How are landscape complexity and vegetation structure related across an agricultural frontier in the subtropical Chaco, NW Argentina?

Abstract

Human-driven alteration of the Chaco strongly affects ecological patterns and associated processes at all spatial scales. To understand these modifications, sufficient methods for describing and quantifying high levels of landscape complexity caused by human activities in the region are urgently needed. Most methods involve the use of passive remote sensors, which capture complexity in only two dimensions (2D). A common 2D approach has been to calculate landscape metrics, such as Shannon's Landscape Diversity Index. But, it is not clear what aspects of three dimensional (3D) vegetation structure are being captured by these metrics. 3D structure is known to be as important as or more important than 2D structure in determining landscape patterns of biodiversity of many groups of organisms. In addition, studies have used a limited number of coarsely defined land-cover classes to calculate metrics. Our question was: how is vegetation structure related to remote sensing attributes in an agricultural frontier in the subtropical dry Chaco, NW Argentina? A secondary question was to explore the relationships between traditional landscape metrics and the semivariogram, a geostatistical tool used to describe 2D complexity. We described landscape complexity from the panchromatic QuickBird band and measured vegetation structure in 22-1 ha plots across an agricultural frontier in the subtropical dry Chaco, northern Argentina. A total of 2683 individual trees in 51 plant species and 21 families were measured in the field and 25,665 points were recorded to estimate foliage height diversity. Four landscape complexity groups were identified by a two-way cluster analysis using the 2D metrics. Four vegetation variables differed significantly among the 2D complexity groups: the standard deviation of the Enhanced Vegetation Index, the coefficient of variation of density per transect (CV density), mean tree diameter (DBH), and foliage height diversity (FHD). Largest patch index and semivariogram range were negatively related to CV density, mean DBH and FHD, while semivariogram sill, mean shape index, landscape shape index and number of patches were positively related to all three vegetation variables. Landscape metrics were not related to tree species diversity or density as previously shown, probably as a result of structural similarity among the dominant tree species in the Chaco biome.