From “separation” to “Integration”: Analyzing the functional transformation of the urban greenbelt in Beijing from the perspective of “parkization”

•Urban greenbelt functional change reflects ecological and social benefits transition.•Parkization trend aids in evaluating urban greenbelt functional transformation.•Urban greenbelt function transformed from separating to integrating effects.•The functional transformation trend of the first greenbelt was more obvious.•Natural amusement and comprehensive parks drove urban greenbelt functional change.