After a millenarian history of overexploitation, most forests in the Mediterranean Basin have disappeared, leaving many degraded landscapes that have been colonized by early successional shrub-dominated communities. Common reforestation techniques see such shrub communities as competitors against newly planted trees, so such shrubs are commonly cleared before tree planting. However, empirical studies suggest that in stress-prone Mediterranean environments, shrubs have a net positive effect on recruitment of other speces. Between 1997 and 2001, this project examined a series of experimental reforestations in the Sierra Nevada Protected Area in southeast Spain aimed at comparing the survival and growth of seedlings planted in open areas and under the canopy of preexisting shrubs species. Over 18,000 seedlings of 11 woody species were planted under 16 different nurse shrubs throughout a broad geographical area, looking to evaluate the effect on the recruitment of other species. The project found that there were differences in the magnitude of interaction between reforestation success and critical periods for planting as well as the nurse effects. The project found that pioneer shrubs facilitated the establishment of woody, late-successional Mediterranean species, which can positively affect restoration success in many different ecological settings.
Sierra Nevada Protected Area, Spain, 37.0924307, -3.162057799999957
Country or Territory:
Temperate Forest - Mixed
Area being restored:
Primary Causes of DegradationAgriculture & Livestock, Fragmentation, Invasive Species (native or non-native pests, pathogens or plants)
This region of southern Spain has seen continual human inhabitation and use for thousands of years, it is difficult to assess or ascribe direct cause.
Reference Ecosystem Description
The historic forest would be difficult to discern with any precision. The combination of long histories of inhabitation and the remnant vegetation indicate some patterns. There are pine trees (Pinus sylvestris), savin and common junipers, while in the lower valleys are gall oaks (Quercus faginaea) and native maples, prickly junipers, and many other forbs and grasses. Along the watercourses are alder, ash, elms, willows, and poplars.
The project south to use the broad range of abiotic and biotic conditions of the Sierra Nevada mountains to consider both variability in space as well as in time, in order to understand the nature and strength of plant-plant interactions related to woody plant establishment. Specifically, the project addressed the following questions: 1) How doe sthe use of shrubs as microsites for planting improve seedling survival and growth?; 2) How does the effect of shrubs on seedling survival and growth vary depending on the shrub and tree species?; 3) How do the sign and magnitude of the interaction between shrubs and woody seedlings depend on spatial characteristics of the study site, such as altitude and aspect?; 4) How do sign and magnitude of the interaction between shrubs and woody seedlings depend on climatic conditions in the year of planting?
The project does not have a monitoring plan.
The loss of primeval forests through a millenarian history of overexploitation in the Mediterranean Basin has left no more than 9-20% of the entire area forested, while on the Iberian Peninsula only .2% is considered natural or seminatural. Simultaneously, the surface area covered by shrublands has increased, representing a stage of degradation of mature forests as well as stages of vegetation recovery in abandoned agricultural lands. In both instances, local and regional characteristics, such as resource availability or the lack of tree propagules, act as barriers to succession and result in self-perpetuating systems that do no return to the structure and complexity of the original mature community. This necessitates some form of human intervention to assist secondary succession at the shrubland or even grassland stage to accelerate restoration of woodlands.
Description of Project Activities:
The project was conducted at seven sites in the Sierra Nevada mountains, for a total of 36 1-ha plots. Project managers planted seedlings of the following target shrub and tree species: Crataegus monogyna, Rhamnus alaternus, Retama sphaerocarpa, Quercus faginea, Q. ilex, Q. pyrenaica, Pinus halepensis, P. nigra, P. sylvestris var. nevadensis and Acer opalus ssp. granatense. These species are either commonly found in natural forests in Mediterranean climates or are endemic species of interest in conservation. One to two year old seedlings were planted in spring (March-April) between 1997-2001 at each study site using two restoration techniques: 1) a traditional technique of planting target seedlings in open interspaces without vegetation (open microsites) and 2) an alternative technique of planting seedlings under the canopy of shrubs intermingled with the open microsites. Seedlings were grown in nurseries under similar conditions, with the most abundant shrub species at each site chosen as nurse plants. In total, the project used 16 nurse plant species and 11 target species for a total of 146 different plot-nurse shrub-target species combinations. Between 50 and 60 individually tagged seedlings were planted per plot-nurse shrub-target species combination. An automatic auger 12 cm in diameter was used to dig the planting holes 40 cm deep in an attempt to minimize disturbance to nurse and soil structure. In the autumn following transplant (after the first drought period) two characteristics per seedling were recorded: 1) survival and the cause of mortality in the case of death; and 2) growth, quantified as the elongation of the apical shoot after the first growing season.
