Spain: Restoration of Quercus pyrenaica Forests,Southern Spain

Project Stage:
Completed

Start Date:
1998-01-01

End Date:
2003-12-31

Primary Causes of Degradation

Degradation Description

Quercus pyrenaica (Melojo oak) is an abundant deciduous species of the western Mediterranean basin, extending through western France, the Iberian Peninsula and northern Morocco. Q. pyrenaica is a major tree in Mediterranean silviculture, which through historic human use and degradation accounts for the serious degradation of the forests seen today. Following disturbance in these ecosystems, they are often replaced by sclerophyllous Quercus species such as Q. ilex and Q. suber, which are better adapted to the poorer soils and drier conditions once the original forest has disappeared. Intensive human intervention is probably responsible for the complete replacement of Q. pyrenaica forests in present times, which is particularly relevant for populations that are locate along the southern boundary of the species distribution area, primarily found in southern Spain. Because of the excellent soil found beneath the canopy of Q. pyrenaica these areas which were cut over were often turned to agricultural uses immediately thereafter, with considerable livestock grazing as one agricultural use. The specific project site, however, has not been grazed since 1997 when the area was declared a National Park.

Reference Ecosystem Description

Quercus pyrenaica forests occur mostly between 400 and 1600 m, rising above 1800 m in southerly locales, and require between 650 and 1200 mm annual precipitation. The species grows predominantly in siliceous soils, but forests are also found on calcareous soils in localities where precipitation is high enough to allow a soil pH to fall close to a neutral value. The species resprouts abundantly from a dense net of shallow roots, which allow rapid regeneration after disturbances and makes it a suitable tree species for copping and preventing erosion.

Project Goals

Three specific goals and/or questions guided the project: 1) What is the effect of shrubs on survival and growth of reforested seedlings?, 2) Are the patterns of survival and growth coupled?, and 3) What is the most critical period fore the establishment of planted seedlings?

Monitoring

The project does not have a monitoring plan.

Stakeholders

The project itself was conceived because natural regeneration of Q. pyrenaica forests are constrained by several factors, particularly in its southern locations. First, the species reproduces mostly by resprouting and acorn production is low and scattered. Second, acorns, when available, suffer extremely high rates of predation by several vertebrates, reaching losses that prevent regeneration. In either case, acorns are highly predated even after sowing and so the most common procedure for reforestation is the planting of seedlings grown in nurseries. However, plantations of Q. pyrenaica suffer heavy losses, primarily from summer drought, that destroy restoration efforts. Such losses have been found to hold true for other species of Quercus and other reforested trees in the Mediterranean area, which are mostly caused by heavy mortality during the first growing season. The rationale for the study was that, for dry environments such as the Mediterranean, the presence of nurse shrubs may increase establishment success by reducing summer drought stress resulting in facilitation.

Description of Project Activities:
One-year-old Q. pyrenaica seedlings were planted in March 1998 in two treatments (microhabitats): (1) Open: seedlings planted in areas of bare soil and (2) Salvia: seedlings planted under the canopy of Salvia lavandulifolia, which was used as a pioneer shrub. The technique used for planting in the Open microhabitat employed the usual procedure followed in the region, consisting of planting seedlings in open interspaces to avoid proximity to shrub species. The planting holes were dug 40 cm deep with a mechanical augur with a 30-cm diameter bit. This is the augur traditionally used in this type of reforestation in the region, as it provides the greatest hole without hampering its manipulation by workers due to excessive weight. After planting, the soil was manually worked in an area of 0.5 m2 around the plant to improve soil structure and increase water retention. For planting in the Salvia microhabitat, we used a smaller augur bit (12 cm diameter) to minimize damage to the shrub roots, and no further cultivation was done around the plants. Initial conditions for establishment and growth were thus poorer in the Salvia microhabitat due to the lower volume of loose soil and lack of further cultivation around the plants. In all cases, the soil was returned to the holes and firmed around the root collar of the seedling. The Salvia plants chosen as planting sites were between 20 and 35 cm high. No further work was done in the reforestation. Planting points were distributed ca. 200 m apart in three plots of ca. 6000 m2. In each plot, we planted 100 seedlings per microhabitat (600 experimental seedlings in total), sampling points being randomly assigned. All seedlings were tagged and mapped. At the end of June 1998, before the onset of summer drought, Quercus seedlings were examined, and those that had died due to transplant shock (2.6% of planted seedlings) were excluded from the experiment. Seedlings were purchased from a nursery located at 1600 m a.s.l. at Sierra Nevada (Soportújar, Granada), grown in plastic trays (18 cm deep, 4 cm side) filled with a substrate of native soil mixed with organic material. The mean length of the seedlings at planting was ca. 5 cm. The acorns came from local populations of Sierra Nevada National Park. The experiment was monitored until the sixth growing season. For each experimental seedling the following data was recorded: (1) Survival, which was sampled at the end of the growing season (October) in 1998, 1999, 2000 and 2003; (2) Cause of mortality; (3) Length of the longest shoot, measured three (October 2000) and six (October 2003) years after planting; (4) Number of shoots, which was also sampled three and six years after planting. Survival was corrected through the years for those seedlings apparently dead but resprouting the following year (a common phenomenon in this and other Quercus species), so that any resprouting seedling was considered alive. Resprouting of aerial biomass was not further recorded after the fourth growing season, and thus the final survival data analysed (six years after planting) can be considered free of spurious results due to resprouting.

