Padilla, F. & Pugnaire, F. 2006. The Role of Nurse Plants in the Restoration of Degraded Environments

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www.frontiersinecology.org The Ecological Society of Americ

Plant interactions strongly influence community struc-ture and dynamics, and are responsible for the pres-ence or absence of particular species in a community.Traditionally, competition has been the most studiedaspect of those relationships, so that ecological modelshave focused for decades on negative interactions, over-looking the existence of positive effects between plants.In the past 15 years, however, research has highlightedthe role of positive plant interactions (facilitation) inalmost all biomes (Bertness and Callaway 1994; Bertnessand Hacker 1994; Callaway 1995; Brooker and Callaghan

1998; Callaway et al. 2002; Bruno et al. 2003; Lortie et al.2004). Despite this increasing recognition, the inclusionof facilitation into mainstream ecological theory has beenslow (Bruno et al. 2003). Facilitation appears to be anessential process, not only for survival, growth, and fit-ness in some plants (Callaway et al. 2002; Tirado and

Pugnaire 2003; Cavieres et al. 2006), but also for diversitand community dynamics in many ecosystems (Pugnairet al. 1996; Kikvidze et al. 2005). Examples are more evdent in harsh, limiting environments, where some specieare able to ameliorate the physical conditions in somway, or to prevent herbivory, thereby providing more suitable habitats for other species (Figure 1). This interactiohas a practical side when applied to ecological restoration. In degraded habitats with extreme environmentaconditions or large numbers of herbivores (Figure 2), tharea near or under the canopy of certain species may pro

vide refuge for the species being restored (target specieswhich otherwise may fail to establish. Here we review thpotential of this procedure for ecological restoration.

Competition and facilitation

Plants growing close to each other influence their neighbors in many ways, resulting in a broad range of detrimental or beneficial outcomes. If negative effects prevail, thinteraction results in competition or interference, a consequence of sharing limited resources (water, nutrientlight, space), or of a release of chemicals that will harmnearby plants (allelopathy). Conversely, nearby plant

may exert a positive influence, termed facilitation, iwhich at least one neighboring species benefits from thinteraction through improved survival, growth, or fitness

Both positive and negative effects occur simultaneously, affect different variables, and change with time anlocation (Armas and Pugnaire 2005). The net balancbetween these effects represents the magnitude and sig(either positive or negative) of the interaction (Callawaand Walker 1997; Holmgren et al. 1997; Figure 3Several factors affect this balance, including physiologcal and developmental traits (Callaway and Walker 1997

REVIEWS REVIEWS REVIEWS

The role of nurse plants in the restoration ofdegraded environmentsFrancisco M Padilla* and Francisco I Pugnaire

Traditional ecological models have focused mainly on competition between plants, but recent research ha

shown that some plants benefit from closely associated neighbors, a phenomenon known as facilitation. Ther

is increasing experimental evidence suggesting that facilitation has a place in mainstream ecological theory

but it also has a practical side when applied to the restoration of degraded environments, particularly dry

lands, alpine, or other limiting habitats. Where restoration fails because of harsh environmental conditions o

intense herbivory, species that minimize these effects could be used to improve performance in nearby targe

species. Although there are few examples of the application of this nursing procedure worldwide, experi

mental data are promising, and show enhanced plant survival and growth in areas close to nurse plants. W

discuss the potential for including nurse plants in restoration management procedures to improve the succes

rate of such projects.

Front Ecol Environ 2006; 4(4): 196202

Estacin Experimental de Zonas ridas, Consejo Superior deInvestigaciones Cientficas, General Segura 1, E-04001 Almera,Spain *([email protected] )

In a nutshell: In limiting environments such as dryland, alpine, or unfertile

habitats, some plants benefit from growing close to others thatameliorate extreme conditions, improve resource availability,or protect against herbivory

The effect known as facilitation has implications for restora-

tion where physical conditions or herbivores constrain plantperformance The application of facilitation to restoration projects may

improve the establishment of target plants, mimicking a nat-ural process

Species traits and site characteristics influence success rate andshould be carefully considered


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The Ecological Society of America www.frontiersinecology.org

Armas and Pugnaire 2005), but abiotic condi-tions seem to be the overriding factor, increasingthe importance of positive effects in harsherenvironments (Brooker and Callaghan 1998;Pugnaire and Luque 2001; Callaway et al. 2002;but see Maestre et al. 2005 and Lortie andCallaway 2006 for discussion of the stress-gradi-

ent hypothesis).