Ecological Outcomes Achieved
Eliminate existing threats to the ecosystem:
In 109 of the 146 experimental cases, shrubs increased seedling survival. The results overall demonstrated a generally positive correlation between woody seedling survival and the use of pioneer shrubs. Seedling survival under shrubs more than doubled in comparison to open microsites and even up to fourfold in some experimental cases. The results found much agreement with the hypothesis that there is little competition between shrubs and tree seedlings in the Mediterranean Basin. The experiment showed that facilitation between shrubs and tree seedlings in Mediterranean environments is not a local or sporadic phenomenon restricted to a few species assemblages and environmental conditions, but is in fact a widespread phenomenon. From an overall conceptual standpoint, the results clearly showed that pioneer shrubs benefit the establishment of woody, late-successional species according to the model of succession by facilitation. The project found it was clear that the planting of late-successional shrubs under primary successional shrubs was the most effective way of accelerating succession in degraded sites were direct recovery of the tree cover can be very difficult, if not impossible. There were also differences between shrubs in their facilitative effect, where the magnitude and sign of the nurse effect on seedling survival varied from large and positive for legumes and small shrubs, through medium and positive for deciduous spiny shrubs, to large and negative for rockroses. Another positive correlation is that the excessive solar radiation present in the Mediterrancean environments is reduced in comparison to open areas when evaluating the shrub canopy shade that favors seedling performance. The micro-climate under the canopy of the nurses is a main facilitative mechanism of woody seedling establishment.
Factors limiting recovery of the ecosystem:
There were differential effects in nurse shrubs that appear to be related to the characteristics of the functional groups. Legumes may increase survival and growth by improving soil nutrient composition due to nitrogen fixation, which is a scarce nutrient in Mediterranean soils. The evaluation of functional groups is critical in the identification of target species because results showed that the identity of the nurse matters. Another factor in the success of this project in Mediterranean environments was the availability of water, which lessened the negative effects that drought had on the survivability of the seedlings.
Socio-Economic & Community Outcomes Achieved
Economic vitality and local livelihoods:
The results of this project clearly showed that the use of nurse shrubs facilitates seedling establishment in many different ecological settings in the Mediterranean mountains. Although the project monitored one-year survival, the benefit of planting seedlings under shrubs could be translated beyond this stage to sapling or reproductive stages, since natural regeneration in Mediterranean ecosystems is mainly limited at the seedling stage and experimental reforestations reveal the bottleneck of first-summer mortality. Since most shrub species studied acted as nurses and most planted species were effectively facilitated, this technique could be used to design multispecific reforestation programs. Since the response of shrub seedlings to the presence of a nurse plant was larger than the response of tree seedlings, this technique could be used to design a two-phase reforestation strategy, mimicking the natural process.
This project established clearly that the removal of shrubs is not appropriate for reforestation in Mediterranean mountains. The ecological reality of Mediterranean forests revealed that the natural spatial patterns of regeneration in woody vegetation (specifically the use of microsites for recruitment) are a major determinant of spatial distribution of woody seedlings. The consequence of this has clear implications for climate change. Given that the facilitative effect increases with abiotic stress, this technique might be more relevant under the predicted rise in temperatures, dryness, and rainfall variability for the Mediterranean region, the benefits of this technique are clear for the future challenges suggested under various scenarios of climate change.
Sources and Amounts of Funding
The project was supported by funding from the Spanish government. It also utilized labor for planting and the seedlings themselves for a local NGO.
Gomez-Aparicio, Lorena et al. 2004. Applying plant faciliatation to forest restoration: A meta-analysis of the use of shrubs as nurse plants. Ecological Applications 14(4): 1128-1138.