Ecological Outcomes Achieved

Eliminate existing threats to the ecosystem:
Seedling survival differed among treatments. The survival percentage under Salvia was 34.8% after six years (all plots pooled), while in the Open microhabitat only 5.5% of seedlings were still alive. The survival rate differed among plots but in all cases followed the same among-microhabitat pattern, with seedlings planted under Salvia showing higher survival in all the plots (no plot â‹… microhabitat interaction). The highest mortality occurred during the first summer, when 49.1% of the initially established seedlings died. Summer drought was the main cause of mortality (99.2%), with very few seedlings dying from other causes (ungulate trampling and vole tunnels). The results from the project show that the survival of Q. pyrenaica seedlings was 6.3 â‹… higher under Salvia lavandulifolia than in the open microhabitat, despite the fact that the planting conditions were ostensibly more favourable for seedlings planted in the latter microhabitat. This huge difference gains further relevance when considering the percentage of surviving seedlings, as only 5.5% of those planted in the open microhabitat survived after six years in comparison to 34.8% under Salvia lavandulifolia. Furthermore, the patterns of growth and survival were coupled, the length of the leader shoot after six years being 1.8 â‹… higher under Salvia in comparison to open areas. The growth of the seedlings, although low, was in accordance with those reported for Q. pyrenaica even in more favourable areas of its range

Factors limiting recovery of the ecosystem:
The first growing season was the most critical period for survival, accounting for approximately half of the deaths. In addition, the survival pattern across treatments was established during the first growing season, with the rank order of among-microhabitat survival rates remaining similar over consecutive years. This supports the contention that the first year is the most critical period for the establishment of reforested tree seedlings, and confirms the robustness of conclusions drawn from this six-year study. Thus, the results show on one hand that the restoration of these Q. pyrenaica forests was not possible in open interspaces, and on the other hand that it is necessary to employ a technique that ensures a high survival rate during the first growing season. The extremely low values of survival in the open microhabitat might be influenced by the fact that the year of sowing was relatively dry. This magnifies the relevance of shrubs as nurse plants, allowing survival even in relatively adverse years.

Socio-Economic & Community Outcomes Achieved

Economic vitality and local livelihoods:
The mechanisms underlying facilitation in Mediterranean type ecosystems are related primarily to a reduction of radiation intensity during summer, which improves the water status of the seedlings through lowering of soil temperature and conserving soil moisture. Experimental field studies indicate that the shade provided by the canopy of shrubs or artificial meshes is the main factor favouring the survival of Quercus seedlings planted in Mediterranean type ecosystems as well as other water stressed habitats. All this suggests that pioneer shrubs, such as Salvia, characterized by a short, open canopy that moderately reduces radiation during the first years after planting, may potentially ensure reforestation success. Salvia species, as well as other shrubby species with similar morphological characteristics, are common in degraded habitats of the Mediterranean mountains, and thus the use of this reforestation technique could be extensively employed for Q. pyrenaica reforestation. Other possible mechanisms, such as a nutritional improvement of soil under shrubs or a richer soil microbial and fungal communities cannot be ruled out, strengthening the role of shrubs as regeneration niches for planted seedlings. In fact, soil beneath Salvia lavandulifolia has a higher concentration of P and K than in open interspaces. In this sense, it is noticeable that mature Q. pyrenaica forests grow mostly on siliceous soils. The species is thus traditionally considered to require decarbonated soils, although it is also present in calcareous soils. Soil properties lose importance in vegetation dynamics with the maturation of the community, given the buffering effect of organic matter on bed rock characteristics. The use of shrubs could thus be particularly critical for the restoration of these forests on calcareous soils. Furthermore, shrubs may protect Q. pyrenaica seedlings from browsing and trampling of vertebrate herbivores. The facilitative effect of shrubs upon seedling establishment has been similarly reported for other Quercus species around the world in environments characterized by a severe moisture stress during the growing season. More particularly, in accordance with these results, this facilitative interaction has been consistently reported for Mediterranean type ecosystems, as for instance Q. douglasi, Q. lobata and Q. agrifolia in California, Q. ilex in southern Spain, or Q. humilis in grasslands of southern France. This suggests that the use of shrubs as nurse plants to boost reforestation success of the genus Quercus might be applicable to a wide range of sites, at least to those environments characterized by moisture stress during the growing season. In the case of Q. pyrenaica, our results clearly show that the use of shrubs as planting sites may help to recover these climax forests with the potential to cover vast areas of the western Mediterranean region.

Sources and Amounts of Funding

The project was supported by a PFPI-MEC grant to Jorge Castro and projects FEDER 1FD97-0743-CO3-02 and HETEROMED (REN2002-04041-CO2-01/GLO).

Primary Contact

Organizational Contact