To cite package ‘rtrees’ in publications use:

  Li Daijiang. rtrees: an R package to assemble phylogenetic trees from
  mega-trees. 2023. Ecography, 2023(7): e06643.
  https://github.com/daijiang/rtrees

A BibTeX entry for LaTeX users is

  @Article{,
    title = {rtrees: an R package to assemble phylogenetic trees from megatrees},
    author = {Daijiang Li},
    journal = {Ecography},
    year = {2023},
    volume = {2023},
    number = {7},
    pages = {e06643},
    url = {https://github.com/daijiang/rtrees},
  }