I didn't really find an answer.
A study[1] in Virgina found that planting multiple varieties of trees was beneficial because it allowed the variety that was most suitable for that location to thrive, and survive problems that might affect other varieties.
A study[2] in Washington State tested a couple varities of common conifers planted in pairs, and found more conventional "trees are affected by competition" result.
This study[3] performed in the inter-mountain West found that some conifers _may_ benefit from being mixed with aspens, but didn't seem nearly as conclusive as the Borneo study.
If anyone can find a more conclusive study about temperate Western forests, I'd love to see it.
[1] https://www.si.edu/newsdesk/releases/tree-species-diversity-...
[2] https://cdnsciencepub.com/doi/10.1139/X09-040
[3] https://besjournals.onlinelibrary.wiley.com/doi/full/10.1111...