In my previous blog, I wrote about the mycorrhizal association of Douglas Fir and Paper Birch in the forests of British Columbia. In California, Douglas Fir has been shown to form similar associations with Bishop’s Pine and Tanoak. These types of mycorrhizal networks are reportedly widespread throughout the world, and I have been wondering if Douglas Fir forms associations with Red Alder, Western Hemlock, or Western Redcedar in the Puget Lowlands.
Douglas Fir and Red Alder
Like Paper Birch, Red Alder is a member of the birch family, so it seems a likely candidate for association with Douglas Fir. Unlike the Paper Birch however, Red Alder hosts bacteria on its roots that fix nitrogen, and high levels of soil nitrogen may deter mycorrhizas. Whereas over a thousand different mycorrhizal species have been found on the roots of Douglas Fir, only fifty have been found on Red Alder, and a survey of alder in Oregon found only thirteen. Though experiments have shown that Red Alder can help Douglas Fir, that may be due to the soil nitrogen rather than mycorrhizal association.
Douglas Fir and Western Hemlock
I think it is likely that Douglas Fir can associate with Western Hemlock since some species of mycorrhizal fungi found on the hemlock are also found on the fir. We know Western Hemlocks are shade tolerant, but perhaps part of their ability to grow in the understory of Douglas Firs is due to sugar tapped from the big firs. I can’t think of a benefit for the firs however, unless it has something to do with the ability of Western Hemlocks to gain nutrients from rotting wood.
Douglas Fir and Western Redcedar
I was surprised to learn that Western Redcedar is unlikely to join in mycorrhizal networks with either Douglas Fir or Western Hemlock because it associates with a completely different group of mycorrhizal fungi. Though most trees in temperate forests associate with ectomycorrhizal fungi, the vast majority of the world’s plants, including Western Redcedar, associate with arbuscular mycorrhizal fungi. Apparently, most species don’t associate with both types, and one study found that this may actually hinder the establishment of redcedar in forests dominated by ectomycorrhizal fungi. Nevertheless, since most plants associate with arbuscular mycorrhizas, maybe Western Redcedars associate with other native plants in our forests.
In conclusion, except for the Paper Birch, I haven’t found references to studies proving mycorrhizal associations between Douglas Fir and Red Alder, Western Hemlock, or Western Redcedar, but I come away from my glimpse into the realm of mycorrhizas with two thoughts. On a practical level, I think the prevalence of mycorrhizal associations is another reason why the practice of dense plantings of mixed species makes sense. And on a more personal level, I find it inspiring to think that forests offer endless paths of exploration for young people today who dream of following in the footsteps of pioneer forest biologist Suzanne Simard.