It is well known that earthquakes and secondary disasters cause major disturbances to the ecosystems in affected areas, resulting in soil physicochemical property changes (Vittoz et al. [
43]; Lin et al. [
44]), vegetation destruction (Cui et al. [
31]; Wang et al. [
45]), landscape fragmentation (Fichera et al. [
46]), and LULC changes (Dewan and Yamaguchi [
47]). Ecological restoration involving biological and engineering countermeasures is carried out to prevent further deterioration and to improve the recovery process in earthquake-affected areas (Lin et al. [
44]; Millington et al. [
48]; Kang et al. [
29]). However, to our knowledge, there are few studies in the literature about LULC changes in a typical basin after an earthquake that reflect in more detail the response of ecosystems to major disturbances. Our study may provide a helpful source of evidence about spatiotemporal changes in LULC over multiple years and a more detailed content to document the process of ecological restoration in earthquake-affected areas.
4.1. Land Use Structure Change in the Longxi River Basin
After the Wenchuan earthquake, the areas of forest and unutilized land decreased and increased, respectively, in Dujiangyan city, to which the Longxi River basin belongs (Nath et al. [
10]). Our study proved that the transition from forest to unutilized land was highest, accounting for 1087.88 hm
2 in the Longxi River basin. However, compared with Dujianyan city, the Longxi River basin had a higher recovery potential in forest because forest area increased gradually between 2011 and 2015, though it also transitioned to other land use types. This may be due to the great resistance and resilience of the large area of natural forests distributed in the basin. This result is consistent with the findings of Qiu et al. [
49] and Knoke et al. [
50], who observed that natural forests with more complex structures and richer biodiversity than plantations showed lesser degrees of earthquake impacts and higher recovery capacities. Overall, forest had a negative single land use dynamic degree after the earthquake until 2009 due to its destruction caused by the earthquake, while it had positive degrees during the periods from 2009 to 2011 and from 2011 to 2015 due to natural recovery. In addition, forest had the second lowest changes in spatial dynamic change degrees during the study period because it dominated all land use types in the Longxi River basin.
In general, environmental factors determine the distribution of cultivated land, while socioeconomic factors often control the direction, quantity, and speed of cultivated land transition (Nath et al. [
10]). In this study, the area of cultivated land decreased continually during the study period due to the bury of secondary disasters such as landslides and debris flow induced by the earthquake and heavy rainfall in rainy seasons. This result is consistent with other studies demonstrating negative impacts of earthquakes and disasters on cultivated land transition (Li et al. [
51]; Zou et al. [
28]). In addition, the local government implemented policies for converting cultivated land into forest or grassland and for reconstruction for the relocation of victims, which also caused the reduction in cultivated land. Therefore, cultivated land had negative single land use dynamic degrees across the study period, with comparatively high spatial dynamic change degrees, showing high transitions to other land use types.
Unutilized land mainly consists of landslides, collapses, and bare land. It accounted for 119.65 hm
2 in 2005 and increased to 1267.49 hm
2 in 2009 due to a large amount of bare land caused by the Wenchuan earthquake, which had a seismic intensity of XI. This result is consistent with a previous investigation, which highlighted a large amount of bare land as the response of LULC to a major earthquake (Li et al. [
52]). However, the area of unutilized land decreased gradually after 2009 due to the high resilience of natural forests as the dominant land use type and strong management by the Longxi Hongkou National Nature Reserve administration. Overall, unutilized land had the highest single land use dynamic degree after the earthquake until 2009 but had negative degrees during the periods from 2009 to 2011 and from 2011 to 2015 due to ecological restoration. Therefore, unutilized land had the highest spatial dynamic change degrees during the period from 2005 to 2009 but had a comparatively low degree after 2009.
The area of grassland reduced after the earthquake but increased due to natural recovery during the period from 2009 to 2011, while it reduced again during the period from 2011 to 2015 due to the transition to forest. During the period from 2009 to 2011, grassland was unstable and easy to transit to other land types under the influence of environment and ecosystem self-recovery because a part of it was transited from bare land induced by the earthquake and secondary disasters. However, the grassland transited from bare land became stable and evolved as forest after 7 years of natural recovery in 2015. This result is consistent with the findings of Luo et al. [
53], who observed that grassland was mainly converted to forest after natural recovery in the Longmen fault zone. Therefore, grassland had a positive single land use dynamic degree with the highest spatial dynamic change degree during the period from 2009 to 2011, while it had negative single land use dynamic degrees with still comparatively high spatial dynamic change degrees during the periods from 2005 to 2009 and from 2011 to 2015.
Compared with 2005, the area of construction land increased in 2009 because the local government carried out the building reconstruction for victim settlement. However, due to the damage caused by a large scale of debris flows in 2010, the area of construction land reduced in 2011 (Xu et al. [
54]). After the debris flows, the building reconstruction once again increased the area of construction land. This result is consistent with a previous study demonstrating the positive effect of reconstruction on construction land (Shi et al. [
55]). Therefore, compared with grassland, construction land had the opposite pattern in the changes of single land use dynamic degrees across the study period but had a similar pattern in spatial dynamic change degrees.
Water area decreased slightly during the period from 2005 to 2009 but intensively during the period from 2009 to 2011. This is also due to the damage caused by debris flows in 2010. Debris flows created a large number of unconsolidated materials deposited onto the riverbed and debris flow accumulation fans on both banks of the river, resulting in the uplift of the Longxi River riverbed, reduction in water area, and difficulty restoring the damaged water area during the period from 2011 to 2015 (Yu et al. [
56]). Therefore, water area had the highest single land use dynamic degree with a comparatively high spatial dynamic change degree during the period from 2009 to 2011.
