Land use and land cover changes, and, more specifically, deforestation, may be the result of a complex set of driving forces, ranging from external (demand from international markets, environmental policies, etc.) to local (population increase, infrastructure development, biophysical variables, etc.) drivers [1
]. Several authors have reported the difficulty of disentangling underlying and proximal driving forces, and often the discussion concerning the causes of deforestation confuses issues across spatial and temporal scales [5
]. Although a consensus has emerged that attributes deforestation to underlying economic, political, and social driving forces acting at the national, regional, and global levels [6
], these drivers alone may be insufficient to understand how deforestation is occurring at the local scale.
According to Geist and Lambin [1
] proximate drivers are human activities at the local and immediate level that directly impact land cover. Agricultural clearing and timber extraction, both authorized and illegal, together with forest wildfires are perhaps the proximate drivers more frequently reported in the literature [1
]. The proximate drivers of deforestation have consistently been associated to population growth, accessibility, topography, and other physical and socio-economic factors that are acting at local scale [4
]. Demographic dynamics have often been identified as an important driver of deforestation, especially in developing tropical countries [2
], while the recovery of forests after crop abandonment (i.e., “forest transition”) has been linked to regional population decline [17
], besides other drivers [20
]. Although, in many regions the relationship between population and land-cover change is straightforward, some studies have shown that the type of change can be modified by socioeconomic and environmental driving forces, independently of population tendencies [1
]. Another driver that is affecting deforestation at local to regional scales is accessibility [12
]. Road construction increases accessibility to remote areas facilitating the expansion of agriculture, logging and deforestation [10
], which leads to the conversion of forests into other cover types. Therefore, roads are often seen as agents of deforestation, accelerating fragmentation, and slowing down the recovery of forests [24
]. Also, physical variables such as topography and precipitation usually play an important role in the deforestation process because of related variations in accessibility, soil properties, temperature, and water availability may promote or prevent changes in land use [27
]. Additionally, government regulations may restrict deforestation above certain altitudes or in steep terrain. Furthermore, it is presumed that those areas with soils suitable for agriculture and a gentle topography permitting tilling, show high rates of forest conversion, when compared to areas with opposite conditions. In this way, it is presumed that sites without limitations for agriculture tend to be more rapidly converted into a more deforested and more fragmented landscape [30
The Gran Chaco is one of the largest seasonally dry subtropical forest in South America, comprising an area of ca. 1,200,000 km2
in Argentina, Paraguay, and Bolivia [31
]. Formerly, an almost continuous forest, during the last decades the Chaco has been converted into agriculture at high rates [33
], and nowadays land cover is dominated by a mosaic of secondary and fragmented forests, shrublands and cultural vegetation, mainly annual crops. Before the Spanish occupation, the Chaco region was covered by primary forests and woodlands, alternating with patches of grasslands that were maintained through traditional management by Amerindians, who used fire in their hunting practices [31
]. This balance between woody and herbaceous vegetation was disrupted when Europeans occupied the region, and, after exhausting the forage in grassland, selectively cleared the forest for extensive livestock raising, a practice that has continued for more than four centuries [38
]. The construction of the railway towards the interior of the Chaco region during the early 20th century was accompanied by intense logging [40
]. More recently, agricultural expansion, including intensified cattle raising and annual crops, has further accelerated deforestation. During the last three decades, the main proximate driver of deforestation in the Gran Chaco has been the expansion of agriculture [32
], which was driven by global trends in technology and soybean markets [44
], which was facilitated by a rainfall increase reported for some territories of the argentine Chaco [32
]. However, recent results that were reported by Gasparri et al. [47
] suggest that in the northern Chaco rainfall has not been a major restriction to cropland expansion, and that the main limitations are imposed by infrastructure and services provided from towns.
A number of studies have been conducted to describe the patterns of deforestation in the southern and more arid extreme of the Gran Chaco, in Córdoba Province, central Argentina [32
]. These studies describe dramatic land cover changes that imply an annual deforestation rate of about 2.75% for the period 1969–2000. Besides rainfall increase, which has been proposed as a primary driver that is promoting deforestation through the expansion of agriculture, other drivers should also be considered to interpret local variations in land use and cover changes. While driving forces that were acting at global scales (international crop prices and demand, technological advances, market trends, etc.) affect the whole area in the same way, a series of physical (topography, soil types) and socio-economic (proximity to roads and to human settlements) drivers could also be influential at the local scale. This could determine, in turn, local variations in the way in which forest conversion has occurred in the past and will proceed in the future.
