To cite package lidR in publications use both:

Roussel J, Auty D, Coops NC, Tompalski P, Goodbody TR, Meador AS, Bourdon J, de Boissieu F, Achim A (2020). “lidR: An R package for analysis of Airborne Laser Scanning (ALS) data.” Remote Sensing of Environment, 251, 112061. ISSN 0034-4257, doi: 10.1016/j.rse.2020.112061, https://www.sciencedirect.com/science/article/pii/S0034425720304314.

Roussel J, Auty D (2021). Airborne LiDAR Data Manipulation and Visualization for Forestry Applications. R package version 3.2.1, https://cran.r-project.org/package=lidR.

Corresponding BibTeX entries:

  @Article{,
    author = {Jean-Romain Roussel and David Auty and Nicholas C. Coops
      and Piotr Tompalski and Tristan R.H. Goodbody and Andrew Sánchez
      Meador and Jean-François Bourdon and Florian {de Boissieu} and
      Alexis Achim},
    title = {lidR: An R package for analysis of Airborne Laser Scanning
      (ALS) data},
    journal = {Remote Sensing of Environment},
    volume = {251},
    pages = {112061},
    year = {2020},
    issn = {0034-4257},
    doi = {10.1016/j.rse.2020.112061},
    url =
      {https://www.sciencedirect.com/science/article/pii/S0034425720304314},
  }
  @Manual{,
    title = {Airborne LiDAR Data Manipulation and Visualization for
      Forestry Applications},
    author = {Jean-Romain Roussel and David Auty},
    year = {2021},
    note = {R package version 3.2.1},
    url = {https://cran.r-project.org/package=lidR},
  }