Improved forest-cover mapping based on MODIS time series and landscape stratification

Figures & data

Figure 1. Study area.

Table 1. Forest map nomenclature.

Class	Vegetation type
Deciduous oaks	Broadleaf forest
Mixed deciduous forest
Mixed mountain deciduous forest
Beech
Pyrenean oak
Holm oak
Evergreen oaks
Mountain pine	Coniferous forest
Aleppo pine
Pyrenean pine
Scots pine
Monterey pine
Silver fir
Beech and silver fir	Mixed broadleaf /coniferous forest
Wooded garrigue	Shrubland
Unwooded garrigue
Spanish matorral
Mountain Spanish matorral
Grassland	Heathland and grassland
Mountain heathland
Alpine heathland
Alpine grassland
Bare soil, rocks	-
Water	-

Table 2. Indices describing the topographical and landscape characteristics of the strata.

Index	Description	Equation
Topographical indices
Area (ha)	Area of a stratum	AS: Stratum area (ha)
Elevation (standard deviation) (m)	Standard deviation of elevation values into a stratum	(1) eij: elevation (m) into the stratum E: Sum of the elevation values (m) into the stratum n: Number of elevation values for the stratum
Slope (average) (°)	Average slope value into a stratum
Area-derived and spatial organization landscape indices
Total forest-cover rate (%)	Total forest-cover rate for a stratum	AF: Total forest-cover area (m^2) AS: Stratum area (m^2)
Patch area (average) (ha)	Patch area average for a stratum	AS: Stratum area (m^2) n: Number of patches
Patch area (standard deviation) (ha)	Patch area standard deviation for a stratum	aij: IJ patch area (m^2) AS: Stratum area (m^2) n: Number of patches
Composition landscape indices
Number of classes	Number of classes for each stratum	-
Shannon Diversity index	Index indicating the class area rate of dispersion for a stratum	(5) Pi: Rate between a class area and the total area of all the classes

Table 3. Indicators used for accuracy assessment.

Processes		Accuracy indicators
Date	Classification	kappa coefficient (κ)	Reject fraction (RF)	Producer’s accuracy (PA)
2007–2009 2009–2011	Without stratification	Global	Global – (per class)	Global – (per class)
		Per stratum	Per stratum
	With stratification	Global	Global – (per class)	Global – (per class)
		Per stratum	Per stratum

Figure 2. PCA results: (a) projection on the two first factors (PC1 and PC2) of the monthly average EVI images; (b) projection on the two first factors (PC1 and PC2) of the five texture indices (Dissi, Dissimilarity; SecMom, Second Moment; Homo, Homogeneity) and EVI for the month April (04).

Figure 3. Results (Johnson’s and Zhang’s curves) obtained using three dates of EVI and each of the five texture indices averaged over five years (2007–2011): (a) second moment, (b) contrast, (c) dissimilarity, (d) homogeneity, and (e) entropy.

Figure 4. Stratification of the study area obtained after merging and eliminating unprocessable strata.

Table 4. Values calculated for the eight landscape indices describing the 25 strata.

Index	Min.	Max.	Mean	Standard deviation
Topographical indices
Area (ha)	13,242	917,244	240,950	212,307
Elevation (standard deviation) (m)	100	543	257	127
Slope (average) (°)	3	22	10	5
Area-derived and spatial organization landscape indices
Total forest-cover rate (%)	29	92	71	20
Patch area (average) (ha)	80	15,715	3054	3543
Patch area (standard deviation) (ha)	19	205	72	52
Composition landscape indices
Number of classes	3	23	11	6
Shannon Diversity index	1.35	2.77	2.07	0.41

Table 5. Accuracy assessment obtained with and without stratification for the two studied periods (kappa coefficient (κ) and reject fraction).

Date	Classification	NDVI Global kappa coefficient (κ)	EVI Global kappa coefficient (κ)	Global reject fraction (RF) (%)
2007–2009	Without stratification	0.53	0.47	6
	With stratification	0.65	0.57	13
2009–2011	Without stratification	0.52	0.44	7
	With stratification	0.64	0.56	13

Table 6. Stratification effect on the producer’s accuracy, NDVI series for the 2007–2009 period.

	Producer’s accuracy (PA) (%)
Class	Without stratification	With stratification	Differences with – without	Group
Beech	74	75	1	1
Alpine heathland	63	64	1
Holm oak	58	59	1
Beech and silver fir	60	63	3
Monterey pine	93	96	3
Spanish matorral	54	60	6
Unwooded garrigue	70	80	10	2
Aleppo pine	61	70	10
Mountain pine	66	77	11
Scots pine	57	69	12
Pyrenean pine	52	64	12
Alpine grassland	54	69	15
Mixed deciduous forest	55	72	16
Grassland	57	74	17
Silver fir	61	79	18
Mixed mountain deciduous forest	20	46	26	3
Mountain Spanish matorral	24	50	26
Wooded garrigue	13	40	27
Mountain heathland	38	67	29
Pyrenean oak	40	69	29
Evergreen oaks	14	48	33
Deciduous oaks	30	64	34
Mean	51	66
St. dev.	20	13

Figure 5. κ and RF values for the 25 strata with and without stratification.

Figure 6. Relation between κ and RF differences with and without stratification for the 25 strata.

Figure 7. (a) Relation between the per-stratum kappa coefficient (κ) value with stratification and the stratum area (ha). (b) Relation between the per-stratum κ value without stratification and the stratum area (ha).

Table 7. Coefficient of determination (R²) between the stratum indicators and the per-stratum κ and RF differences for the 2007–2009 period (bold: significant for α = 0.01; (+): positive correlation; (-): negative correlation).

Index	κ difference	RF difference
Topographical indices
Stratum area (ha)	(-) 0.32	(-) 0.35
Elevation (standard deviation) (m)	(-) 0.38	(-) 0.31
Slope (mean) (°)	(-) 0.28	(-) 0.18
Area-derived landscape indices
Total forest-cover rate	(-) 0.09	(-) 0.11
Patch area (average) (ha)	(+) 0.07	(+) 0.003
Patch area (standard deviation) (ha)	(+) 0.64	(+) 0.59
Composition landscape indices
Number of classes	(-) 0.34	(-) 0.42
Diversity Shannon index	(-) 0.10	(-) 0.15

Figure 8. Relation between stratum area (ha) and κ difference.

Figure 9. Classified vegetation area (ha) – strata order by group and increasing with stratum area.

Figure 10. Relation between the standard deviation of elevation (m) and the κ difference considering the strata groups A, B, and C.

Figure 11. Relation between standard deviation of the patch area (ha) and κ difference considering the strata groups A, B, and C.

Figure 12. Relation between number of classes and κ difference considering the strata groups A, B, and C.

Table 8. Segmentation results according to 15 and 20 scale parameter values.

Scale parameter		15	20
number of HLU:	total	56	39
number of HLU:	area < 100,000 ha	30	15
number of HLU:	100,000 < area < 450,000 ha	25	22
number of HLU:	area > 450,000 ha	1	2
number of unprocessed strata		39	31