Fungi up High: Biodiversity and Canopy Soils

© Jennifer Kerekes
Original publication: Mycena News, April 2014

Forest canopies, defined as the sum of all the tree crowns, are among some of the least studied habitats on our planet (Orlovich et al. 2013). But forest canopies are hosts to a variety of biodiversity, including vascular flora and animal biota. Within these forest canopies, primarily in wet tropical forests, organic matter derived from decomposed epiphytic plants and decomposed tree litter accumulates as soil mats within tree branches. This accumulated soil is often referred to as “canopy soil,” also known as arboreal soil, aerial soil, crown humus, and canopy organic matter. Canopy soils can also be found in old growth temperate rainforests.

Tropical Rainforest Canopy—Canopy in the Clouds
Photo by Drew Fulton

Not surprisingly, within these canopy soils exist microbial communities, including bacteria, ammonia-oxidizing bacteria, and fungi. In fact, one recent study found that canopy soils had higher bacterial and fungal richness than forest floor soils (Cardelus et al. 2009), and another noted that microbial biomass in tropical canopy soils was just as high, and sometimes even higher than the microbial biomass in forest floor soils (Vance and Nadkarni, 1990). While there has been very little research into the roles played by fungi in these habitats, a few recent studies have confirmed the presence of fungi in canopy soils. Arbuscular mycorrhizal fungi (AMF) have been reported in epiphytic plants, along with dark septate endophytic fungi in the roots of epiphytic plants (Grippa et al. 2007; Rains et al. 2003). Interestingly, AMF are not always found in epiphytic plants, and some researchers hypothesize that high atmospheric inputs of nutrients reduces the requirement for mycorrhizae in canopy soils (Mafia et al. 1993 and citations within). These studies suggest that spore dispersal and inoculum potential in canopy soils is adequate for mycorrhizal formation, and the lack of mycorrhizae in other epiphytes is not due to spore dispersal limitation. A recent study by Orlovich and colleagues (2013) in the temperate rainforests of New Zealand noted the presence of ecto­mycorrhizal fungi (ECM) in adventitious canopy roots of old growth Silver Beech (Nothofagus menziesii). Genera of ectomycorrhizal fungi found in the canopy roots included Cortinarius, Russula, Cenococcum, Thelephora/Tomentella, Lactarius, Laccaria, Clavulina, and Leotia. Stephenson and Landolt (2011) reported the pres­ence of cellular slime molds (dictyostelids) in canopy soils. Although slime molds are not true fungi, they are often studied by mycologists.

The characteristics of canopy soils have been studied and compared with forest floor soils. In general, canopy soils have been reported to have higher concentrations of organic matter, lower pH and higher nitrogen levels, and experience more ex­treme fluctuations in their moisture conditions, especially at certain times of the year (Bohlman et al. 1995; Cardelús et al. 2008; Nadkarni et al. 2002).

Indeed, canopy soils are unique microcosms among the tree canopy. These microcosms, or microhabitats, are known to play an important role in the subsystem of tropical forests (Nadkarni and Matelson, 1991). Vance and Nadkarni (1990) suggested that microbial communities in canopy soils may play an important role in regulating nitrogen availability in some tropical forests. These microhabitats remain understudied for microorganisms. Within these canopy soils may be yet another important source of fungal biodiversity. For example, in Stephenson and Landolt’s 2011 paper investigating dictyostelids in canopy soils, at least three of the new species described were not known from ground soils. Continued studies of canopy soils will provide a greater understanding of the ecological roles of fungi in rainforest ecosystems and will contribute to our understanding of fungal diversity and biogeography. Next time you happen to be out looking for fungi in an old growth temperate rainforests or tropical rainforest, don’t forget to look up!

Further reading:

Canopy in the Clouds

Works Cited:

About the Author:
Jennifer Kerekes received a Ph.D. in Microbiology in December, 2011 from the University of California at Berkeley, where she worked with Dr.Tom Bruns. She is interested in the ecology and diversity of saprotrophic fungal communities.