Speaker
Dr
Joao Carreiras
(National Centre for Earth Observation (NCEO), University of Sheffield)
Description
Land use and land cover change, particularly conversion from forest to non-forest, is the second largest source of greenhouse gases emissions. Global deforestation is mostly concentrated in the tropics, with severe consequences for carbon stocks depletion and major impacts on biodiversity, with most of the cleared land occupied with annual or perennial crops, plantations or cattle ranching. The Brazilian Amazon is one of the world’s major deforestation hotspots, although deforestation rates reported by the Instituto Nacional de Pesquisas Espaciais (INPE) show a decreasing trend from its highest value in the mid-1990s (~30,000 sq km/yr) to a record low of ~4,500 sq km/yr in 2012.
Abandoned croplands and pastures in tropical regions give rise to rapid establishment of secondary forests, with consequent carbon accumulation and potential restoration of biodiversity. However, the age, type and composition of these forests established on abandoned lands are a consequence of several factors, such as land use history, soil fertility, and distance to mature (primary) forests. Therefore, it is important to know the age and land use history of regrowth areas to better understand patterns of carbon accumulation and impacts on biodiversity.
The land use history prior to land abandonment can be retrieved by analyzing time series of land cover maps derived from classification of high resolution optical data. This approach is nonetheless often hindered by cloud cover, leading to poor coverage by optical data. However, all-weather SAR data availability is increasing with consequent benefits for land use/land cover change monitoring over tropical regions.
The objective of this study was to assess the capability of C- and L-band SAR data to discriminate and map the age of secondary forests across the Brazilian Amazon. Available 3-class (mature forest, non-forest, secondary forest) land cover maps over three selected sites of the Brazilian Amazon (Manaus: 1973-2011; Santarém: 1984-2010; Machadinho d’Oeste: 1984-2011) were used to generate maps of age of secondary forests (ASF) at any given year in those periods. ASF classes were defined as initial (≤ 5 years), intermediate (6-15 years) and advanced (≥ 16 years). Japan Aerospace Exploration Agency (JAXA) Advanced L-band Observation Satellite (ALOS) Phased Array L-band SAR (PALSAR) data acquired over those sites between 2007 and 2010 were processed and used to test its ability to discriminate stages of tropical secondary forests. Additionally, the current concurrent availability of both C-band (Sentinel-1A) and L-band (ALOS-2 PALSAR-2) data makes it possible to test the synergy of multi-frequency SAR data to improve the classification of stages of tropical secondary forests.
Fitted models of the type I = a0 + a1 ln(ASF) (where I = HH or HV backscatter intensity in dB) were significant (α=0.05) at the site and time-step level (with the exception of those with HH polarization at Manaus in 2007 and 2009); model goodness of fit (R2) was low but better at the HV polarization, ranging from 0.02-0.04, 0.13-0.34 and 0.10-0.23 at Manaus, Santarém and Machadinho d’Oeste respectively. Models fitted with aggregated data at the site level were also significant (α=0.05) and with a better goodness of fit (R2), ranging from 0.28 to 0.43; however considerable inter-annual variability is noticeable.
The analysis of variance and the Kruskal-Wallis rank sum test of the model relating ASF classes to L-band HH or HV backscatter intensity at the site scale showed that overall all models were significant (α=0.05). The exceptions are those with HH in 2009 and 2010 (Manaus), and with HH in 2007 (Santarém). Non-significant (α=0.05) pairwise combinations of ASF classes, according to Dunn’s test are: i) Manaus, initial-intermediate; initial-advanced; ii) Santarém, intermediate-advanced. Data from aggregating all sites showed that according to the analysis of variance and the Kruskal-Wallis rank sum test all models were significant and only the combination initial-intermediate in 2007 was rejected as significant according to Dunn’s test.
L-band SAR data alone does not accurately discriminate the age and stages of tropical secondary forests in the Brazilian Amazon; however, better results were obtained when data from different sites were aggregated. The concurrent use of data acquired by L- and C-band SAR sensors may improve the discrimination of these forests; structural differences among different ages/stages of tropical secondary forests might be better captured with SAR data representing the interaction of radiation with different parts of the tree.
Primary author
Dr
Joao Carreiras
(National Centre for Earth Observation (NCEO), University of Sheffield)
Co-author
Prof.
Shaun Quegan
(National Centre for Earth Observation (NCEO), University of Sheffield)
