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Professor of Botany. University of Navarra
If you are accustomed to frequenting the bush, you may have noticed an increase in native forests and shrublands, and a reduction in herbaceous communities such as grasses. This process, called scrubThe consequences are especially important in arid and semi-arid ecosystems, as described above. in the western United Statesthe semi-arid forest of Australia, southern Africa, or the in the Mediterranean basin.
The predominance of this subject of vegetation can affect the soil and the development of other species and increase the risk of fires.
Causes of the increase in thickets
At a global level, the causes that trigger thickening in one ecosystem cannot explain it in others. Moreover, it is the result of multiple factors that interact in complex ways at different spatial and temporal scales.
programs of study The main causes of scrubland degradation, such as overgrazing, the cessation of agricultural or livestock activities, increased carbon dioxide and nitrogen deposition, the presence of exotic plants, and rising temperatures due to climate change, are described in a number of studies conducted in different ecosystems around the world.
Focusing on Spain, during the second half of the 20th century there was a significant exodus of the population from rural areas to industrialized cities. As a consequence, agricultural and livestock activities were drastically reduced, and with it, grazing and the use of fire to keep woody species at bay.
Thinning is particularly accentuated in the mountainous regions, due to the great reduction of livestock in mountain pastures and the decrease in agricultural activities in the valleys.
Consequences for biodiversity
Generally speaking, scrubland increases the carbon, nitrogen and acidity content of the soil, while reducing the cover of herbaceous plants. However, the consequences on ecosystems and, consequently, on the goods and services they provide, are highly variable and even contradictory.
While widespread degradation has been shown in semi-arid scrub-shrub ecosystems in the southwestern United States, positive effects of shrublands have been detected in the Stipa tenacissima-dominated esparto grasslands of southeastern Spain.
The negative effect of scrubland on former extensive grazing areas is more evident. In Mediterranean Europe, a reduction in the area of pastures, their productivity and diversity has been detected. As a consequence, areas of scrub and woodland are increasing, homogenizing the landscape and reducing diversity at local and regional scales. It also increases the biomass and continuity of the forest mass, providing a fuel that under conditions of drought and strong winds increases the probability of fire risk.
Some shrub species exert a special influence on the advance of scrub and its consequences in the western and central Pyrenees. Among them, the ollaga (Genista scorpius), the otabera (Genista hispanica subsp. occidentalis), the blackthorn (Prunus spinosa) or the hedgehog (Echinospartum horridum) stand out.
The erizon is one of the most important species in the reduction of pastures in the central Pyrenees. This species forms large and dense patches that can cover large areas where only a few species are able to survive in small clearings. Its expansion speed is very fast, with speeds of about 2 meters per year, as has been quantified in the pastures of the Ordesa and Monte Perdido National Park.
Can we control it?
It does not seem feasible to try to recover all the former grazing areas, but only the most productive areas for livestock or those that could have the greatest consequences in case of fire. The combination of grazing, mechanical clearing and controlled burning seems to be the most appropriate way to stop the colonization of trees and shrubs.
In the south and west of the Pyrenees, programs of study recent studies recommend incorporating intensive grazing after the use of fire or mechanical clearing, because it accelerates nutrient cycling.
Although the process of scrub and forest encroachment continues in our forests, the combination of these techniques helps to maintain areas of pastureland that are key to generating heterogeneity in the landscape, an aspect that is core topic for reducing the risk of fire and maintaining biodiversity.