Spatiotemporal patterns of planted forests on the Loess Plateau between 1986 and 2021 based on Landsat NDVI time-series analysis

Figures & data

Figure 1. Location of the Loess Plateau and the spatial distribution of the reference samples.

Figure 2. Flowchart of the DCE framework for extracting planting year and conversion patterns of planted forests: (i) data pre-processing, (ii) standard NDVI time-series construction, (iii) planted forest mapping, (iv) planting year and conversion patterns extraction, and (v) accuracy assessment.

Table 1. Numbers of samples and their sources for each land cover type.

Land cover types		Field survey data	7^th National Forest Inventory	Image data		Total number
Vegetation	Planted forests	1265	305	0	1570
	Natural forests	265	2065	0	2330
	Cropland	0	0	1200	1200
	Grassland	0	0	980	980
Non-vegetation	Bare area	0	0	720	720
	Built-up	0	0	670	670
	Waterbody	0	0	280	280
Total number		1530	2370	3850	7750

Table 2. Accuracy assessment for the land cover classification in 2021.

	Planted forests	Natural forests	Crop-land	Grass-land	Non- vegetation
User accuracy (%)	87.11	97.55	83.20	89.10	99.68	Overall accuracy = 92.32%
Producer accuracy (%)	88.22	93.99	88.75	95.92	94.33	Kappa = 0.90

Table 3. Accuracy assessments for change detection in the conversion patterns types.

Detection types of the conversion patterns
	Correctly identified	Undetected	Changed from cropland misclassified as grassland	Changed from grassland misclassified as cropland	Others	Total
Num of identified pixels	252	25	17	13	7	314
Percentage	80.25%	7.96%	5.41%	4.14%	2.23%	100%
CA = 80.25%

Figure 3. Land cover classification results: (a) area (*10⁶ha) and percentage of each land cover type in the study area, and (b) land cover map in 2021.

Figure 4. Temporal characteristics of the conversion to planted forests: (a) total area of planted forests and (b) number of newly planted forest pixels in each year forming four temporal phases (TPs), and (c) spatial distribution of the newly planted forests in TP1, TP2, TP3, and TP4, respectively.

Figure 5. Conversion patterns of planted forests (i.e. from previous land cover type to planted forests, including persistence, afforestation, and deforestation): (a) chord diagram of conversion patterns, (b) spatial distribution of conversion patterns.

Figure 6. Conversion patterns for planting trees on grasslands (a–d) and returning croplands to forests (e–h), (a) spatial distribution of the grasslands converted to planted forests, (b) newly planted forest pixels from grasslands in each year, (c) inter-annual NDVI time-series and trend in MK statistics (UF) of a conversion process from grasslands to planted forests, (d) intra-annual NDVI time-series and corresponding Google Earth image in 2011 and 2021, and (e) spatial distribution of the croplands converted to planted forests, (f) newly planted forest pixels from croplands in each year, (g) inter-annual NDVI time-series and trend in MK statistics (UF) of a conversion process from croplands to planted forests, and (h) intra-annual NDVI time-series and corresponding Google Earth image in 2011 and 2021.

Figure A1. Number of Landsat observations in each year.

Figure A2. Typical standard inter-annual NDVI time-series for seven land cover types: (a) planted forests, (b) natural forests, (c) cropland, (d) grassland, (e) built-up, (f) bare area, and (g) waterbody.

Figure A3. Typical standard intra-annual NDVI time-series for seven land cover types: (a) planted forests, (b) natural forest, (c) cropland, (d) grassland, (e) built-up, (f) bare area, and (g) waterbody.

Data availability statement

The land cover map in 2021 and the core code for clustering standard NDVI time-series and mapping planted forests are available on the Google Earth Engine platform at the following website, respectively (https://code.earthengine.google.com/?asset=users/mengyy225/landcovermapofLoessPlateau; https://code.earthengine.google.com/80f9d3af9b63b2bd2444e33e8ba57bef?asset=users%2Fmengyy225%2FlandcovermapofLoessPlateau; https://code.earthengine.google.com/25331e665bb971836265b92cedac11a6?asset=users%2Fmengyy225%2FlandcovermapofLoessPlateau).