"Tree" is (apparently) not a very well defined term as far as scientific classifications are concerned, so I'll put my own restrictions on it for this question:
Typical healthy adult specimens must reach 15 feet or 5 meters (yeah, I know it's not an exact conversion, and I don't really care because it's "close enough" for me)
Typical healthy adult specimens should have (or appear to have) a single trunk for each individual tree. (the occasional odd individual tree with 2 or 3 trunks emerging from the ground at or near the same spot is ok, as long as it's the except and not the rule for that species. Also, clonal colonies, such as quaking aspen, which could theoretically be composed of a single genetic individual taking up the entire forest population of trees also qualifies, as it "appears" to have individual specimens with individual trunks for each one, even though it's not)
The species must make use of Secondary Growth, not just primary growth.
The species must be made of actual Wood, though the wood need not be useful as lumber.
"Density" in terms of a forest, can also have some interesting variations in its definition, so for this question the key factors will be:
The average space in between the surfaces of the neighboring tree trunks, as opposed to the distance between their center-points, as measured from a height of 6 feet or 2 meters off the ground or below. Branches are not taken in to consideration for this density measurement. (so if two species both grow an average of 30 feet apart at the center-points of their trunks, the species with the thicker trunk will have the shorter distance between surfaces, and would edge out the other for that reason)
Only Old Growth areas of forest should be considered, and only the Adult individual trees (those individuals that have reached the minimum height of 5 meters or 15 feet) are included in the density, while shorter/younger trees don't count toward the density.
Dead individual trees are included, even if they have partially tipped over, as long as they are still more vertical than not (pointing more than 45 degrees up from horizontal). Individuals that have fallen over (less than 45 degrees from horizontal) are not counted.
I also understand that environmental factors will play a role, so feel free to adjust those environmental factors to whatever extreme is needed, as long as the species could still realistically grow, to meet the rest of the requirements, in the described environment, and so long as the environment does not require active influence from humans in order for the environment to exist on Earth. Also, descriptions of the related environments are not required, unless they are significantly different from the "real" environment of the species described.
I also understand that it may not be realistic for a forest to be comprised entirely of a single species. For the sake of simplicity, feel free to assume that the forest is composed of only that species, unless a specific mixture of specific species, which can all grow in the same environment, would make a more dense forest for a specific reason.
Finally, undergrowth is not considered relevant to the question. Only the proximity of the trees, themselves, to each other is relevant.
TLDR, Recap: What species of tree yields the most dense forests?