Water-holding capacity of canopy soil mats and effects on microclimates in an old-growth redwood forest

The evergreen fern Polypodium scouleri growns in thick mats high above the ground. Photo by Stephen Sillett, Institute for Redwood Ecology,  Humboldt State University
The evergreen fern Polypodium scouleri growns in thick mats high above the ground. Photo by Stephen Sillett, Institute for Redwood Ecology, Humboldt State University

Based on their research in Pairie Creek Redwoods State Park, Anthony Ambrose and Stephen Sillett have found that mats of humus soil deposited as high up as 265 feet in the crowns of coast redwood trees moderate the climate around them. This makes the mats habitable to a wide variety of insects and animals more commonly found on the forest floor.

The crotches and limbs in redwood canopies trap falling leaves and branchlets. When this organic matter settles and becomes stable to the point that it decomposes no further, it is referred to as humus. If conditions do not change, the matter might remain as it is for centuries, or millennia.

Humus mats, which in redwoods can have the volume of a small school bus, hold much more water than bare portions of the tree. This causes the air above the mats to be more humid more consistently than the air at the tops of trees. Just as the ocean moderates climate fluctuations on the coast, the moist humus mats cause temperature to fluctuate much less than it does above the canopy. It remains cooler during the day and warmer at night.

What the researchers make of this is that arboreal humus functions like a sponge, absorbing and storing water and releasing it gradually to canopy ecosystems. The water-storing capacity of arboreal humus allows organisms sensitive to drying out that are normally found on the forest floor, such as segmented worms, mollusks, and salamanders to survive in the canopy.

In fact, a recent survey of wandering salamanders—a species of salamander restricted to coastal Northern California that feeds on microscopic bugs in redwood-housed humus mats—provides strong evidence for their living year round in the canopy. Based on sightings of wandering salamanders in tree crowns during the dry and wet seasons, the researchers of this separate study have good reason to believe that at least some wandering salamanders live out their lives without ever touching ground. Underneath the surface of the bigger humus mats are complex tunnels formed when roots and rhizomes of plants have decayed. For an animal that needs to stay moist and avoid predators to survive, this is ideal habitat.

These findings have major conservation implications. Arboreal salamanders in old-growth forests may be predators at the top of a diverse community of nutrient dependant organisms fueled by the productivity of epiphytes—plants that grow in the humus mats—and the trees themselves. Such ecosystems don’t exist in young second growth forests on logged-over land. The small trees bearing thin branches in these forests lack the cavities and the bulk to catch falling matter. Organisms associated with humus mats are restricted to just a few national parks that protect larger trees with the structural complexity—crotches and large limbs—to trap soils and create habitat. Ensuring protection of the existing old-growth forest and supporting regeneration of second-growth to old-growth forests is the way to conserve and expand the limited range of arboreal species in redwoods.

Ambrose’s study was peer-reviewed by the biology faculty of Humboldt State University and accepted as a partial fulfillment of the requirements for an M.S. thesis.

The results of the second study mentioned were published in the journal Herpetological Conservation and Biology under the title “Evidence of a new niche for a North American salamander: Aneides vagrans residing in the canopy of old-growth redwood forest.” It can be accessed from http://www.herpconbio.org/.

Grant Details

Grant Applicant Organization:

Grantee: Stephen C. Sillett and Anthony R. Ambrose

Amount: $58,000

Date: 2000

Web: Project Site / More Information

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