that the effects of temperatures, PAR, and soil water on net CO2 uptake are multiplicative, the predicted net CO2 uptake of a seedling of F. acanthodes under the canopy of H. rigida was only 36% of that of an exposed seedling. However, areole production by seedlings of F. acanthodes located under the nurse plant was 68% of that measured in exposed areas. This discrepancy probably reflects the 60% higher soil nitrogen levels under H. rigida. Therefore, nurse plants facilitate seedling establishment by reducing high temperatures near the soil surface and provide a microhabitat with a higher soil nitrogen level. However, shading and competition for water with the nurse plants markedly reduce seedling growth. The magnitude of the reduction depends on seedling size and location under the nurse plant.