Previous studies reporting land cover changes in the southern Chaco have emphasized forest conversion into agriculture [32
], but processes such as forest conversion into other non-agricultural cover types, forest and crop persistence, and crop abandonment, have rarely been explored despite some results of those same studies suggesting they might be important. All of these processes may occur spatially associated or dissociated across the region, depending on how the drivers are influencing at the local scale. Because of the interdependence of the different processes, it is important to consider their spatial association when analyzing drivers, which may be achieved through multivariate analysis. The main objectives of this study were to (1) analyze the main trends in the land use and land cover changes (conversion, abandonment, forest persistence) in the southern extreme of the dry Chaco through multivariate analysis of remote sensing data, and (2) examine how physical drivers (precipitation, topography, soil type, and agricultural suitability of soils) and indicators of socio-economic driving forces (distance to roads and human settlements) have influenced those trends.
The first broad trend reported in this study for the southern extreme of the Great Chaco is a conversion of forests (both closed and open forests) into cultural vegetation and shrublands, both before and after the beginning of our study period. Our data report that abandonment (cultivated areas that became shrublands and/or forests) may reach high proportions in some areas, since some of the circular samples exhibit more than 50% of their surface affected by this type of process (Table 1
and Figure 2
c). This paper also shows that, even when precipitation is a major driver of forest conversion in the southern Chaco, changes in land use and land cover that were observed before and after 1979, may have also been influenced by several other factors acting at different scales. Additionally, we show how multivariate analyses can be used to analyze trends in local land cover changes and their spatial associations across a region, and how these trends can be related to the potential local drivers of change.
The dry Chaco has a long tradition of land use: until the 1970’s production was centered in extensive cattle ranching, selective logging, and charcoal, with great impact in the domestic market [62
]. After technological advances and the mechanization of agriculture, strong efforts were devoted to improve agriculture production. This resulted in effective conversion of the most suitable lands within the study area (the eastern sector). However, the lack of good soils and several other factors, such as reduced precipitations, restricted the development of agriculture in the western areas, where the abandonment and persistence of woody vegetation predominated during the study period. Consequently, shrublands and forests that were converted into cultural vegetation in the south-western part of the area, are now back under cattle grazing activities. Our data show that the main trend of variation along axis 1 of the PCA ordination discriminates samples located at the northeast sector of the study area that were converted into cultural vegetation both before and after 1979, from samples that have at present woody vegetation, either because they never were converted, or because they were converted and then abandoned (at the west of the study area). Caldas et al. [64
] reported that in the western Paraguayan dry Chaco cattle ranching is a major driver of forest loss, while in the more suitable eastern Chaco, soybean expansion is the main driver of land cover change, as also stated by Fehlenberg et al. [35
Agriculture expansion has been repeatedly reported as the major cause of deforestation in subtropical areas of South America [14
], and a similar trend has been described for the Argentine dry Chaco, where soybean cultivation has rapidly expanded during the last two decades following precipitation increase and technological developments [32
As shown by PCA axis 1 (Figure 2
) and its correlation with the conversion rate, the conversion of woody to cultural vegetation in the southern Chaco during the study period was spatially associated to places that had already been cultivated before 1979. This means that the increase in precipitation that was reported by different authors during the last decades [32
] only intensified the activity in adjacent areas to those that were already converted, but hardly produced an advance of the agriculture frontier.
The pattern described in this study is consistent with previous results reported for the study area [32
], as well as for other areas from north western Argentina [44
], where the loss of forests has been more intense in areas with higher rainfall. However, our results also suggest that even when deforestation in the southern extreme of the Great Chaco is associated with precipitation (Table 3
and Table 4
), which follows an east-west gradient, other drivers that were related to accessibility and/or favorable environments (mainly productive soils and gentle slopes/lower altitudes) are also important at the local scale. In agreement with our results, Gasparri et al. [47
] report that the main variables explaining the spatial patterns of cultivated areas in the northern Chaco of Argentina are the distance to main towns and soil suitability.
