A nurse plant is one that serves the ecological role of helping seedlings establish themselves, and of protecting young plants from harsh conditions. This effect is particularly well studied among plant communities in xeric environments.
Effects Overstory trees and shrubs have a facilitative effect on the establishment of understory plants. This effect is also seen in some interactions between herbaceous plant species. Nurse plants are important in xeric environments because they provide shaded microhabitats for the survival of several other plant species, buffering temperature extremes and reducing moisture loss. For example, in the Sonoran Desert, nurse plants canopies provide reduced summer daytime temperatures, soil surface temperatures, and direct sunlight, higher soil fertility, protection from the wind and browsing animals, reduced evapotranspiration rates in the nursed species, elevated nighttime temperatures, and post-fire resprouting in some species... This means that nurse plants provide a positive interaction between themselves and the organisms they protect, and are often crucial in maintaining biodiversity in water-scarce environments. It has been suggested that the assistance provided by nurse plants can enhance the performance of stress intolerant species on green roofs. There is also evidence suggesting that phenotypic traits affect how protégé plants that grow on large green roofs react to nurse plants. Nurse plants also aid with recovery after herbivore grazing, because they provide higher levels of resources to the plant. The effect of nurse plants on any particular species is dependent upon species richness and the dispersal strategy of the organism. Nurse plants can help with seedling recruitment and protect plants from granivory. A saguaro's root system is restricted to 15 cm of soil surface and the Palo Verde's (Cercidium microphyllum) roots go deeper under the surface. Studies suggest that a saguaro's network of roots intercept moisture before it can reach a Palo Verde's roots. When analyzing the contributions of Nurse trees, the prevention of herbivory is reduced in arid environments because the herbivores are at a much lower density, so the contribution of herbivore defense is excluded from arid environments. Nurse plants also have better soil under their canopies than what is out in the open. "Soil properties under nurse plants were always better than outside them, which are in concordance with the generalized existence of fertility islands in high mountains" Palo Verde (Cercidium spp.), mesquite (Prosopis spp.), and ironwood (Olneya tesota) trees all provide positive interactions among other plants species like facilitating seedling survival and germination. The richness and abundance of many plant species is greater under the canopies of these trees than in surrounding areas The density of plant species that depend on nurse plants depends on the number of nurse plants in a community. For example, the density of the senita cactus (Pachycereus schottii) was higher when there was a higher density of nurse plants. But the study by Holland et al., found that "there was not a significant main factor effect of nurse plants on the germination and seedling recruitment of senita cacti." The positive effects of nurse plants with this plant species depended on rainfall
Examples
Olneya tesota
Study of ironwood trees (O. tesota) has shown that a nurse plant's importance is not only as a temperature buffer, but also as a water buffer. In terms of water stress, there was a difference in the facilitative effects between mesic and xeric sites. In xeric sites, the richness and abundance of perennial plants was higher, whereas, ephemerals saw no difference. In mesic sites, the abundance of perennial and ephemeral plants was no different, but the ephemeral richness was lower. The size of the canopies of ironwood trees was no different between xeric and mesic sites. But the canopy size did affect perennial plants more than ephemeral plants. With the perennial plants, there was a positive effect. The richness, abundance, and size of the plants was greater under the canopies. With ephemeral plants the richness was unaffected, and the abundance increased in xeric sites. Ironwood tree canopies have provided facilitative effects on plant species richness and abundance in xeric sites in the Sonoran Desert. Two factors, water stress and benefactor size, had effects on facilitation and are factors to consider when looking at the richness, abundance, and size of the plants under nurse plant canopies in xeric and mesic habitats. The ironwood was often the only tree growing in xeric areas and their canopies had the largest effect on plant community structure and richness even when water stress was high. Thus, ironwood trees creative diversity that is absent in other desert microhabitats.
Palo verde (Cercidium microphyllum) and saguaro (Carnegiea gigantea)
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