data(bei)bei is an object of class "ppp"
  representing the point pattern of tree locations.
  See ppp.object for details of the format.  bei.extra is a list containing
  two pixel images, elev (elevation in metres) and
  grad (norm of elevation gradient). These pixel images are objects
  of class "im", see im.object.
bei gives the positions of 3605 trees
  of the species Beilschmiedia pendula (Lauraceae)
  in a 1000 by 500 metre rectangular sampling region
  in the tropical rainforest of Barro Colorado Island.  The accompanying dataset  bei.extra gives information
  about the altitude (elevation) in the study region. It is a list
  containing two pixel images, elev (elevation in metres) and
  grad (norm of elevation gradient).
These data are part of a much larger dataset containing the positions of hundreds of thousands of trees belong to thousands of species; see Hubbell and Foster (1983), Condit, Hubbell and Foster (1996) and Condit (1998).
The present data were analysed by Moller and Waagepetersen (2007).
Condit, R., Hubbell, S.P and Foster, R.B. (1996) Changes in tree species abundance in a neotropical forest: impact of climate change. Journal of Tropical Ecology 12, 231--256.
Hubbell, S.P and Foster, R.B. (1983) Diversity of canopy trees in a neotropical forest and implications for conservation. In: Tropical Rain Forest: Ecology and Management (eds. S.L. Sutton, T.C. Whitmore and A.C. Chadwick), Blackwell Scientific Publications, Oxford, 25--41.
Moller, J. and Waagepetersen, R.P. (2007) Modern spatial point process modelling and inference (with discussion). Scandinavian Journal of Statistics 34, 643--711.