Nurse and Target plants

Nurse and Target plants

Introduction

With plants competing for nutrients, water, and sunlight, different plants in the same physical environment affect the other negatively and sometimes positively. The effects of such an ecosystem often are harmful due to the competition. However, there are cases where the survival of some plants would otherwise be in jeopardy without other plants supporting them. This is the case for plants that takes advantage of the canopy of other mature plants for them to germinate and grow. Such plants thrive on the change in temperature, light, humidity that the other mature plants offer by providing a cover to such plants. In the desert and other regions with a harsh climate, such dependency can be used to restore the environment. Despite the advantages nurse plants offer, other factors such as human disturbance can affect the facilitation, which requires the nurse plants to be different depending on the specific region. This review studies plant facilitation in restoring the desert environment. Case examples of plan facilitation methods are discussed.

Nurse and Target plants

A nurse plant is a plant that offers a conducive environment for other plants, known as target plants, to survive the harsh conditions underneath them. There are both positive and negative sides to nurse plants. On the negative side, growing under the shade of another plant may reduce the amount of sunlight received by such plants, thus affecting photosynthesis. One of the main determinants in the restoration of an arid region is the determinant of the nurse plant (Ren, Yang, & Liu, 2007). The nurse plant offers the ideal environment for the target plant to thrive, depending on the conditions for survival for the target plant.

Another factor to consider is the age and size of the target plant. The nurse plants have more positive effects when the target plants are young, with older and bigger target plants offering competitive interaction (Ren, Yang, & Liu, 2007). In cases where the environment is hugely degraded, the best plants to act as nurses are the native species (Ren, Yang, & Liu, 2007).

The distance between the nurse and target plants is another factor that affects facilitation. There is competition for water and nutrients between the two plants affecting both negatively. Also, some plants draw water from underground and to the surface soil with the plants nearby being the significant benefactors in the process. Protection from animals is better for plants growing near the nurse, as explained in the paragraphs below

Plant facilitation methods

The extreme temperatures, solar radiation, and shortage of water in arid regions make it impossible for many plants to thrive. Also, with human interference and animals, both domesticated and wild grazing, restoration of plant cover in the arid regions becomes harder. Desert plants that survive such climatic conditions can improve such conditions, offering an ideal habitat for other plants to survive. Some of the states and nurse plants that protect against the extremes are discussed below.

Lack of Rainfall

Some nurse plants are known to improve the availability of water in the soil by the use of the hydraulic lift. This is where the deep-rooted plants lift water from the moist soil to the upper soil layer, which is dry via the roots during the night (Wang et al., 2018).  Such nurse plants are drought resistant due to this characteristic and thus are vital for the survival of shallow-rooted plants that utilize the water during the night. An example of such plants is the Populus euphratica Olivier, which grows in the desert region and helps in the hydraulic redistribution of water (Wang et al., 2018).  In addition to this, other plants help during prolonged dry spells. Studies conducted on Acervate plants show that it helps in the survival of the target seedling during rainless years (Ren, Yang, & Liu, 2007).

Extreme Solar Radiation and Soil Moisture

The extreme solar radiation in the desert regions can destroy the reaction centers of photosystems and produce oxidative damages in healthy plants (Ren, Yang, & Liu, 2007).  Also, shade torrent species cannot survive the desert sunlight due to photoinhibition. Such target plants require nurse plants that can offer the shading that can sustain them. In addition to providing shading, such types of nurse plants help in lowering the transpiration of target plants and in maintaining high soil humidity (Ren, Yang, & Liu, 2007).  This shading thus offers the environment for other plants to grow under them in the desert. Such is the case for desert succulents, which are often found under the canopies of the desert bunchgrass (Franco & Nobel, 1988). Also, the cacti know to withstand the harsh arid climates, usually grows under trees and shrubs during its seedling days (Withgott, 2000).

Shielding from Grazing

Nurse plants can shield the target plants from herbivores (Soliveres, Eldridge, Hemmings, & Maestre, 2012). Such is the case for nurse plants that are non-palatable or are thorny, thus offering physical protection, that the herbivores can’t graze near them.  Some nurse plants offer associative resistance where the herbivore has less preference for target plants when it is associated with another plant as compared to the target in isolation (Acuna-Rodríguez, Cavieresi, & Gianoli, 2006).  A case example is the Atriplex patula, where studies conducted showed 50% less survival when growing in isolation than when associated with Juncus Gerardi (Acuna-Rodríguez, Cavieresi, & Gianoli, 2006).  This makes the target thrive without the threat of the herbivores. In addition to protection, the nurses help in the recovery of the target plants after grazing by providing higher levels of resources (Soliveres, Eldridge, Hemmings, & Maestre, 2012).

