LEDD issues in forests & shrubland: Spain, Portugal and Baixo Guadiana
Authors: Michiel Curfs, Anton ImesonEditors note: Text source D311 section 3.1.3
Land desertification
The main function of 88 percent of Spanish forests is to protect against soil erosion and desertification, and to regulate the hydrological cycle (FAO 2011). Dehesa and montado forests protect the soil from the drying effects of wind and solar radiation (shadow and windblocks). This type of forest also facilitates the condensation of fog (natural fog harvesting effect). Deep root systems extract nutrients from the lower layers of soil and through leaf litter, return these nutrients to soil surface horizons. Furthermore, litter works as mulch, shading the soil and helping to create favourable moisture conditions which protect the soil from erosion. Livestock manure is also introduced to the system via agroforestry practices (Olea and San Miguel-Ayanz 2006). This increases soil organic matter and makes the soil less vulnerable to erosion.
Just as in other areas, however, Dehesa and Montado forests are degrading into shrublands with thinner and hotter soils. As the forest canopy becomes more open, there is positive feedback which leads to more runoff, less infiltration and degradation of the hydrological cycle and other ecosystem functions and services. As degradation increases, there is an associated progressive decline in natural and economic capital. This general degradation of the forests is exacerbated by specific issues that include inappropriate soil conservation issues (poor construction of terraces), introduction of exotic tree species such as eucalypts that produce toxic substances in the soil, local contamination from mining and a drop in groundwater levels.
Forest productivity decline
The 12 percent of forests in Spain that are used for production purposes suffer from many degradation issues. Issues include the use of floodplains for growing trees that use phreatophytic water, so that less water is available for other river functions. A major issue in many forests in Spain and Portugal is the use of bulldozers and heavy machinery and as a result the soil profile is inverted, placing sensitive soil on the surface. This issue is present in a number of regions in Spain. This issue occurs where the main source of income is subsidies for planting trees on land that has been burned or used for other agri-environmental schemes. Trees fail to grow because of the way they are planted and managed, and entire landscape eventually becomes desertified. The land loses its natural soil and water regulation function because of soil compaction from heavy machinery.
Loss of biodiversity
The loss of biodiversity in relation to plant species is discussed below, as most of the key LEDD issues and their drivers impact directly on biodiversity. For example, it is clear that forest fires lead to biodiversity loss, particularly in the short term. Fragmentation and soil deterioration are also directly linked to biodiversity loss (COST 341 2003). A special emphasis must be given to indirect loss of biodiversity in dehesa and montado systems. These systems are an important habitat for a number of endangered animal and bird species, such as the Spanish imperial eagle (Aquila adalberti) and the Iberian Lynx (Lynx pardinus). As the area of dehesa and montado declines, so does the available habitat for these threatened species (Olea and San Miguel-Ayanz 2006).
Forest fragmentation
As reported in the COST 341 report ‘Wildlife and traffic’ (2003), one of the major impacts of infrastructure development is habitat fragmentation and associated biodiversity loss. Transportation, agriculture and urbanisation are three key causes of fragmentation. Spain and Portugal have seen a significant expansion of the road network over the past 20 years, with a 640 percent increase in its road network (Eurostat 2009). Urban expansion is also causing significant forest fragmentation in Portugal and Spain (MAOTDR 2007).
Most dehesa and montado forests are located in rural areas with low populations, or in mountainous areas used for hunting grounds. In Portugal, ecological corridors or green bands are being created in order to prevent forest ecosystem fragmentation. In the Spanish part of the Baixo Guadiana study area, solutions are being sought to enable these green corridors to be expanded from Portugal into Spain.
Soil erosion
In the Baixo Guadiana area, soils in general are shallow. Dehesa and montado land use management includes ploughing between trees. This happens, in general, twice per year and causes accelerated (tillage) erosion. The main reason for ploughing is to clear undergrowth as a fire prevention tool. This leads to excessive erosion rates in the form of sheet, wind, rill and gully erosion and soil deterioration. On the Spanish side of the study area, landowners are held responsible for any fires starting on their land. As a result, preventative ploughing is deeply entrenched as a cultural activity, to prevent severe fires.
Soil sealing
In Portugal, there is a generalised and increasing trend in soil sealing. Between 2000 and 2006, soil sealing increased by 10 percent, mainly as a result of dispersed urbanisation. An important issue is the impact of surfaced and unsurfaced roads on surface water drainage. Hard surfaces lead to rapid runoff and the loss of the water that trees require. Rapid runoff also leads to soil erosion and the sedimentation of water courses, which can contribute to flooding downstream.
The forest can also provide a protection function (See Doran et al. 2005) which protects people from rockfall and landslides. The disturbance of the forest because of road building or agriculture leads to the loss of this function.
Water stress and phytosanitary deterioration of forest cover
A recent study by Garcia et al. (2003) showed that all Mediterranean forests suffer from a permanent shortage of water. Large areas of cork and holm oak mortality have been observed in Spain in recent years. Early symptoms of impending mortality range from progressive signs of defoliation with partial or total dead branches to rapid death in a short period of time (sudden oak death or ´seca´). Surviving trees appear unhealthy for a considerable length of time. Mortality occurs in pockets with no apparent geographic patterns.
At the Evora conference in 2006 on the vitality of cork and holm oak stands and forests, it was stated that in Portugal, data suggests that the area of cork and holm oak (montados) increased between 1902 and 1956, from approximately 800,000 ha to 1,200,000 ha following a relatively stable period (with cork oak areas occupying a slightly larger area than holm oaks). Analysis of the health of these areas in Portugal suggests that area and tree densities are in relatively fair condition but health status, such as levels of defoliation at the plot level, suggest that a high proportion of plots may be in poor condition.
Among the main causes of mortality, pests, fungi and water stresses were found to be the most critical. Pests such as Phytophthora spp. occur in association with all Quercus species and are perhaps an important factor of cork and holm oak decline. This process is called ´seca´ in Spain (Figure 1). Oak endophytic fungi can act as pathogens to their host.
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Figure 1. La ‘Seca’ affecting holm oaks in the Baixo Guadiana. Source: (Author Michiel Curfs). |
The lower Guadiana basin, represents a typical semi-arid region where human activity and modification of the hydrological regime over previous decades have led to increasing water scarcity and the identification of water shortage as a ‘structural characteristic’ of the system. Future climate change will act to amplify existing water stress, with important consequences for the availability and distribution of water between different land uses (ADAM 2009). Water stress is related to irregular rainfall patterns, which are predicted to worsen on the Iberian Peninsula as a result of climate change scenarios. The decline of forest productivity of the montado and dehesa forest type is likely to be negatively affected as a result of changing climatic conditions and biological pest attack.
Socio-economic issues
Economic and social issues that affect the forest include reductions in the value of forest products such as cork, and the labour and knowledge needed to produce it. Crops that were sustainable and integrated into the economy and social structure are replaced by eucalyptus for cellulose production, which provides some labour but which degrades the hydrological functions of the soils.