Changes in landscape heterogeneity resulting from spatial differences in succession rates after farm land abandonment in a mountain environment

We performed a multi scale analysis about the recovery of natural vegetation at the Central Spanish Pyrenees. After a period of intense human pressure, this area undergone a process of land abandonment during the second half of the XX^th Century, which promoted the recovery of the natural vegetation. Our hypothesis is that, due to spatial differences in the conditions which control the rate of vegetation recovery, the process of vegetation succession after land abandonment results in an increase of the landscape heterogeneity during the first years or decades; however, as the process of vegetation recovery attains a mature state in an increasingly large proportion of the area, it eventually leads to a higher landscape homogeneity.

To test this hypothesis we used two contrasting methodologies. First, a series of satellite images—Landsat TM and ETM+ from August 1984, 1991 and 2001—was processed to obtain a time series of the Normalized Vegetation Index (NDVI) for the whole study area of about 2000 km². Since the NDVI is a continuous variate, changes in the heterogeneity of the vegetation activity were assessed by a spatial variance filter. Second, a time series of more detailed aerial photographs from a smaller area were classified into vegetation types, and the time variation of several landscape indexes was analyzed. Although the nature of the variables and the scale and spatial resolution of the analyses differ, the results from both methods supported the hypothesis about the effect of the vegetation succession process on the landscape heterogeneity. Understanding the driving factors of natural vegetation recovery in complex landscapes can be of great interest to foresee the effects of land use change in many similar mountain areas of the World, specially in the developed countries in which the abandonment of marginal farm land has become a common process.