Spain: Quercus Restoration on Abandoned Agricultural Fields

Project Stage:
Completed

Start Date:
2001-02-01

End Date:
2003-12-31

Primary Causes of Degradation

Degradation Description

Because of declining productivity of the land, intentional setting aside of old cropland for reserve lands, and the movement away from strictly agrarian livelihood in this part of Spain has contributed to many of these lands being abandoned. While succession on these lands occurs, this project sought to evaluate how forest species that were known historically in the oak forests of this part of Spain might be utilized to restore the forests.

Reference Ecosystem Description

This part of central Spain has been cultivated for hundreds if not thousands of years. While there are several biosphere reserves in the area, even these ecosystems are not without the influence of human beings. The granitic mountains and rugged landscapes to the northeast of Madrid give rise to holm oak, oak and ash forests. These woods alternate with pasturelands used for grazing and cropped land that is the traditional use of the land.

Project Goals

The objective of the project was to assess the combined effects of radiation load and weed competition on the performance of these Quercus species in order to suggest management practices that optimize investment in active restoration of abandoned farmland in central Spain. The study was notable because it was relatively long (3 years) under field conditions. The project tested several hypotheses: first, that shading and mowing would increase water availability to the planted seedlings by decreasing evaporation from soil and transpiration from weeds; second, that the environments with lower radiation and water stress will show little and sustained seedling mortality through time. Similarly, the most stressful environments will show a high mortality that is concentrated in the earliest stages of seedling establishment. The project sought to ask what environments are the most stressful by the seedlings of each species in the study; third, that mortality differences among species under stressful conditions will correspond with ecological soil water preferences shown by the species distribution in the field and the amelioration of the environmental harshness will dilute or eliminate the differences among species; fourth, the project expected that stress release would chiefly favor the performance of the most mesic species (Q. faginea) because environmental conditions will become more similar to its threshold of stress tolerance with respect to the other two species.

Monitoring

The project does not have a monitoring plan.

Stakeholders

Large areas of cropland have been abandoned in recent years in this area of Spain due to productivity loss, social changes, and also set-aside programs. These deforested areas can be left to undergo secondary succession or passive restoration; or they can be actively restored by planting and managing native shrubs and trees to reduce soil erosion, increase biological diversity and create carbon sinks. The environmental conditions of these areas usually differ from those where natural regeneration of shrubs and trees occur because different abiotic and biotic factors hinder the establishment of the introduced woody seedlings. Another factor influencing this project is that revegetation projects usually require an appropriate management plan which is important because large amounts of public and private funds are being invested in cropland reforestation, making this project an important component in regional restoration efforts.

Description of Project Activities:
The experiment was laid out in a random plot design. The four treatments resulted from the factorial combination of artificial shading (shaded versus full-light plots) and weed mowing (mowed versus weed plots). There were four replicate plots per treatment (16 plots in total). Size of each plot was 7.5 m x 7.5 m. The planted seedlings were cultivated from acorns collected in localities climatically similar to the experimental area to minimize the variation in genetic composition. Thirty 1-year-old seedlings, 10 per species, were planted with a regular distribution in each of the 16 plots, being separated from each other by at least 1 m. The plantation scheme followed always a constant species sequence to avoid that two seedlings of the same species were adjacently planted. The seedlings were planted with their 5-cm diameter and 18-cm deep plugs. They were protected with a plastic mesh of 1 cm x 1 cm. Seedling mortality within the first month of the experiment was attributed to transplanting problems, and these dead seedlings were replaced. Seedlings were randomly distributed among treatment plots in the experiment. Some of the main species of the weed community were Tragopogon pratensis, Sylibum marianum, Fumaria officinales, Medicago sativa, Hordeum muralis, Dactylis glomerata and Chenopodium album. Shading was achieved with a neutral shade cloth placed 1.9 m above the ground to cover the entire plot area, and three additional net pieces on all plot orientations except northwards that were placed 0.5 m above the ground to avoid border effects. The current reduction in radiation is not limiting for plant growth even for exemplary heliophillous species such as Retama sphaerocarpa. Weeds were mowed eight times during the 3 years of the experiment, mainly in spring, to continuously reduce the competition on planted seedlings. Weed height was always maintained lower than seedling height.

