Simple, and often used, tree species density measures are trees per unit area and basal area per unit area. These can be separated by tree species. Field plots are best for determining these. Remote sensing might be used, but species below the main canopy might be missed.
Number of stems of a particular species per unit area. Eg. If you set a plot of 20m x 20m, you have to calculate the plot area which is 400sqm. Then convert into ha. Iha is 10,000sqm. Then 400sqm/10000sq equals to 0.04. Finally divide number of stems by 0.04ha
Additionally, remember to divide the total number of individuals by the total sample plots, what you get is density /Ha. If you do not divide the number of stems of each species, what you get will be density of the particular species in all plots.