Interactions Between Seedlings of Agave Deserti and the Nurse Plant Hilaria Rigida

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Abstract
Seedlings of the succulent crassulacean acid metabolism (CAM plant Agave deserti in the northwestern Sonoran Desert were found only in sheltered microhabitats, nearly all occurring under the canopy of a desert bunchgrass, Hilaria rigida. Apparently because soil surface temperatures can reach 71°C in exposed areas, seedlings were generally located near the center or on the northern side of this nurse plant. Both species have shallow root systems, about half of the roots of H. rigida and all those for seedlings of A. deserti occurring above soil depths of 0.08 m. To examine competition for water between the nurse plant and an associated seedling, a three—dimensional model for root water uptake was developed. The model divided the soil into 17 conentric sheaths each subdivided radially into eight wedges and vertically into nine layers. Predicted pre—dawn soil water potentials at the mean root depth and total shoot transpiration agreed well with field measurements. Simulated annual water uptake by a seedling of A. deserti was reduced °50% when the seedling was moved from an exposed location to the center of the nurse plant. Shading by the nurse plant reduced total daily photosynthetically active radiation (PAR) by up to 74% compared with an exposed seedling. On the other hand, soil nitrogen under the canopy of H. rigida was 60%higher than in exposed locations. Assuming that the effects of nitrogen, temperature, PAR, and soil water on net CO2 uptake are multiplicative, the predicted net CO2 uptake by a seedling of A. deserti under the nurse plant was only °45% of that for an exposed seedling. Thus, although the nurse plant facilitates seedling establishment by reducing maximum soil surface temperatures and provides a microhabitat with higher soil nitrogen levels, its shading and competition for water reduce seedling growth.

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