Nurse Shrubs Promote Ecosystem Regeneration

a mountain with snow is in the background of a landscape with trees and shrubbery

Biological Strategy

Mediterranean montane ecosystem

AskNature Team

Image: Kewl / Pixabay / Free non-commercial use

Distribute Solids

Solids include resources such as food, seeds, spores, and leaves, which living systems must sometimes move to accomplish various activities such as pollinating, feeding their young, or building shelters. To effectively distribute solids, living systems must use strategies appropriate to the sizes and types of solids and the media in which they exist (air, water, or another medium). This may entail strategies beyond simply holding and carrying solids. For example, some seeds are carried by the wind to new locations for germination. Others–like the burdock plant’s hooked seeds–cling to animal fur to be transported.

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Cooperate Within an Ecosystem

An ecosystem is a community of organisms (plants, animals, and microbes) interacting with one another and the nonliving components of their environment (such as air, water, and mineral soil). This interaction can be passive or active, and can cooperatively enhance the functioning of the ecosystem as a whole. For cooperation to contribute to maintaining communities within an ecosystem, it must be beneficial to at least some members of the community. Cooperation consists of symbiotic relationships, such as mutualism (in which two or more species in an ecosystem benefit) and commensalism (in which one species benefits and the effect on others is neutral). An example of a commensal relationship is that between bromeliad plants and trees: bromeliads live on trees without harming them. Bromeliads have mutualistic relationships with other species, including insects, frogs, and worms. The plants capture water in their base, forming a pond that these organisms join. The nutrients that these organisms excrete in their droppings nourish the bromeliad.

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Regulate Climate

Individuals and ecosystems manage local atmospheric conditions, thus regulating local climates. The local scale refers to climates such as those found under a shrub or within a forest. This local climate is called a microclimate. Regulating local climate creates favorable conditions, such as increased humidity or less severe winds, and contributes to species diversity. Microclimate development occurs over time: just from their presence, a few species initially able to tolerate the local conditions start to modify those conditions, resulting in microclimates that support others. For example, plants that live in alpine areas are subject to strong ultraviolet light, drying winds, and freezing temperatures. By growing together into dense clusters, these plants create a favorable microclimate that supports survival of a diversity of species.

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Manage Disturbance in a Community

When environmental conditions change, they can disrupt an ecosystem’s equilibrium. Excessive rain can cause flooding and drought can cause forest fires. An ecosystem must be resilient to such disturbances. Disturbances are unpredictable in location, size, and intensity, so ecosystems must be able to regrow and must have a variety of duplicate forms, processes, or systems that are dispersed in location. For example, a forest ecosystem can recover from fire because diverse organisms play different roles in different ways and in different locations. Many organisms can resprout or grow from seeds triggered by fire, and their dispersed distribution ensures that an entire population isn’t decimated. Though the recovered ecosystem may look totally different from the pre-fire one, the ecosystem as a whole remains healthy.

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Generate Soil/Renew Fertility

An ecosystem can’t survive long without building up its soils and preventing them from being depleted of fertility. Organic matter, minerals, and other nutrients in soil support organisms, from microorganisms to the largest mammals. Raw soil consists of weathered minerals, such as silica. But organisms need more than minerals to survive, and minerals alone are poor at storing water needed by organisms. Therefore, soil has organic matter to support an ecosystem of organisms that, in turn, contribute more organic matter, make nutrients more accessible, and hold water. The dung beetle is an organism that enhances soil fertility. This beetle gathers and transports animal dung, feeds on it, buries it in the soil, and lays eggs on it. The dung provides its young with food when they hatch. At the same time, it adds nutrients and organic matter to the soil, benefitting the ecosystem as a whole.

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Ecosystems

Eco- ("household"), system ("organized whole")

An ecosystem consists of all the biological and nonbiological elements that make up a defined area. That definition might seem vague, but that means it’s flexible: We can apply the word across different scales and times. Ecosystems can be small, like a single tide pool, or large, like coastal wetlands. Ecosystems can include plants, animals, bacteria, fungi, rocks, temperature, light, soil, and climate—all linked through the flow of energy and cycle of nutrients. Each part of an ecosystem interacts with, and is influenced by, other members of the system. These interactions are dynamic, leading to changes to any ecosystem over time.

