Abstract:

Objective: This study aims to clarify the overall morphological characteristics of urban tree canopy (UTC) patches in the urban area of Beijing, and reveal the spatio-temporal dynamics of UTC patch morphology during the urbanization process, so as to provide the scientific reference for the planning and construction of urban forest in the future. Method: In this study, we first extracted the UTC patches within the 6^th Ring road of Beijing in 2002, 2013 and 2019 based on the high-resolution remote sensing images, and then analyzed the composition of patch morphological characteristics with the morphological spatial pattern analysis (MSPA) method, and finally revealed the spatial discrepancy of UTC transition using transition matrix and the relative contribution index method. Result: The UTC within the 6^th Ring road of Beijing increased from 387.93 km² in 2002 to 771.14 km² in 2019, showing a large expansion of UTC during the study period. However, there existed differences between the period of 2002–2013 and 2013–2019. From 2002 to 2013 the UTC reached to 282.92 km², which was apparently larger than that from 2013 to 2019 (97.39 km²). Furthermore, the morphological composition was rather steady throughout the study period. The islet, core and bridge were the main morphological types of UTC patches, since the total area of these three types comprised of over 80% of UTC in 2002, 2013 and 2019. The edge, loop, branch and perforation were the secondary types. During the study period, the transition between other urban land types and the islet of UTC was the main transition type of UTC, accounting for 30% and 25% of the total UTC conversion area during the periods of 2002–2013 and 2013–2019, respectively. Nevertheless, there were differences in UTC dynamics between these two sub-periods. Between 2002 and 2013, the transition related to the core of UTC patches had a relatively large areal ratio (approximately 23.12%), while the transition related to the bridge of UTC patches had a relatively large areal ratio (approximately 35.22%) between 2013 and 2019. It was suggested that the UTC transition before 2013 was to a larger extent characterized as the dynamics of UTC core, while UTC transition after 2013 was characterized as the dynamics of UTC bridge. From the perspective of spatial changes, the transition related to the UTC islet mainly clustered within the 4^th Ring road, while the transitions related to the UTC core and bridge were located between the 4^th and 6^th Ring-road. Conclusion: In the past two decades, the UTC within the 6^th Ring road in Beijing has expanded, mainly due to the urban greening projects. The islet, core and bridge are the main morphological characteristics of UTC patches within the 6^th Ring road in Beijing. With the development of the city, the main transition type of morphological characteristics of UTC patches gradually changes from the transition related to the core to the transition related to the bridging. The transitions of UTC patch morphological features present a gradient distribution between urban and rural areas, that is, from the city center to the outside, the main transition type of UTC patch morphological characteristics changes from the transition related to the islet to the transition related to the core and bridge. In the future, urban forest construction should focus on maintaining and improving the connectivity between urban forest patches and strengthening the protection of remnant forest patches in the city center.

Key words: urban tree canopy patches, morphological spatial pattern analysis, spatio-temporal dynamics, Beijing.