Ecological Outcomes Achieved

Eliminate existing threats to the ecosystem:
Overall, artificial shading and weed mowing, two techniques intended to facilitate the establishment of native Quercus species in abandoned Mediterranean cropland, ameliorated the environmental harshness of the treated plots and improved oak seedling performance. The results of the experiment are further evidence that high radiation and weed competition limit tree establishment in this type of environment because they reduce water availability to the seedlings. This holds true even for species that are very characteristic of dry sites such as Q. coccifera. Shading was found to reduce the incident photosynthetic active radiation (PAR) that reached the seedlings, while mowing increased it during the dry season. Weeds provided a natural shad equivalent to one third of the artificial shade that was used and might be beneficial for the introduced seedlings because of the reduction of photo-inhibition damage. Shading and mowing reduced effective precipitation, but increased soil moisture in the dry season. This occurred because shading reduced evaporation, but did not significantly increase weed production and the reduction in weed transpiration due to mowing appeared to outweigh the increase in evaporation due to the lack of shading by weeds. Additionally, the increase in soil moisture was found to be more prominent in deeper soil layers where water was more accessible to seedlings rather than the weeds. Shading and mowing also increased seedling performance, but there were differences between treatments and species. Weed competition reduced more survival and resprouting capacity than high radiation, suggesting that water shortage is an important limiting factor in this habitat. Survival under stressful conditions was similarly low for all species, which means that the environmental harshness imposed by full-light or weed competition was usually above the capacity of these species to survive. The performance between species under the most favorable conditions was inversely related to their soil water preferences. The results highlight the importance of the first growth season in the recruitment of woody plants.

Factors limiting recovery of the ecosystem:
There was evidence that water storage is an important limiting factor in this type of habitat. Additionally, while there were differences between species in survival rates because of different growth volumes in some species. It was also shown that other Mediterranean species and their capacity to withstand severe drought decrease in the shade due to increased below-ground competition for water with established plants. The first dry season was seen as a bottle-neck for the survival of the introduced seedlings, and close management decreased mortality up to five times for the three species during the first year. Weed competition limited seedling survival in all species more than high radiation. The relative importance of these factors in limiting growth depended on the growth measure and species but, overall, shading increased growth more than mowing.

Socio-Economic & Community Outcomes Achieved

Economic vitality and local livelihoods:
Across the world, cropland is undergoing abandonment for a number of reasons. Developing active restoration strategies helps to mitigate the changed abiotic and biotic conditions that result from years of active cultivation. This project demonstrated that active restoration is feasible and desirable and when combined with other strategies can provide an effective approach to cropland restoration.

Long-Term Management

There is a clear synergetic effect of shading and mowing. For management of Quercus plantations, it is important that the introduced seedlings of these species that advantage of a low competition environment from weeds before the first dry season. Once seedlings have been established, an artificially shaded environment would provide benefits in terms of plot cover by the oaks, the actual target pursued in this type of reforestations, particularly for the most mesic species. While management is expensive and concentration of management reduces investment, for instance shading may be effective and practical in a nursery setting but it is far too expensive to use on a large scale. This approach is feasible for creating little reforested patches in vast deforested agricultural regions. Plots may then act as sources of propagules of Quercus species, thus aiding their later natural establishment. Similar approaches could be tried for other species and/or regions where high radiation and/or herb competition limit the establishment of natural woody vegetation.

Sources and Amounts of Funding

This project was funded by the grand projects REN2000-745 and CGL2004-355-BOS (CICYT, Ministerio de Educación y Ciencia, Spain) and GR/AMB/0757/2004 (Comunidad de Madrid).

Other Resources

Rey Benayas, Jose M. et al. 2005. Effects of artificial shading and weed mowing in reforestation of Mediterranean abandoned cropland with contrasting Quercus species. Forest Ecology and Management, 212: 302-314.

Primary Contact

Organizational Contact