Soils overhead: Characterizing canopy soils

When you tell people to look at the soil around them, most will look down toward their feet. But in some forests, it would also be appropriate to look up – into the trees. Up in those trees is another type of soil, one unseen and unnoticed by most people. It’s called canopy soil.

Canopy soils develop when epiphytes, plants that grown on other plants but are not parasitic, start to decompose. “As with soils on the ground, the formation of canopy soils is also aided by soil organisms such as mites and bacteria, the climate, the input of organic material,” says Camila Tejo Haristoy, lead author of a new study. The canopy soils, also called arboreal soils, can then provide nutrients and water to the epiphytes, such as ferns and moss. Even the host trees themselves can draw from the canopy soils directly by growing roots called canopy roots.

soil on maple tree branches

Tejo Haristoy’s study of canopy soils was recently published in a special section of the Soil Science Society of America titled “North American Forest Soils Conference Proceedings.” The study focuses on canopy soils in the Queets River Watershed, Olympic National Park, Washington. Tejo Haristoy and her colleagues wanted to characterize canopy soils that form on both Sitka spruce and bigleaf maple trees.

“A large amount of research has been done on the ground of these forests, and the soils that accumulate on these two trees had been quantified but not characterized,” she explains.

Characterizing canopy soils is an important step in fully discovering their impact in forests. Canopy soils act as reservoirs of nutrients – a savings account for the forest. The nutrients in the canopy soils are cycled into the ecosystem when the epiphytes fall to the ground, or when the nutrients leach out of the canopy soils or are taken up directly by the host tree through canopy roots. Tejo Haristoy and others want to fully describe these characteristics and roles of canopy soils so they can better understand the complete forest ecosystem.

In their recent study, the research team found that soils on the maple trees were thicker than soils on the spruce trees. When maple trees drop their leaves, that nutrient-rich litter creates more opportunity for epiphytes to grow, form mats, and decompose into soil. Additionally, maple trees are shorter trees, and litter from neighboring trees can fall on them and add to the formation of canopy soils. Spruce trees, on the other hand are taller, and only their own litter contributes to the soils.

The litter that fell on the trees wasn’t the only factor causing differences in the canopy soils. The age of the trees also affected the characteristics of the soils.

closeup of soil on spruce tree

“One of the most striking differences was how time affected the soils – how much time the canopy mats had to accumulate and then decompose to form canopy soils,” says Tejo Haristoy.

Sitka spruce trees in the study area of the Queets River Watershed are around 300 years old. Maple trees, however, are younger – closer to 200 or 250 years old. While both canopy soils were classified as the same type of soil (called Histosol), there were clear differences due to the amount of time they had had to decompose. The soils on the older spruce trees were more decomposed, and more of the fibrous material had already been broken down. The soils on the maple trees, conversely, were less decomposed and contained more fibrous material.

The type of tree litter, beyond affecting the amount of canopy soil that forms, can also affect chemical characteristics of the soil. “When the litter decomposes, the nutrients bound in it are released into the canopy mats affecting the soil pH, the nutrient availability, and other characteristics of the canopy soils,” explains Tejo Haristoy. Those chemical differences could, in turn, influence what can grow in the canopy soils and what the soils provide to the broad forest ecosystem. The impacts of canopy soils are likely to be seen far beyond the treetops.

Says Tejo Haristoy, “The importance of the canopy environment as a whole needs further attention, but we are beginning to open the ‘black box.’ ”

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