History
Contenido
El cambio de uso de suelo (o cambio de uso de la tierra) es posiblemente uno de los factores antropogénicos más antiguos que han tenido impacto en el medio ambiente.[4] De hecho, aproximadamente la mitad de la superficie del planeta tierra ha sido alterada a causa de las acciones humanas.
Se estima que durante el período 1700-1990, 1.206 millones de hectáreas fueron convertidas en terrenos agrícolas") y 2.927 millones fueron transformadas en áreas de pastoreo. Por su parte, Ramankutty & Foley (1999) estimaron que, entre 1700 y 1992, 1,621 millones de hectáreas fueron habilitadas para la agricultura de las cuales 885 millones correspondían a bosque, 565 millones a sabana/ praderas/estepa, 150 millones a matorrales y 21 millones a tundras/desiertos[5]”.
El cambio de uso de suelo ha sido un proceso progresivo a través del tiempo, pero a las vez que se ha realizado de manera rápida e intensa. A nivel global, para el año 2015, un 12% de la tierra se utilizaba para el cultivo de regadío, el 37% para pastoreo (19% extensivo y 2% intensivo) y 16% sabanas y tierras arbustivas, 2% plantaciones forestales"), 20% de los bosques para extraer madera y otros usos, 7% ecosistemas no forestales con un uso humano mínimo, 9% bosques intactos o primarios y 12% otras tierras.[6] De los datos antes mencionados, es posible observar que solo el 28% se podría decir que no se había “alterado”.
En Chile, se cree que los primeros estudios para determinar las “transformaciones antropogénicas en el paisaje” se realizaron a fines de la década de los 70 en la zona central del país. Posteriormente, en los próximos años, durante la década de los 80, se dieron a lugar nuevas investigaciones que colaboraron en el estudio del cambio de uso de suelo, incorporando ahora también en la zona de estudio, además de la zona centro, a la zona sur del país. Y, en la década de los 90, con la ayuda del gobierno por medio de CORFO (Corporación de Fomento de la Producción) y CONAMA (Comisión Nacional del Medio Ambiente), se comienza a “cuantificar los recursos vegetacionales nativos existentes en el país e inicia una serie de monitoreos para detectar los cambios de cobertura[5]”.
Según ciertos estudios se estima que la superficie original de los bosque nativos que había en el país a la llegada de los españoles se ha visto disminuida en más del 50%,[7] es decir, que en comparación del año 1540 con la época actual, se ha perdido más de la mitad del auténtico bosque nativo del territorio chileno. En los años posteriores a 1540, entre 1955 y 2016, la transformación del cambio de uso de suelo en estos lugares fue una realidad, lo que era procedente de un territorio de arboleda nativo fue cambiado (o reemplazado) en un 47% representando las nuevas praderas y matorrales y/o el 40% por plantaciones forestales, todo esto a causa de actividades antrópicas.[7].
En el centro-sur de Chile a fines del siglo , este proceso tuvo sus inicios asociado al desarrollo o “expansión de la frontera agropecuaria que permitió satisfacer tanto las necesidades internas como la creciente demanda externa por productos agrícolas[5]”. El terreno para agricultura y la expansión de las plantaciones forestales fueron, en un comienzo, las principales causas. Hay quienes opinan que esta última mencionada se vio fuertemente impulsada y desarrollada en el área por un subsidio estatal: el Decreto Ley 701 (DL701).
El Decreto de Ley 701, promulgado el 15 de octubre de 1974,[8] impulsado por Fernando Leniz (quien en aquel entonces tenía el cargo de ministro de Economía) y por Julio Ponce (director de CORFO en ese año), con el fin de “impulsar el desarrollo forestal de Chile; para este efecto se establecen incentivos a la actividad forestal[9][10]”. Sin embargo, hubo una ley antes que dio inicio al desarrollo forestal en el año 1931, la Ley de Bosques. “Dicha norma legal fue el motor de esta actividad, al reconocer el valor de las plantaciones en relación con su aporte de mitigar los procesos erosivos y, por sobre todo, iniciar el fomento estatal a la forestación como negocio a largo plazo[11]”. Cuarenta y tres años después, el 28 de octubre de 1974, se publicó el Decreto Ley 701 sobre Fomento forestal.[8] Posteriormente, en el año 1995, los incentivos fueron aplazados, perdurando por medio de algunas modificaciones, tales como la Ley N°19.561[12] (en el año 1998) y la Ley N° 20.488[13] (en el año 2011) , hasta el 31 de diciembre del 2012.[11].
En Chile, para poder realizar un cambio de uso de suelo se deben seguir cierto protocolo y cumplir con algunos requisitos, el cual va a ser autorizado por parte del ministerio de agricultura y de SEREMI del sector en el cual se quiera realizar la acción.