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Pioneering nurse shrubs in Mediterranean montane forests promote ecosystem regeneration by increasing shade above ground and potassium below ground.

Recovering from large disturbances, such as deforestation or forest fires, is a lengthy process. A clearing goes through multiple steps, with different plants starting to regrow in a process called succession. Smaller, weedier plants typically grow first, priming the area for larger, more long-lived ones (like shrubs and trees). This type of multi-step succession occurs in Mediterranean montane forest environments as well, with “nurse” shrubs providing protected conditions for larger trees.

These nurse shrubs prepare the area in two ways: by creating a shaded environment and by depositing higher concentrations of potassium into the soil. The shaded environment promotes tree seedling growth through the canopy effect: a group of effects including lower air temperatures, lower soil temperatures, and higher water levels. All of these effects together help create the perfect environment for seedlings.

Second, nurse shrubs increase soil potassium concentrations by trapping windblown particles in their branches and by generating organic litter such as broken twigs and fallen leaves. During the dry season, when water is scarce, higher levels of potassium help protect the new tree saplings from drought by reducing transpiration rates (the plant equivalent of sweating). This accumulation of potassium reduces transpiration losses by affecting the osmotic gradient (the comparable concentrations of two solutions separated by a semipermeable membrane). By increasing the potassium levels within the plant, the gradient is increased, causing water to diffuse into the plant as it tries to maintain equal concentrations on each side of the membrane. In essence, by increasing its own potassium concentration, a plant can hold on to water better. In such a way, these nurse shrubs play a key role in ecosystem succession.

This summary was contributed by Thomas McAuley-Biasi.

Nurse shrub catches leaves and twigs which eventually decompose in the soil, releasing potassium (K) to be absorbed by tree roots

Increased potassium (K) in the soil creates an osmosis gradient that moves water into the roots

Image: John Tann / CC BY - Creative Commons Attribution alone

Last Updated June 18, 2020

References

“Is the facilitative effect of nurse shrubs on early recruitment of trees mediated by a ‘canopy effect’ (micro-climate amelioration and protection from herbivores), a ‘soil effect’ (modification of soil properties), or both?…Both effects benefited seedling performance. However, microclimatic amelioration due to canopy shading had the strongest effect, which was particularly pronounced in the drier site. Below-ground, shrubs did not modify soil physical characteristics, organic matter, total N and P, or water content, but significantly increased available K, which has been shown to improve seedling water-use efficiency under drought conditions. Conclusions: We propose that in Mediterranean montane ecosystems, characterised by a severe summer drought, pioneer shrubs represent a major safe site for tree early recruitment during secondary succession, improving seedling survival during summer by the modification of both the above- and below-ground environment.” (Gómez-Aparicio et al. 2005:191)

“The canopy effect due to microclimate amelioration was the main mechanism enhancing seedling survival and growth (see also Valiente-Banuet & Ezcurra 1991; Valiente-Banuet et al. 1991; Greenlee & Callaway 1996; and Maestre et al. 2001). Lower air and soil tempera- tures, radiation and vapor pressure deficit measured below shrub canopies in comparison to open areas re- duced leaf temperature and transpiration losses, conse- quently inducing a more favourable water balance in tree seedlings.” (Gomez-Aparicio et al. 2005:197)

“Shade appears to be a necessary condition for seedling establishment of several Mediterranean woody species (Gómez 2004; Castro et al. 2004a; Gómez-Aparicio et al. 2005).” (Gomez-Aparicio et al. 2005:192)

“However, nurse shrubs effectively modified one soil property, available K [potassium], being in all cases higher under shrubs than in open interspaces. Potassium seems to increase plant resistance to drought, mainly due to os- motic adjustments and a reduction in transpiration rates, which in turn involves a higher water-use efficiency (Bradbury & Malcom 1977; van den Driessche 1991; Egilla et al. 2001).” (Gomez-Aparicio et al. 2005:196)

Journal article