The nurse effect

In some habitats, seedling establishment may beenhanced in the vicinity of adult plants thatameliorate extreme environmental factors (egCavieres et al. 2006). The positive influence ofthe adult plants on seedlings is called nurseplant syndrome (Niering et al. 1963), and is oneof the first recorded examples of close spatialassociation between plants being more advanta-geous than detrimental. This effect is more com-

mon in environments where abiotic factors orherbivory limit plant performance, such as in arid (Floresand Jurado 2003) or alpine habitats (Cavieres et al. 2006).The underlying mechanisms relate mainly to theimprovement of microclimatic conditions, increasedwater and nutrient availability, and protection againstherbivory (Panel 1; also see Callaway 1995; Callaway andPugnaire 1999).

Although some authors have suggested that this nurseeffect could potentially play a role in restoration(Bradshaw and Chadwick 1980), by the mid-1990s only afew anecdotal reports on this topic were available (eg

Mitchley et al. 1996). However, experimental evidenceaddressing the role of nurse plants in restoration hasincreased in the past few years (Table 1). Here we reviewrestoration experiments in which seeds or seedlings ofrestored species were placed both near adult plants thatacted as nurses and in control gaps (Figure 4), and pro-vide suggestions for management. We have not includedexamples from natural or planted forest systems or fromnurse crops (ie when nurse plants are cultivated, either inadvance or simultaneously, with restored plants).

Role of facilitation in restoration

The first published research looking at the use of naturalnurse plants for restoration purposes was carried out atthe end of the 1990s in southeast Spain (Castro et al.2002; Gasque and Garca-Fayos 2004). Since then sev-eral experiments have been conducted in alpine areas,semiarid steppes, arid shrublands, coastal wetlands, anddegraded and burnt sites.

In the Sierra Nevada range (Spain), at an elevation of1800m, Castro et al. (2002) found that nurse shrubsdecreased mortality in two mountain pines withoutinhibiting their growth. After two growing seasons, sur-vival of Scots pine (Pinus sylvestris) and European black

pine (Pinus nigra) was markedly better under sage (Salvialavandulifolia) than in control gaps (55 versus 22% and 82versus 57%, respectively). Differences were still presentafter four growing seasons (Castro et al. 2004); survivalrates were 1.8 to 2.6 times higher under sage than in gaps.When the nurse plants were thorny shrubs such as Prunusramburii, establishment differed between the north andsouth aspects of the plant; while results in the north weresimilar to survival levels seen under sage, in the south theresults were similar to those seen in open areas.

In the same Sierra Nevada range, but including a wider

Figure 1. Fertile area under the canopy of the leguminous shrub Retamasphaerocarpa in the Tabernas desert (Almera, Spain). Retamafacilitatesthe growth of understory plants, leading to the development of a communityconsisting of numerous small shrubs and herbaceous species.

Panel 1. The advantages of growing close to nurseplants

Nurse plants may buffer non-optimal environmental condi-tions. Shade reduces soil water evaporation, lowers soil and airtemperature, and decreases the amount of radiation reachingthe plants, thus protecting seedlings from the damaging effectsof extreme temperatures and low humidity in arid environ-ments.Canopy protection also prevents salt enrichment in soilmarshes and wetlands, and may reduce frost injuries in coldareas.