Los trámites a realizar tienen un costo que es establecido por resolución (Nº 3816 1996 SAG ), y se deben tener ciertos criterios en cuenta para poder otorgar por completo el cambio uso de suelo, en el cual se debe proteger el ecosistema de interés, ya que independiente del proyecto a realizar se puede ver afectada la biodiversidad de la zona, y por último disminuir los efectos secundarios que pueda generar el proyecto.[14].
Measurement
There are different methods for measuring land use change, in which various software such as ArcGis, QGis, Idrisi, Python, among others, are used; for the processing of maps obtained from geographic information sources. Although each of the software is used in a different way, the purpose of each one is the same: to generate a matrix that shows the change in land use in a given sector. Below, a method will be presented taking as reference a measurement carried out with the ArcGis and Idrisi software and general steps that are important to take into account in land use change measurements will be highlighted.
The measurement of land use change over recent years is carried out mostly with remote sensing systems as a source of geographic information. As an example, an important project in this field is the Landsat satellites that have played an important role in collecting images of the Earth's surface and have been operating since 1972.
Remote sensing allows monitoring the Earth's surface through sensors that are installed on balloons, satellites, helicopters, drones, airplanes, among others. These sensors capture energy in the form of electromagnetic radiation that can be emitted or reflected by the Earth's surface depending on whether the energy source is passive or active respectively. In the first case, electromagnetic radiation is emitted in the form of sunlight and in the second, the sensor is the emitter of the radiation and it is reflected by the Earth's surface and then captured by the system. In both scenarios, the sensor will receive radiation according to the different physical characteristics and energy reflection capacity of the different structures of the land cover. After capturing the information, it is stored or sent to a reception system where it will be ready to be interpreted by experts[15] through competent software such as, for example, ArcGis, which among its functions allows delimiting different land use areas. After this stage, the process of creating digital maps ready to be used to solve problems in the scientific community will be completed.
Before starting the measurement, it is important to verify that all the cartographies of the dates studied meet certain criteria, such as: having the same classification system, being on the same scale and that the images have coherence in relation to the area covered, station and sensor.[15].
Once the previous criteria have been verified, the process begins with the help of ArcGis software. The first step to carry out is that both cartographies must be converted to raster format with the intention of generating a new column in the attribute table previously generated by ArcGis, to assign a value to the polygons that contain the different land use codes and that are present in this table, it is vitally important that the values of each land use are the same for both cartographic layers. Then, the information layers converted into raster must be generated, paying special attention to the Pixel size that will be configured for the studied layers and that will be represented in square meters. Next, the procedure will be saved as an image, taking care that both raster maps are composed of the same number of columns and the same number of rows.
Causes
The development, expansion and maintenance of forestry and agricultural activities along with the construction and expansion of urban and/or industrial infrastructure are commonly mentioned as the main causes of land use change. It is evident that those actions generate an immediate change—that is, without intermediate procedures—in the soil, which is why they have been classified as direct causes.
Agricultural activity has used more than a third of the planet's land surface,[3] which translates into around 5,000 mega hectares.[16] Of the land dedicated to agriculture, approximately one third is used for the production of crops for food purposes and the remaining two-thirds are used for the maintenance of meadows and pastures required for grazing.[16] However, mentioning agriculture and livestock as the main direct causes of the Land use change implies encompassing an area that incorporates diverse cultures, companies of all sizes and different methodologies. Therefore, in order not to reflect on the generality, the forms of impact presented by agriculture and livestock activities are described below:
Livestock farming, in the first instance, requires the preparation of the land to operate it as grazing areas and growing forage for livestock. On many occasions, this implies the need to deforest forests or even modify agricultural crops used for the food industry. Its way of manifesting a change in land use includes processes of defoliation, trampling of the land and the return of nutrients through livestock feces and urine.[17].
Agriculture, understood as the “set of techniques and knowledge related to the cultivation or tillage of the land[18]”, has generated changes in land use from the need to increase production. This entails using methods to improve production yield, increase crop intensity (i.e., harvest frequency) and expand the tillage area.[19] Practices that are not sustainable, such as monocultures, imply land degradation, salinization and reduction of agricultural genetic diversity.[20] On the other hand, good agricultural techniques allow for the enhancement of ecosystem benefits such as carbon storage and the conservation of rural landscapes and their biodiversity.
Forest development is understood as the certainty of conservation and use of the diversity of natural resources that it can give us. Forest management produces different impacts, depending on the interest of a given forest area; either the preservation of species within the area or the planting of species that will be exploited to obtain natural resources (such as wood, resin, among other natural products). Due to the different impacts obtained by forest management, during the years 2001 to 2015 it could be said that 3.5 to 6.5 million hectares per year were lost worldwide, which has generated a great loss of ecosystem services such as carbon sequestration.[7].
Given the high productivity rate, that is, due to the harvesting or felling of forest trees, a change in land use can be generated since the trees have the action of protecting the soil from erosion as well as from dehydration. Therefore, when they are cut down, the soil is exposed with a high probability of being eroded.[21].
