To cite rtrees in publications use:

  Li Daijiang. rtrees: an R package to assemble phylogenetic trees from
  mega-trees. 2023. Ecography. 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},
    url = {https://github.com/daijiang/rtrees},
  }