Nurse plants may improve the availability of soil resources.Through the process known as hydraulic lift, roots of certainspecies lift water stored in deep soil layers and release it nearthe soil surface. Once in the surface layers, the water can beused by understory plants, and improves their water status and

growth rate. Nutrients in the understory are enhancedthrough litter and sediment accumulation, higher mineraliza-tion rates, and larger microorganism populations.Positive rootinteractions between facilitator and facilitated plants allownitrogen transfer between legumes and non-leguminous plants,increase ectomycorrhizal infection, and make possible theexchange of nutrients and carbon via mycorrhizal fungi.

Nurse plants may protect against grazing. In heavily grazedareas, plants growing beneath non-palatable or thorny plantshave an advantage,as compared to unprotected plants.

Nurse plants that are highly attractive to pollinators mayincrease pollinator visits to the target plants.


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altitudinal range (5002000 m elevation), Gmez-Aparicio et al. (2004) conducted a series of experiments totest the effect of 16 native shrub species over 11 shrub andtree species. One year after planting, establishment suc-cess under shrubs was more than double that seen in thegaps, reaching fourfold higher numbers in some cases.However, the outcome differed, depending on the targetspecies, type of nurse plant, and year. The observed nurseeffect of shrubs was considerable for evergreenMediterranean species, such as Holm oak (Quercus ilex),shrubs such as prickly juniper (Juniperus oxycedrus), and

deciduous species like maple (Acer opalus), but was nosignificant for pines (Scots and black pine). The most successful nurse plant species were native brooms (such aGenista spp), and small and thorny shrubs. In contrast, significant negative influence was seen with rockrose(Cistus spp), probably the result of allelopathy. In fact, thharsher the ecological conditions, the stronger the faciltative effect of the nurse plants.

A large number of experiments have been carried out ttest the potential of esparto grass (Stipa tenacissima), widespread perennial tussock-forming grass, as a nurs

plant on degraded semiarid steppe

in southeast Spain. However, thresults differed depending on sityear, and target species involvedGasque and Garca-Fayos (2004found that the favorable conditionnear esparto grass tussocks increasegermination rate of Aleppo pin(Pinus halepensis; 43% under Stipversus 8% in control gaps) as well aearly establishment (19% under Stipversus 3% in control gaps); after thsummer drought, however, all thplants died. Similar results wer

obtained by Navarro-Cano et a(pers comm) with seedlings oKermes oak (Quercus coccifera) anRhamnus lycioides, and by Maestreal. (2002) with Kermes oak. Espartgrass increased germination and suvival before the drought period, buagain no plants survived beyond thsummer. In other experiments usinseedlings of moon trefoil (Medicagarborea), lentisc (Pistacea lentiscus

Figure 2. In past centuries, intense pressure from human activities, including agriculture, overgrazing, burning, and logging, has resultein the deforestation of most mountainous areas in SE Spain, such as the Sierra Alhamilla foothills shown here. Woodland restoration asuch sites is frequently impeded by drought and grazing; using nurse plants may improve the success of restoration projects.

Table 1. Experimental reports in which facilitation by nurse plants was usedin restoration projects

Environment Nurses Targets Reference

Mediterranean Shrubs, legumes Shrubs,trees Castro et al. (2002);mountain (Salvia, Genista) (Pinus,Acer) Gmez-Aparicio et al. (2004)

Semiarid s teppes Perennial g rass Shrubs, trees Maestre et al. (2001,2002);(Stipa) (Quercus, Pinus) Gasque and Garca-Fayos (2004);

Navarro-Cano et al. (pers comm)

Marshes Perennial grass Deciduous shrub Egerova et al. (2003)(Spartina) (Baccharis)

Tropical sub- Trees Tree Snchez-Velsquez et al. (2004)

humid forest (Acacia,Acalypha) (Brosimum)

Arid shrubland Succulent shrubs Succulent shrubs Blignaut and Milton (2005)(Drosanthemum) (Drosanthemum)

Arid rangelands Shrub Grasses Huber-Sannwald and Pyke (2005)(Artemisia) (Agropyron)

Semiarid Leguminous shrub Shrubsabandoned fields (Retama) (Olea,Ziziphus) Padilla and Pugnaire (unpublished)

This is not an exhaustive list of the species used


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and Kermes oak, Stipa did improve survival after thedrought period, and did not affect plant growth (Maestreet al. 2001).