Across the study period, the speed of land use change fluctuated in the Longxi River basin, reflecting the impacts of natural disasters, natural succession, and human activities in different periods. During the period from 2005 to 2009, land use change was mainly affected by secondary disasters, resulting in a value of 81.37% in comprehensive change index of land use degree. Forest was the main land use type transiting to other types. During the period from 2009 to 2011, land use change was mainly controlled by natural succession, reconstruction, and secondary disasters, resulting in the highest value of 162.55% in comprehensive change index of land use degree. Unutilized land was the main land use type transiting to other types. During the period from 2011 to 2015, land use change was mainly affected by natural succession and reconstruction, resulting in the lowest value of 64.73% in comprehensive change index of land use degree. Unutilized land was also the main land use type transiting to other types. During the period from 2005 to 2015, the comprehensive land use dynamic degree was 39.35%, which was higher than the findings of Shu et al. [
57] and Liu et al. [
58], who observed that the comprehensive land use degrees of the Salaxi demonstration area and the China–Vietnam border zone ranged from 1.33% to 2.72% and 0.43% to 4.50%, respectively. Therefore, land use types in the Longxi River basin changed relatively rapidly during the period from 2005 to 2015.
The comprehensive change index of land use degree also showed great change during the period from 2005 to 2015. It had an average value of −14.87 during the period from 2005 to 2009, suggesting that a high intensity of land use change occurred, especially a great transition from forest to unutilized land after the earthquake. After the post-earthquake 3 year recovery, the index reached 8.15, indicating that restoration in unutilized land increased the areas of grassland and forest. However, the index changed slowly during the period from 2011 to 2015, suggesting that the speed of natural recovery decreased. Across the study period, the index had an average value of −4.02, showing that negative effects of the earthquake on land use change still existed after a 7 year recovery. The regions with a high comprehensive change index of land use degree were distributed in the northeast and south of the Longxi River, especially along both sides of the lower reaches of the Longxi River, showing that flat land in the downstream alluvial fan is suitable for reconstruction in the basin due to its convenience for transportation, construction, and development. This result proved the conclusion of Zou et al. [
59] that flat land in mountainous area had the potential for good economic foundation, convenient transportation, and a dense population.
4.2. The Changes of Land Use Gravity Centers
The gravity centers of land use types would undoubtedly migrate due to human activities and the changes in environmental factors (Chu et al. [
60]). Our study found that the gravity centers of all land use types changed across the study period. The gravity center of grassland continued to move southward due to the reduction in its damaged area at high altitude and the transition of unutilized land at the early stage of natural succession.
The gravity center of cultivated land shifted from north to south but then suddenly eastward in 2009. This was due to the original distribution and victim settlement. Cultivated land was scattered in the middle and lower reaches of the basin before the earthquake. However, the destruction of cultivated land induced by the earthquake was severe in the middle reaches, resulting in its southward shift. In addition, the settlement of the population around Longchi town caused the reduction in cultivated land in the west of the basin, resulting in its sudden eastward shift.
The construction land scattered along the riverside for agritainment in the basin was seriously damaged by debris flows and was difficult to rebuild, resulting in its southward shift toward the lower reach of the Longxi River.
Forest, as the dominant land use type, was affected by many factors, including secondary disasters and natural recovery. Its gravity center was similar than unutilized land, and both of them shifted from south to north during the study period. However, forest shifted its gravity center from north to south during the period from 2009 to 2011 due to the reduction in forest area caused by debris flows in 2010.
Water area also transferred its gravity center from north to south. This may be due to the decrease in water storage in the Longchi lake in the north of the basin and the burying of the riverbed caused by unconsolidated materials of debris flows (Zou et al., 2019).
4.3. Land Use Transit Characteristics across the Study Period
During the period from 2005 to 2009, forest and grassland were destroyed after the earthquake, resulting in the increase in unutilized land; the areas of grassland, forest and water area decreased, while that of unutilized land increased. The transition between forest and grassland was high due to the simultaneous degradation of forest and grassland succession to forest. Construction land increased with the transition from forest, grassland, and cultivated land due to victim settlement.
After a 3 year recovery, the transitions among forest, grassland, and unutilized land still dominated the land use transit matrix. However, the areas of forest and grassland increased, while that of unutilized land decreased, though the debris flows that occurred in 2010 caused heavy losses (Yu et al. [
56]). In particular, water area intensively decreased from 14.09 hm
2 in 2009 to 5.16 hm
2 in 2011, suggesting that the riverbed was buried by debris flows (Zou et al. [
28]).
After 7 year recovery, the transitions among forest, grassland, and unutilized land still were the main transit types. However, Unlike in 2011, the area of grassland decreased in 2015, indicating that the Longxi River basin maintained positive succession due to the high resilience of natural forests and strong management by the Longxi Hongkou National Nature Reserve administration.
However, though the area of forest almost reached its original status in 2015, all land use types except unutilized land still had lower areas than in 2005 (
Table 6), indicating that recovery was not complete 7 years after the earthquake (Zhao et al. [
61]; Li et al. [
62]). Hence, the management of land use, afforestation, and forest protection should be carried out to improve the restoration in the Longxi River basin.
In this study, we used four different remote-sensing satellite images to analyze the spatiotemporal dynamic characteristics of land use change in the Longxi River basin. However, we were not able to consider other environmental indicators (e.g., regional meteorological data, soil properties, population, and socioeconomic development data) as they were not concerned during the study period. Therefore, elucidating the effects of these and other indicators on land use change would require further research.