Nurse plants


Nurse plants are trees that serve as protection to smaller plants. Xeric environments can experience extreme high temperatures and extreme low temperatures. In these environments, nurse plants provide shaded microhabitats for the survival of several other plant species. 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... 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 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.
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, mesquite, and ironwood 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 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''

Joshua Tewksbury’s study discusses the different interactions between nurse plants and those under their canopy. It showed that the presence of ironwood trees did not explain the significant variation of plant community structure. 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. These trees contribute to habitat heterogeneity that can’t be found in other desert microhabitats.

''Palo verde'' ''(Cercidium microphyllum) and saguaro (Carnegiea gigantea)''

An example of a nurse plant would be the Palo Verde tree, found in the Sonoran Desert, that may have saguaro cacti underneath its canopy. Other examples of nurses are grasses and cacti. Trees and shrubs are the more common nurse plants.
Nurse plants provide the ideal microclimatic environment for species like the saguaros. They allow them to extend their ranges “in otherwise inhospitably cold areas".. Some of the benefits described above can limit saguaros during establishment, but subfreezing temperatures is one variable the cactus is susceptible to. These temperatures in the northern part of Arizona are why saguaros haven’t established there. 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"
Saguaros are established on the south side of a nurse plant’s canopy more than the north side. According to Drezner and Garrity, the south side of canopies have higher minimum temperatures and the north sides have colder temperatures. Saguaros establish under denser canopies than plants with a more open canopy because of better microclimatic conditions. It might have been unexpected that the saguaro established on the south side because of higher minimal temperature. Saguaros can handle higher temperatures but are susceptible to subfreezing temperatures. With the temperatures on the south side of the nurse, the risk of experiencing subfreezing temperatures is reduced in the winter.
Ambrosia deltoidea and Cercidium microphyllum were the two main nurse plants observed. The study found that maximum temperatures under C. microphyllum were lower and the minimum temperatures were higher, showing that nurse plants provide a microclimate under their canopy and protects the saguaros from extreme cold or hot temperatures.
The death of a nurse plant generally precedes the plant species it’s protecting. There is evidence of competitive interactions between saguaro cacti and paloverde trees. The saguaros under a paloverde’s canopy declined the vigor of the tree. Trees that didn’t have saguaros under them didn’t die as fast. One factor of this competitive interaction is root competition. The saguaro’s roots exist in shallow soil, whereas, a paloverde’s roots go deeper. The saguaro’s roots are like an umbrella and capture most of the moisture before it can reach the paloverde’s roots.

Invasibility and nurse plants

Badano et al used two hypotheses to look at the invasibility by alien species with nurse plants in the area. They used the biotic resistance hypothesis where a new species that arrives is more likely to find strong competitors that impede their success as the number of native species increases and local diversity acts as a barrier for biological invasions, and the biotic acceptance hypothesis, which is described as the main force that regulates native and alien species’ performance and diversity and increased availability of resources and habitat heterogeneity associated with increased surface area.
Neither of these hypotheses considered the variations of harsh environments. This harshness could reduce competition in plant communities and the overall performance of plants.
The nurse plant in this study was the field chickweed, Cerastium arvense L. For natural assemblages, there was a positive relationship between C. arvense and the abundance and diversity of plant assemblages growing within cushion plants, or cushions. Cushion plants are plants that grow a few inches in height, three meters in diameter, and form a compact mat of closely spaced stems. It was found from this study that invasive species grew under nurse plant canopies and that they were providing protection for those species.
According to Badano et al, “This study indicated that the performance of the invasive plant C. arvense was positively affected by increasing diversity of native species within the habitat patches created by the cushion plant A. madreporica, while these relationships were negative or absent in the surrounding open areas.”

Nurse plants and ants

These nurse plants also help the composition of ant communities. They provide protection and food to the different ant communities in the Sonoran Desert. Four ant species “were associated with tree habitats, whereas Pheidole sp. A was associated with open areas".
In the Sonoran Desert, ant species are greater than in the Mojave Desert, Chihuahuan Desert, or Chihuahuan desert grassland, and that is due to greater precipitation. When rainfall increases, so does the ant diversity.