Abstract
Shrubs growing in former burnt areas play two diametrically opposed roles. On the one hand, they protect the soil against erosion, promote rainwater infiltration, carbon sequestration and support animal life. On the other hand, after the shrubs’ density reaches a particular size for the canopy to touch and the shrubs’ biomass accumulates more than 10 Mg ha−1, they create the necessary conditions for severe wild fires to occur and spread. The creation of a methodology suitable to identify former burnt areas and to track shrubs’ regrowth within these areas in a regular and a multi temporal basis would be beneficial. The combined use of geographical information systems (GIS) and remote sensing (RS) supported by dedicated land survey and field work for data collection has been identified as a suitable method to manage these tasks. The free access to Sentinel images constitutes a valuable tool for updating the GIS project and for the monitoring of regular shrubs’ accumulated biomass. Sentinel 2 VIS-NIR images are suitable to classify rural areas (overall accuracy = 79.6% and Cohen’s K = 0.754) and to create normalized difference vegetation index (NDVI) images to be used in association to allometric equations for the shrubs’ biomass estimation (R2 = 0.8984, p-value < 0.05 and RMSE = 4.46 Mg ha−1). Five to six years after a forest fire occurrence, almost all the former burnt area is covered by shrubs. Up to 10 years after a fire, the accumulated shrubs’ biomass surpasses 14 Mg ha−1. The results described in this paper demonstrate that Northwest Portugal presents larger shrubland areas and greater shrub biomass accumulation (average 18.3 Mg ha−1) than the Northeast (average 7.7 Mg ha−1) of the country.
Highlights
Portugal is an European country with a constituent land mass and 4 separate archipelagos.The former is located in the east of the Iberian Peninsula with an area of approximately 90,000 km2 .From the mainland area (52%) there are: forest stands (39%), dense shrubland (12%), and sparse shrubland (1%) [1,2]
The landcover characterization work carried out was based on false colour RGB482 composition visual analysis (Figure 4) and used normalized difference vegetation index (NDVI) images (Figure 5) for interpretation
As the vegetation red reflectance is always lower to the near-infrared reflectance, the positive NDVI achieved values could indicate vegetation density
Summary
Portugal is an European country with a constituent land mass and 4 separate archipelagos.The former is located in the east of the Iberian Peninsula with an area of approximately 90,000 km2 .From the mainland area (52%) there are: forest stands (39%), dense shrubland (12%), and sparse shrubland (1%) [1,2]. Portugal is an European country with a constituent land mass and 4 separate archipelagos. The former is located in the east of the Iberian Peninsula with an area of approximately 90,000 km. Between the mid-1980s and 2020, due to increasing human rural abandonment and edaphoclimatic conditions, a large number of forest fires occurred in mainland Portugal during the summer. The intensity of these fires increased dramatically each decade [3,4,5,6,7]. Published results [8,9,10,11,12] demonstrate that, Forests 2020, 11, 555; doi:10.3390/f11050555 www.mdpi.com/journal/forests
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