Orchids belong to a recent, extremely diverse family of plants-the Orchidaceae. Not only will you find these species on the forest floor, but living on other larger plants as epiphytes as well as being attached to rocks, known as a lithophytic lifestyle. Today, orchids encompass nearly 10% of all the plants on Earth! This expansive and unique group of organisms are notorious for their patchy distribution. Many orchids I find, (yes there are native orchids in western New York) are accompanied by other individuals nearby. For some time now, we have linked this patchiness to the orchids specific mycorrhizal fungus. Without particular mycorrhizal fungi living in the soil, orchid seeds will not germinate. These plants are quite temperamental, having evolved some of the most obligatory relationships with orchid mycorrhizal fungi, hereafter OMF.
OMF promote seed germination, and nourish the plant during the seedling stage without receiving a sugar reward. Before an orchid produces its first leaves, the orchid protocorm exclusively depends on its fungal symbiote to supply it with everything it needs to survive. All orchids initially go through a mycoheterotrophic stage in which the plant receives carbon and mineral nutrients solely from the fungus. This can last a month, or this stage may persist for years. Orchids and their mycorrhizae have a truly fascinating evolutionary relationship, that boggles the mind of even experts in the field.
Most other mutualists living among the forest floor don’t stick around a partner that isn’t providing for them. However, the fungi associated with orchids invest nutrients and energy in young unproductive plants only to collect satiable sugar rewards once the plant becomes more established. This relationship is hypothetically similar to a squirrel cultivating an oak tree, defending it from herbivores for years, so it can finally reap the rewards of an acorn bounty. Doesn’t quite make sense. Most species interactions exist in the now, and don’t invest resources for future benefit. When a species provides another species with resources, it expects an immediate reward.
Some orchids carry out this mycoheterotrophic juvenile trait for their entire life, never giving their fungal “symbiote” a sugar reward. These orchids are parasites. To this day we don’t know how this interaction really works. These orchids are either somehow tricking the fungus into thinking it is receiving a reward, they can be stealing sugars and nutrients without the fungus realizing, or these fungi are just oddball species in which orchid selection over millions of years has taken place. Some think that mature parasitic orchids chemically mimic exudates released during the protocorm stage, to entice OMF into nurturing them. Whatever the case, orchids make out like a bandit. Even though their specialist fungal counterpart doesn’t receive much, or in some cases any sugar reward, OMF still exist, so that says something.
To better understand how OMF interacts with orchids, we should look at what makes some orchid species common, and look into what makes some species rare. With around 28,000 currently accepted species of orchid, yes, I am generalizing here, but there are noteworthy ecological trends that must be expressed. Now there are plenty of selfish orchid collectors out there who sell rare and other sought-after species on the black market, but for now, I’m comparing orchids that are rare without the influence of humans.
One widespread species of orchid I encountered on a daily basis working in the southern Appalachia became my favorite plant for months to come, the downy rattlesnake plantain- Goodyera pubescens. This small charismatic orchid is one of, if not, the most common species of orchid on the East coast. In middle, to late successional forest ecosystems, it occurs as frequent as common weeds. The seeds of this species are not long-lasting so unlike other orchid species, there isn’t a strong seed bank that supports germination once proper conditions ensue. Still, this species does great!
In one study, Melissa McCormick and her team found that this orchid species switched mycorrhizal partners during a drought. That’s right, some species in this seemingly mycorrhizal specific plant family can change its underground symbiote with environmental perturbations. Also, the OMF that associate with G. pubescens where found to carry out a saprotrophic lifestyle, not solely depending on the orchid for energy. This means that these OMF exist all over the forest floor, feeding on dead organic plant material. For these reasons, Goodyera pubescens has large niche width, and occurs throughout forest ecosystems on the east coast. Other, more rare orchids are limited to one single fungal species, that aren’t the strongest decomposers. These orchids are extremely patchy, because they are nested in their symbionts niche, and their OMF too is nested in the niche of their botanical counterpart.
In a previous blog entry, I explain how mycorrhizae helped plants get a foothold on land, and talk about the trajectory of both symbionts. Originally, land plants and mycorrhizae both were generalists. Over time, species representing both kingdoms evolved more specific relationships. Again, this trajectory of species evolving more specialized relationships is at play, because specialization ultimately reduces competition-the enemy of fitness. The epitome of plant and fungal specialization is expressed by the orchids and their OMF. This specialization has created the extremely patchy distribution of orchid species, which reduces genetic drift in many orchids and probably explains the vast number of orchid species. Isolated patches of orchids, with their limited fungi remain in one environmental pocket, and go off on an evolutionary tangent. Other orchids like G. pubescens that have reverted to a more generalized relationship with fungi secure their place on a warming, ever-changing anthropogenic Earth. A common stat I express here is that 90% of land plants form mycorrhizae. If you take away just one family of plants, the Orchidaceae, then only 80% of land plants would form mycorrhizal relationships. Pretty crazy, right?