Nurse plants have also helped in the restoration ofcoastal marshes in Louisiana. Egerova et al. (2003) foundhigher survival and growth rates in groundsel trees(Baccharis halimifolia) growing among clones of the peren-nial smooth cordgrass (Spartina alternifolia) than in gaps(45 versus 11%, respectively), as a result of the morefavorable microclimate and soils.

In a secondary tropical dry forest, Snchez-Velsquez etal. (2004) looked at four different types of nurse plants forbreadnut seedlings (Brosimum alicastrum). Breadnut estab-lishment after one year differed depending on the type ofspecies of nurse tree. It was higher under Acalypha cinctaand guayabillo (Thouinia serrata; 55 versus 40%) andmuch lower (


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where soil resources and climatic conditions did not con-strain establishment (reflected by 100% survival in control

plots) sagebrush seedlings growing close to legumes wererestricted rather than favored by nurse plants.

The importance of facilitation increases with increas-ing severity of the abiotic conditions (Pugnaire andLuque 2001; Callaway et al. 2002), and therefore the pos-sibility of benefiting from nurse plants should alsoincrease under such conditions. Gmez-Aparicio et al.(2004), for example, found that facilitation effects werestronger in dry locations and on the south-facing slopes ofa dry Mediterranean mountain.

Rainfall variability

In dry areas, changes in water availability may make interac-tions among plants shift from competition to facilitation andvice versa, thereby increasing the importance of facilitationduring drought (Holmgren et al. 1997). This shift betweenpositive and negative effects may be relevant for nurse plantsuccess, since different results could be obtained at the samesite in different years, depending on rainfall. Furthermore, inwet years the nurse effect may not be as critical as in dryyears, because establishment may occur without a nurseplants protection (see Kitzberger et al. 2000). As describedabove, Gmez-Aparicio et al. (2004) found that shrubbynurse plants have considerable influence on seedling sur-vival in dry years, but not in wet years. Similar results have

been reported by Ibaez and Schupp (2001), in an experi-ment conducted in Logan Canyon, Utah, where they placedseedlings of curl-leaf mountain mahogany (Cercocarpus ledi-

folius) under big sagebrush; facilitation was apparent in a dryyear whereas negative effects were seen during a wet year.

Nurse species

Selection of the best nurse species is an important deci-sion in restoration projects, as this will determine the suc-cess of the project (Gmez-Aparicio et al. 2004; Snchez-

Velsquez et al. 2004). In extreme environments, thmost suitable choices are native species that are able t

improve environmental conditions for seedling establishment. Although some exotic species, such as black locus(Robinia pseudoacacia), have been used successfully anurse crops in the south of England (Nimmo anWeatherell 1961), such options should be scrutinizecarefully because of the risk of biological invasions. Iheavily grazed sites, thorny, non-palatable species are recommended, although some herbivory and seed predatiomay still occur, since the nurse plants may provide refugfor small animals. Species that release allelopathic compounds should be avoided.

The nurse plants canopy structure may also influencestablishment success, in particular in relation to shad

intensity and rainfall interception. The location of targets under the canopy also affects seedling surviva(Castro et al. 2002), which is often higher in the shadiepositions. In a tropical, sub-humid forest, the varying levels of shading created by the nurse plants appeared to bresponsible for the variations in seedling establishmenreported by Snchez-Velsquez et al. (2004).

Many shrubs may limit water availability in theunderstories by intercepting rainwater during small precipitation events, making the soil under shrubs dryer thain open areas (Tielbrger and Kadmon 2000

Nonetheless, during moderate to heavy rainfall, somshrubs enhance water availability by directing wate

intercepted by the canopy to the understory througstemflow (Garca 2006). Distance from the nurse plant another important factor; amelioration of negative condtions and improved availability of resources has beeshown to decrease from the canopy center outward(Moro et al. 1997; Dickie et al. 2005).

