Appropriate citations include:

  Thornley, J H M (1998). Modelling shoot : root relations: the only
  way forward? Annals of Botany, 81(2): 165-171.
  doi:10.1006/anbo.1997.0529.

A BibTeX entry for LaTeX users is

  @Article{,
    title = {Modelling shoot : root relations: the only way forward?},
    author = {J H M Thornley},
    journal = {Annals of Botany},
    year = {1998},
    volume = {81},
    number = {2},
    pages = {165-171},
    month = {FEB},
    doi = {10.1006/anbo.1997.0529},
  }

  Higgins, Steven I., O’Hara, Robert B., Bykova, Olga, et al. (2012). A
  physiological analogy of the niche for projecting the potential
  distribution of plants. Journal of Biogeography, 39(12): 2132-2145.

A BibTeX entry for LaTeX users is

  @Article{,
    title = {A physiological analogy of the niche for projecting the potential distribution of plants},
    author = {Steven I. Higgins and Robert B. O’Hara and Olga Bykova and Michael D. Cramer and Isabelle Chuine and Eva-Maria Gerstner and Thomas Hickler and Xavier Morin and Michael R. Kearney and Guy F. Midgley and Simon Scheiter},
    journal = {Journal of Biogeography},
    year = {2012},
    volume = {39},
    number = {12},
    pages = {2132-2145},
    issn = {1365-2699},
  }

  Higgins, Steven I. I., Conradi, Timo, and Muhoko, Edward (2023).
  Shifts in vegetation activity of terrestrial ecosystems attributable
  to climate trends. Nature Geoscience, 16(2): 147+.
  doi:10.1038/s41561-022-01114-x.

A BibTeX entry for LaTeX users is

  @Article{,
    title = {Shifts in vegetation activity of terrestrial ecosystems attributable to climate trends},
    author = {Steven I. I. Higgins and Timo Conradi and Edward Muhoko},
    journal = {Nature Geoscience},
    year = {2023},
    volume = {16},
    number = {2},
    pages = {147+},
    month = {FEB},
    doi = {10.1038/s41561-022-01114-x},
  }

  Higgins, Steven I., Burkhardt, Arne, Conradi, Timo, and Muhoko,
  Edward (2024). TTR.PGM: An R package for modeling the distributions
  and dynamics of plants using the Thornley transport resistance plant
  growth model. Methods in Ecology and Evolution.

A BibTeX entry for LaTeX users is

  @Article{,
    title = {TTR.PGM: An R package for modeling the distributions and dynamics of plants using the Thornley transport resistance plant growth model},
    author = {Steven I. Higgins and Arne Burkhardt and Timo Conradi and Edward Muhoko},
    journal = {Methods in Ecology and Evolution},
    year = {2025},
    volume = {100},
    number = {1},
    pages = {000-000},
  }

The TTR model was proposed and described by Thornley 1988. Higgins et
al. 2012 added environmental forcing to the TTR model and interpreted
it as a species distribution model. Higgins et al. 2023 interpret the
TTR model as a time series model. Higgins et al. 2025 describe the R
package.