Despite the effect of rainfall, at the local scale the drivers of land cover changes may be complex and vary with context [5
]. The intensity of forest loss appears to be significantly related to some of the bio-physical conditions of the area, different than precipitation. Our data showed a significant positive correlation of PCA axis 1 (reflecting old and recent forest conversion) with soil quality (Table 3
), indicating that the likelihood of forest loss is greater at most suitable lands, with better soils and higher soil productivity indexes. Such biophysical control on the expansion of agriculture has been observed in many places of the world, in which soil quality, elevation, and slope gradient were found to be important predictors of deforestation [47
]. The impacts of human activities on forest loss have more frequently been reported in flat lands and at low elevations, both because of more suitable physical conditions and the proximity to towns and infrastructure. In agreement with our results, numerous studies in different parts of the world have reported the higher likelihood of deforestation in low elevation areas and in flat landscapes with gentle slopes [2
] (but see some exceptions in Mon et al. [67
], Htun et al. [73
], and Rojas et al. [74
Another finding of this study is that forest conversion was negatively correlated with human settlements and the distance to roads, which may indicate that the high rates of deforestation occur along roads and in the proximity to towns, probably due to the better accessibility and market availability. Infrastructure development has been repeatedly mentioned as a socio-economic factor that promotes deforestation [10
] and/or agriculture expansion [47
]. Our results confirm that the distance to human settlements has a significant effect on land use and cover changes, as reported previously for different places of the world [13
]. Distance to roads appears with a significant effect in the multiple regression models, which means that this variable influences land cover changes, when the effect of precipitation and physical setting (slope) is controlled. The results that were published so far for different ecosystems of the world, especially tropical forests, show contrasting effects of the distance to roads on deforestation: while some authors report a significant effect [13
], other studies indicate no evidence that roads contributed to deforestation [77
Another pattern claiming for further explanations is the abandonment of cultivated lands and the old conversion of natural vegetation into agriculture. Sites showing old conversion into cultivated lands correspond mostly to circles that are located in the eastern part of the study area. These sites had more suitable conditions even before the recorded increase in precipitations during the second half of the past century, and are still under cultivation. On the other hand, some circles located in the western part of the study area show either the persistence of woody vegetation (without conversion during the study period) towards the north, or the abandonment of previously converted sites towards the south. As pointed out above, the sites of the study area with less rainfall and poorer soil quality are less suitable for agriculture (Table 2
and Table 3
). All of these factors determine that crops are not profitable in the western sectors and fail to provide the expected benefits, and after a short period of cultivation, the land is abandoned. As a result, some sites that were deforested previously to 1979 were soon abandoned and were generally devoted again to cattle grazing. Human migration from rural to urban centers, with the consequent abandonment of agricultural land, is another mechanism explaining abandonment [78
]. Our results suggest that sites closer to human settlements showing temporary increases in precipitation may undergo opportunistic conversion of natural vegetation to agriculture, but once precipitation decreases, these sites are abandoned. As pointed out in previous studies that were carried on in the southern extreme of the dry Chaco [32
], the area has also been subjected to migration of rural population to towns and cities as a consequence of both, lack of profitable crops and changes in land tenure regime. This may, in turn, favor land abandonment and reduce land-pressure [71
], resulting in the recovery of woody vegetation [79
]. Unlikely, sites with low precipitation and far from settlements have never undergone conversion (Figure 2
, Table 4
We detected two main local trends in land-cover change along the southern extreme of the Great Chaco through multivariate analysis. The primary local trend was permanent conversion, either old or recent, of the land into agriculture; and, the secondary trend was old conversion, whether permanent or not. On the basis of the combination of both trends, we identified four alternative types of processes that were experienced by local landscapes along our study area: (1) conversion into agriculture before 1979 and persistence as agricultural land until present; (2) persistence of woody vegetation until 1979 and later conversion into agriculture; (3) conversion into agriculture before 1979, but later abandonment; and, (4) persistence of woody vegetation until present.
Geographical, biophysical, and socio-economic drivers determined which of the identified processes predominate in a given area. Increasing precipitation towards the east was the major driver of conversion into agriculture both before and after 1979, and precipitation shortage was the main driver of agriculture abandonment. Additionally, conversion has been more likely to occur in landscapes with better and more productive soils, and at the proximity to human settlements and roads. These results show that even when precipitation seems to be a determinant driver of woody vegetation loss in the Chaco, other factors that are controlling patterns of conversion into agriculture may vary at different spatial scales and contexts, and should also be taken into account in order to understand local causes and to establish general frameworks that are concerning the drivers of land cover change.