Wind Protection and Soil nutrients

With most arid regions being bare for miles, the strong wind that is prevalent in these regions carries anything in its way and uproots plants that have a weak root system. For target plants that have such roots, they are offered protection by other plants that can withstand the wind and thus spared from the wind. Also, the nurse plants offer soil oxygenation to other plants when the soil PH is not conducive for some target plants (Withgott, 2000). For soil nitrogen, legumes are useful nurse plants, offering such nutrients to targets in arid regions (Ren, Yang, & Liu, 2007). The nitrogen levels in the dry areas are higher around desert trees and shrubs, making good ground for the growth of other target plants (Franco & Nobel, 1988).

Soil Microbiota and Soil Stabilization

Nurse-associated soil microbiota leads to changes in plant characteristics such as chlorophyll content, flower, and seed content of the target plants (Rodríguez-Echeverría, Lozano, & Bardgett, 2016). For a start, there is a more substantial litter input under nurses. Arid areas are a hotspot for microbial activities, with faster rates for decomposition and nutrient mineralization (Rodríguez-Echeverría, Lozano, & Bardgett, 2016). This leads to more rapid rates for nutrient mobilization with activities of the micro-organisms, to the advantage of the target plants. Bacterias such as Proteobacteria brought about by the nurse plants are related to increased nutrient mobilization mineralization (Rodríguez-Echeverría, Lozano, & Bardgett, 2016). In addition to this, Proteobacteria taxa solubilize phosphate by releasing enzymes and organic acids into the soil. This makes nutrients readily available to the target plants (Rodríguez-Echeverría, Lozano, & Bardgett, 2016).

Also, Mycorrhizal fungi, associated with nurse plants, are known to contribute to the formation of stable aggregates of soil particles, which leads to soil stabilization (Rodríguez-Echeverría, Lozano, & Bardgett, 2016). These soil aggregates also contain nutrients and water which protect micro-organisms from extreme weather conditions.

Conclusion

Nurse plants play an essential role in the success of the restoration of a poor environment, such as arid regions. The layer of protection offered to plants from extreme climate and stabilizing the soil is enough to change apart from bare arid to a region covered with vegetation.

References

Acuna-rodriguezi, I.S., Cavieresi, L.A., & Gianoli, E. (2006). Nurse effect in seedling establishment: facilitation and tolerance to damage in the Andes of central Chile. Revista Chilena de Historia Natural, 79(3), 329-336. Retrieved from http://www.redalyc.org/articulo.oa?id=369944279005

Franco, A.C., Nobel, P.S. (1988). Interactions Between Seedlings of AGA Ve Deserti and the Nurse Plant Hilaria Rigida. Ecological Society of America. 69(6), 1731-1740. https://doi.org/10.2307/1941151

Ren, H., Yang, L., Liu, N. (2007). Nurse plant theory and its application in ecological restoration in lower subtropics of China. Science Direct, 18(2), 137-142. https://doi.org/10.1016/j.pnsc.2007.07.008

Rodríguez-Echeverría, S.,  Lozano, Y.M. & Bardgett, R.D. (2016). Influence of soil microbiota in nurse plant systems. Functional Ecology, 30, 30–40. doi: 10.1111/1365-2435.12594

Soliveres, S., Eldridge, D.J., Hemmings, F., & Maestre, F.T. (2012). Nurse plant effects on plant species richness in drylands: the role of grazing, rainfall, and species specificity. Europe PMC Funders Group, 14(6), 402–410. doi:10.1016/j.ppees.2012.09.003

Wang, F., Xu, Y., Yang, X., Liu, Y., Lv, G., Yang, S. (2018). Soil water potential determines the presence of hydraulic lift of Populus euphratica Olivier across growing seasons in an arid desert region. Journal of Forest Science, 64(7), 319–329. https://doi.org/10.17221/49/2018-JFS

Withgott, J. (2000). Botanical Nursing: From deserts to shorelines, nurse effects are receiving renewed attention. Bioscience, 50(6), 479-484. https://doi.org/10.1641/0006-3568(2000)050[0479:BN]2.0.CO;2