Consequences
The change in land use has consequences within the ecosystem, whether at a global or local level. However, it is worth mentioning that this is presented as a multidimensional problem, which implies consequences in both environmental and social and economic aspects. When carried out for agricultural, livestock or industrial purposes, it can have different consequences. Both positive and negative are distinguished.
One of the positive consequences of the change in land use is that the irreversible loss of agricultural soils with the highest productive potential is avoided, since the installations of activities other than forestry and livestock are regulated because it is one of the sectors with the highest productivity, which is why the installation of non-agricultural activities on the soils of classes with lower production is favored.
Within agricultural activities, described in the Direct Causes item, good management of these is capable of keeping the ecosystem stable or favoring its functionality. Livestock farming, for example, allows the return of nutrients from livestock feces and urine.[24] On the part of agriculture, the maintenance of vegetation under good management contributes to the storage of carbon and the conservation of rural landscapes and their biodiversity.
Land use change is one of the direct drivers with the greatest impact worldwide in the face of global change in nature in the last 50 years, being the main driver that has triggered crises in terrestrial ecosystems.[3] These consequences can be described as: loss of biodiversity, soil degradation and carbon loss.
This is one of the most important consequences. This can generate the fragmentation of habitats and the extinction of species, be it flora or fauna of the area in which a change in land use is made, and environmental services, also without forgetting that it leaves ecosystems and people more vulnerable to any natural disaster (this could be cited with the source of the land in figures).
Land use change is one of the actions that could be said to have the greatest impact on soil degradation. It is an anthropic process that causes the loss of biodiversity, productivity and structure of forests, and that largely causes the emission of carbon dioxide into the environment, thus being direct consequences.
Another direct consequence that has not been mentioned and is of great importance is the loss of carbon which has been determined through research[6] which is a 53% to 58% loss that is produced by the change in land use which influences deforestation,[25] this is caused by the degradation of peatlands which store up to ⅓ of the carbon which is contained in the soil.
Just as there are direct consequences, “indirect consequences” are also highlighted. These are changes in soil properties that produce favorable conditions for the direct negative consequences described in the previous item and are related to the change in soil properties, whose impacts can transform both the surface layer of the earth and its other horizons;[26] depending on the magnitude of the cause (degradation, erosion, among others).
Causes and consequences in Chile
In Chile, the native forest has been modified by anthropogenic activities; according to official figures, it is believed that in the period from 1995 to 2016 alone, most of it has been replaced, by 47%, by grasslands and bushes or replaced, by 40%, by forest plantations, followed by approximately 6% by land for agricultural use.[7].
Among the main human-driven causes are real estate projects, the expansion of agricultural land, the extraction of minerals, the growth of the forestry industry and last but not least, are illegal logging, which results in both the degradation of native forests and the change to other land uses, such as bushland or agriculture.[7] All of these have resulted in the loss or reduction of forests in Chile, which in turn has harmed "services." ecosystems that they offer, which, in addition to generating damage at an ecological level, have caused a great negative impact at a sociocultural level in the country [7].”
Due, in large part, to the plantations of exotic species, urban expansion and expansion of the agricultural frontier, eleven ecosystems have been distinguished so far that are in the situation of "threat in the Mediterranean biome", specifically in the central zone of Chile.[7] This highlights the consequences that are being generated with events closely associated with the change in land use, which should not be taken lightly, since an "example of the final result of the threat process of an ecosystem is the collapse of the Aculeo lagoon, which generated the complete disappearance of the ecosystem due to the synergistic effects of climate change and poor territorial management[7].”
In the central-southern area of the country alone, more than 130,000 hectares of native forest have been supplanted by forest plantations. In fact, it is believed that between 1974 and 1992 more than 200 hectares of the native forest of Chile were replaced. This is reflected in scientific evidence, since in the regions of Maule and Biobío, between 1978 and 1987, 48,000 hectares of this type of forest were replaced by plantations of radiata pine[5] (pinus insigne), a species of Californian origin (therefore, an exotic species in Chile), which due to its rapid growth is highly used for the use of its wood (FAO. “El pino de Monterrey as an exotic species.” Unasylva, vol. 14, no. 1, 1960, https://www.fao.org/3/x5394s/x5394s03.htm .). And, in subsequent years, in the period 1975-2000, it was reported that between the Maule and Cobquecura rivers, approximately 80 hectares of native forest had been lost, being replaced by forest plantations.[5].
As a result of the aforementioned, in south-central Chile “the remaining native forest areas are currently located in sectors with difficult access, steep slopes, altitudes above 800 meters, or inside protected wild areas[5].” Therefore, it can be seen that biodiversity has been affected and decreased, that is, biodiversity has been lost in Chilean territory, with this area of Chile being known and considered by many “as one of the areas with the greatest biodiversity and endemism in the world[27]”, with its habitat area reduced.