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.
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
