The Intensity int column has the ability to store all the treeID values. There is a slightly different format for copying rows to another and you can find the locations using str(las1).  Additionally, some NA values were discovered from points not allocated, which I have remedied below as this initially failed to enable writing the .las successfully. Once imported into ArcPro adjust the Symbology to draw using intensity creating a custom value source and random Color scheme. The data clearly displays individual trees satisfactorily with some adjustment lowering the Symbol scale.

 

```{r}
library(lidR)
library(dplyr)
## TESTING SMALL FILE  #############################################################################
#  Import the las and segment the trees
LASfile <- system.file("extdata", "MixedConifer.laz", package="lidR")
#las <- readLAS(LASfile)
las <- readLAS(LASfile)
las1 <- segment_trees(las, li2012())
##################################################################################################
# remove points that are not assigned to a tree
trees = lasfilter(las1, !is.na(treeID))  #  remove NA
col <- random.colors(100)
plot(trees, color = "treeID", colorPalette = col)

las1@data$Intensity = las1@data$treeID  #  copy 
head(trees)  # checking first few rows copied

trees = lasfilter(las1, !is.na(Intensity))  # remove all NA values from here too
writeLAS(las = trees, file = "ntrees.las")

#plot(las1, color = "treeID", colorPalette = col)
```
Screenshot below of my zoomed ArcPRO working with over 65,000 treeID from the above code but loading a significantly larger .las file than the example, this consisted of of 130mb or 30 million data points
[![Screenshot of zoomed ArcPRO working with 65,000 treeID from the above code loading a larger .las file][1]][1]


  [1]: https://i.sstatic.net/itUZo.jpg