The forests of the Pacific Northwest are known for the grandeur. This region is home to one of the greatest temperate rainforests in the world. A hiker is both dwarfed and enveloped by greenery as soon as they hit the trail. One aspect of these forests that is readily apparent are the carpets of epiphytes that drape limbs and branches all the way up into the canopy. Their arboreal lifestyle is made possible by a combination of mild winters and plenty of precipitation.

We are frequently taught that the relationship between trees and their epiphytes are commensal - the epiphytes get a place to live and the trees are no worse for wear. However, there are a handful of trees native to the Pacific Northwest that are changing the way we think about the relationship between these organisms in temperate rainforests.

Though conifers dominate the Pacific Northwest landscape, plenty of broad leaved tree species abound. One of the most easily recognizable is the bigleaf maple (Acer macrophyllum). Both its common and scientific names hint at its most distinguishing feature, its large leaves. Another striking feature of this tree are its epiphyte communities. Indeed, along with the vine maple (A. circinatum), these two tree species carry the greatest epiphyte to shoot biomass ratio in the entire forest. Numerous species of moss, liverworts, lichens, and ferns have been found growing on the bark and branches of these two species.

Epiphyte loads are pretty intense. One study found that the average epiphyte crop of a bigleaf maple weighs around 78 lbs. (35.5 Kg). That is a lot of biomass living in the canopy! The trees seem just fine despite all of that extra weight. In fact, the relationship between bigleaf and vine maples and their epiphyte communities run far deeper than commensalism. Evidence accumulated over the last few decades has revealed that these maples are benefiting greatly from their epiphytic adornments.

Rainforests, both tropical and temperate, generally grow on poor soils. Lots of rain and plenty of biodiversity means that soils are quickly leached of valuable nutrients. Any boost a plant can get from its environment will have serious benefits for growth and survival. This is where the epiphytes come in. The richly textured mix of epiphytic plants greatly increase the surface area of any branch they live on. And all of that added surface area equates to more nooks and crannies for water and dust to get caught and accumulate.

Photo by SuperFantastic licensed under CC BY 2.0

When researchers investigated just how much of a nutrient load gets incorporated into these epiphyte communities, the results painted quite an impressive picture. On a single bigleaf maple, epiphyte leaf biomass was 4 times that of the host tree despite comprising less than 2% of the tree's above ground weight. All of that biomass equates to a massive canopy nutrient pool rich in nitrogen, phosphorus, potassium, calcium, magnesium, and sodium. Much of these nutrients arrive in the form of dust-sized soil particles blowing around on the breeze. What's more, epiphytes act like sponges, soaking up and holding onto precious water well into the dry summer months.

Now its reasonable to think that nutrients and water tied up in epiphyte biomass would be unavailable to trees. Indeed, for many species, epiphytes may slow the rate at which nutrients fall to and enter into the soil. However, trees like bigleaf and vine maples appear to be tapping into these nutrient and water-rich epiphyte mats.

A subcanopy of vine maple (Acer circinatum) draped in epiphytes. — A subcanopy of vine maple (*Acer circinatum*) draped in epiphytes.

Both bigleaf and vine maples (as well as a handful of other tree species) are capable of producing canopy roots. Wherever the epiphyte load is thick enough, bundles of cells just under the bark awaken and begin growing roots. This is a common phenomenon in the tropics, however, the canopy roots of these temperate trees differ in that they are indistinguishable in form and function from subterranean roots.

Canopy roots significantly increase the amount of foraging an individual tree can do for precious water and nutrients. Additionally, it has been found that canopy roots of the bigleaf maple even go as far as to partner with mycorrhizal fungi, thus unlocking even more potential for nutrient and water gain. In the absence of soil nutrient and water pools, a small handful of trees in the Pacific Northwest have unlocked a massive pool of nutrients located above us in the canopy. Amazingly, it has been estimated that mature bigleaf and vine maples with well developed epiphyte communities may actually gain a substantial fraction of their water and nutrient needs via their canopy roots.

Photo Credits: [2]

Further Reading: [1] [2] [3] [4] [5]

I have often remarked that working in the southern Appalachian Mountains during the summer feels more like working in a rainforest than it does an eastern deciduous forest. Lots of rain, high humidity, and a bewildering array of flora and fauna conjure up images of some far away jungle. Only winter can snap this view out of ones head. I recently learned, however, that these feelings are not misplaced. Indeed, this region of southern Appalachia is considered a temperate rainforest.

These mountains are old. They arose some 480 million years ago and have been shaping life in this region of North America ever since. Another thing these mountains are quite good at is creating their own weather systems. Here in southern Appalachia, warm, wet air from the Gulf of Mexico and western Atlantic blows northward until it hits the Appalachian Mountains. The mountainous terrain comprising parts of Pisgah, Nantahala, and Chattahoochee National Forests has been referred to as "the Blue Wall" and is responsible for the unique conditions that created this temperate rainforest.

As this air rises over their peaks, it begins to cool. As it does, water in the air condenses. This results in torrents of rain. On average, this area receives anywhere from 60 to 100+ inches of rain every year. The Appalachian temperate rainforest is second only to the Pacific Northwest in terms of rainfall in North America. All of this water and heat coupled with the age and relative stability of this ecosystem over time has led to the explosion of biodiversity we know and love today.

Life abounds in the southern Apps. The plant diversity can be rather intimidating as species from the north mix with those coming up from the south. For instance, there are more tree species in these mountains than in all of Europe. Rates of endemism in these mountains, both in terms of flora and fauna, are remarkable. There are relics of bygone eras that never expanded their range following repeated glaciations. What's more, a multitude of species combinations can be found as you go from low to high elevations.

At lower elevation, forests are dominated by American beech (Fagus grandifolia), yellow birch (Betula alleghaniensis), maple (Acer spp.), birch (Betula spp.), and oak (Quercus spp.). Magnolias cover the humid coves. Mid elevations boast birches, mountain ash (Sorbus americana), and mountain maple (Acer spicatum). High elevations contain fraser fir (Abies fraseri) and redspruce (Picea rubens). Both the understory and the the mountain balds are home to a staggering array of different Heaths (Ericaceae). From Rhododendrons to azaleas and mountain laurels, the colors are like those lifted from an abstract painting. The forest floor is where I focus most of my energy. It is hard to capture the diversity of this habitat in only a few paragraphs. What I can say is that I haven't even scratched the surface. It seems like there is something new to see around every corner.

The point I am trying to make is that this region is quite special. It is something worth protecting. From development to mining and changes in temperature and precipitation, human activities are exacting quite a toll on the Appalachian Mountains. The system is changing and there is no telling what the future is going to look like. Conserving wild places is a must. There is no way around it. Luckily there is a reason people love this place so very much. There are a lot of dedicated folks out there working to protect and conserve everything that makes southern Appalachia what it is. Get out there, enjoy, and support your local land trust!