The present study is aimed at exploring native and alien plant species diversity patterns in distinct types of the urban forest environment by documenting differences in species density, level of invasion, and species-area relationships across four spatial scales (1, 10, 100, and 1,000 m2). The research was conducted in deciduous urban woodlands in Kyiv, Ukraine. The total set of sampling units (n = 60) was divided among three forest types representing semi-natural and planted woodlands. We observed significant differences in plant species diversity patterns between semi-natural and planted urban woodlands across structurally and functionally complex disturbance regimes. Specifically, broadleaved plantations had lower number of species at small spatial scales but steeper species accumulation within a designated area. Steep species accumulation resulted in a lack of differences with semi-natural forests at 100 m2 and 1,000 m2. Mesotrophic oak-hornbeam woodlands were relatively resistant to plant invasions. However, an existing pool of alien plant species coupled with a constant flow of newly arrived species at the city scale may create a sufficient propagule pressure to “break down” this resistance. Ash and poplar semi-natural forests occupied “an intermediate position”, not differing from oak-hornbeam forests and from deciduous plantations in terms of mean density of native and alien plant species, respectively. The multi-scale approach we applied in this study revealed the multifold aspects of plant species diversity patterns in the urban forest. We encourage a further development of plant diversity monitoring techniques that encompass multiple spatial scales and analyses.