Factors such as competitive ability, use of resources bthe nurse plants themselves, and the potential for roooverlap between nurse plants and target plants (Blignauand Milton 2005; Huber-Sanwald and Pyke 2005) musalso be taken into account. Competition or interferenc

Figure 4. (a) A planted Aleppo pine thrives under the canopy of the drought-deciduous shrub Anthyllis cytisoides, which provideshelter against (b) high radiation levels in experiments on nurse plants conducted in dry mountains in Almera (SE Spain).

(b)(a)(a) (b)


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caused by species that occur naturally under nurse plantcanopies (eg understory herbaceous species) may alsoaffect the outcome.

Target species

Interactions among plants depend upon species charac-

teristics, so the selection of target species (ie those beingrestored) may influence the outcome of a restoration pro-ject. Furthermore, the balance of an interaction could bedetermined by the ecological requirements of the speciesinvolved and their ability to deal with unfavorable abi-otic conditions (see Liancourt et al. 2005; Bertness andHacker 1994).

Walker et al. (2001), for example, reported higher sur-vival rates of Ambrosia dumosa in the open than undershrubs in an arid environment, becauseAmbrosia can suc-cessfully cope with the conditions that exist in open areas.

Ambrosia was also subjected to competition from the nurseshrub. Gmez-Aparicio et al. (2004) reported that shade-

tolerant species and late-successional shrubs showed amore positive effect in response to nurse plants than didpioneer shrubs and shade-intolerant pine trees (Castro etal. 2002, 2004). In spite of this positive influence, thenurse effect may be insufficient to increase plant establish-ment if target species have a low tolerance for the preva-lent abiotic conditions, or if these are particularly severe.For example, Kitzberger et al. (2000) and Maestre et al.(2002) found no seedling establishment, either with orwithout nurse plant protection, during especially dry years.

The age and size of target species must also be consid-ered, since several studies have shown that the balancebetween facilitation and competition varied with the life

history of plants. Nurse plants had strong positive effectswhen the target species were relatively young, but pre-dominantly competitive interactions were observed witholder, larger individuals (Callaway and Walker 1997;Holmgren et al. 1997; Gasque and Garca-Fayos 2004;Armas and Pugnaire 2005). The use of plants of similarage and size, both as nurse plants and target species, couldhave exacerbated the negative effect of clumpingreported by Blignaut and Milton (2005).

Positive and negative effects of nurse plants

High recruitment rates close to nurse plants do not pre-

clude negative effects on target species, but do ensurethat the positive effects outweigh the negatives ones.This may lead to higher survival rates under nurse plantsthan in gaps, but lower survival rates than those seenwhen using other procedures, such as artificial shading(Barchuk et al. 2005) or watering (Snchez et al. 2004).

Conclusions

Published reports show that nurse plants improveseedling establishment in some systems, and that they

may have potential for use in restoration projects.Restoration ecologists and land managers should takefacilitation effects into account, not only because the roleof facilitator species is key in restoring the characteristicsand functions of the original system (Bruno et al. 2003),but also because facilitation is believed to drive succes-sion in many habitats, particularly at disturbed sites

(Walker and del Moral 2003).Additional experiments, conducted under a variety of

environmental conditions and using different nurse plantspecies, are needed to identify the potential of this processand to encourage long-term monitoring of targetnurseplant interactions. Research aimed at determining thenurse species zones of influence and their effects onneighboring plants under differing conditions of resourceavailability, will provide us with a valuable technique forimproving the success of restoration projects.

Acknowledgements

This work was funded by the Spanish Ministry of Scienceand Technology (grant AGL2000-0159-P4-02). We thankSerfosur SL for assistance during this project. FM Padilla wassupported by a PhD grant within the CSIC-I3P Program(supported by the European Social Fund). JS Gray madehelpful comments on an earlier draft of this manuscript.

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Padilla, F. & Pugnaire, F. 2006. The Role of Nurse Plants in the Restoration of Degraded Environments