| Nurse plants alter the temperature, photosynthetically active radiation (PAR), nitrogen and water availability for associated seedlings of Agave deserti, Carnegia gigantea, and Ferocactus acanthodes in the Sonoran Desert.; Soil surface temperatures can reach 71{dollar}spcirc{dollar}C in the summer, compared to 57{dollar}spcirc{dollar}C under a common nurse plant, the bunchgrass Hilaria rigida. Shading by nurse plants reduced total daily PAR by up to 77% depending on nurse plant species, seedling size and location under the nurse plant. Soil nitrogen content was significantly higher under nurse plants than outside. For F. acanthodes and A. deserti, the presence of the most common nurse plant, Hilaria rigida, reduced simulated annual water uptake by an associated seedling by up to 75%, depending on annual rainfall, seedling size, and seedling location. Thus, although the nurse plant reduces maximum soil surface temperatures, thereby facilitating seedling establishment, and provides a microhabitat with higher nitrogen levels, its shading and competition for water reduce seedling growth.; The ground area explored by roots was determined for three sizes of plants of Hilaria rigida in monospecific stands over a period of 4 years. For half of the selected plants, possible competition from adjacent plants was eliminated so that the effects of competition on root and shoot growth could be assessed.; Over the 4-year study period, the rooting area of small, medium and large plants increased by an average of 17% in undisturbed sites but by 125% when the adjacent vegetation was removed. Similarly root length, root dry weight and shoot dry weight increased proportionally when the adjacent vegetation was removed. Thus the relative lack of growth of all plant sizes in undisturbed monospecific stands of H. rigida, indicates that the relative sizes of various clumps may remain constant for many years, but opportunistically they could increase when competition for water is alleviated. |
