It's quite logical to assume, even without requiring a peer-reviewed double blind placebo controlled 100 million study, that nature has made it so, otherwise we would have single groups of organisms ruling the planet unhindered.

The fact that life on earth displays tremendous diversity and balance, from the macro level down to the micro level, means almost every organism has an equal and opposite adversary somewhere.

Plants and trees have a tremendous number of compounds developed over millions of years to fight diseases with. There is a reason, for example, that the Materia Medica for Chinese medicine is over 1300 pages of microscopic text. It's over 2000 years of accumulated experience on plant substances, while our modern pharmaceutical medicine is barely a few hundred years old.

There is also a reason why many pharmaceutical companies spend resources investigating and cataloging native plant substances. It's not that they want to sell herbal medicine, it's because they want to develop synthetic analogs for patenting purposes.

Which is also the reason you will never find as many scientific articles on herbal medicine, as you will find on pharmaceutical medicine. One is a billion dollar patent-protected industry, while the other is not.