I often see small woodland areas that are dominated by just two fern species. Marginal and evergreen wood ferns on a rocky mountainside; or bracken and hayscented ferns; marginal wood fern and polypody on boulders; marsh and royal in a very wet swamp, or cinnamon and New York. Sometimes bracken or hayscented or cinnamon ferns can dominate as a single species. I wonder whether one can infer very specific habitat information from the pairings in particular, say the distinctions between cinnamon / New York vs. cinnamon / interrupted.
Even more intriguing to me is the tendency of some relatively uncommon ferns to grow near one another. Here are some examples:
Dryopteris goldiana and Diplazium pycnocarpon (damp talus slopes)
Deparia acrostichoides and Phegopteris hexagonoptera (wet stream banks)
Thelypteris simulata and Woodwardia areolata (old bogs)
Botrychiums, including all three subgenera, tend to grow together, perhaps due to shared mycorhizal requirements.
It often seems that the best place to look for uncommon pteridophytes is near other uncommon pteridophytes.
It is not surprising that a good place to look for Bootts wood fern is in the presence of crested wood fern.
Recently at the White Memorial Forest in Litchfield CT I was looking for broad beech fern, as it had been collected 60 years ago for their herbarium. I found it by following a long population of silvery glade fern down a stream and then up one sloping bank.