There’s more to forest matter than meets the eye
August 1, 2006
Homeowners installing BMPs on their properties are often advised to spread a layer of pine needles over bare ground. Mats of needles absorb the impact of raindrops on soil. This prevents erosion of that chunk of soil and, it’s believed, keeps nutrients out of Lake Tahoe. But, evidence is building that suggests that this might not be such a good idea.
Soil experts classify soil into layers, or horizons. The A horizon is the top layer of mineral soil, with horizons B, C, and so on, below it.
Each layer has distinct characteristics and is differentiated by changes of color, texture, roots, structure, rock fragments, etc.
Covering the soil surface, the A horizon, in forested areas is an organic layer, appropriately called the O horizon. The O horizon can be divided into layers reflecting the degree of decomposition of the organic material.
At Tahoe, according to Wally Miller, professor of soil and hydrology at the University of Nevada at Reno, the O horizon is divided into three layers. The bottom layer, called the Oa horizon, consists of very highly decomposed humus-like material. It’s not often found in the Tahoe Basin. The next layer, the Oe, consists of actively decomposing organic material, commonly referred to as duff. The top layer, the Oi horizon, consists of pine needle litter that hasn’t yet decomposed very much.
Miller and his colleagues have been studying the A and O horizons in Truckee and in the Tahoe Basin’s forested areas, and they’ve uncovered some fascinating bits of information. The researchers reported in “California Agriculture, Volume 60, number 2” that “heavy accumulations of decomposing layered organic deposits are now predominant in the Tahoe Basin and can be as high as 83,000 pounds per acre in some areas.” This is one of the results of 80 to 90 years of fire suppression in the Tahoe Basin.
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Collections made of surface runoff from the interface between these organic layers and the underlying mineral soil surface were reported in a paper published last year in the journal “Water, Air and Soil Pollution.” That runoff was surprisingly rich in nitrates, ammonium and phosphates ” nutrients that we’re trying to keep out of Lake Tahoe.
Ammonium and phosphate concentrations in runoff from organic layers were about a thousand times greater than concentrations in soil water, and the ratios were almost as large when compared with concentrations in snowmelt. Nitrates were roughly the same in soil and from the organic layer.
Miller and his associates investigated this further by removing two-by-eight-foot sections of the organic layer from the forest floor and placing them in large pans. With some of the sections, they also separated the two organic layers ” the heavily decomposed layer and the pine needle litter layer. They leached nutrients from those blocks of organic material with mists of water, simulating rainfall, and with snowmelt water.
“If you look at just the litter layer,” Miller said, “we were getting about one kilogram per hectare of phosphate phosphorus. And of the more highly decomposed layer, we were getting an average of 1.7 kilograms phosphate per hectare. The two combined (when the layer was intact) was about 2.2 kilograms phosphate per hectare. That’s quite a bit.” A kilogram is about 2.2 pounds and a hectare is about 2.5 acres ” you do the math.
What happens after a wildfire, when the decomposing nutrient-producing forest duff is burned off? The area that Miller and his colleagues had chosen for their study of the A and O horizons was partly burned over by the July 3, 2002, Gondola Fire, which charred 670 acres uphill of South Shore casinos and east of the Heavenly Valley gondola. The fire left the researchers with some burned study plots and some unburned plots, so they could compare.
The fire consumed the forest duff, removing that source of nutrients. But it also altered the character of the A horizon. Fires volatilize the organic coating on soil particles that make them water-repellent. The organic vapor condenses on soil a few inches below the surface, leaving the top few inches more friable and less water-repellant. That allows water to leach nutrients from the soil, which in turn adds nutrients to runoff. But it might be an improvement in the long run. In the California Agriculture report, Miller explains that “fuel reduction due to fire may cause an immediate increase in the surface mobility of nutrients, but the long-term effect may be a decrease in nutrient discharges due to the reduction of the source, the heavy surface deposits of decomposing organic litter.” Also, the researchers found that cooler prescribed burns don’t mobilize nutrients as the hotter wildfires do.
So, is the advice to leave a layer of a duff on the ground really counterproductive?
“Well,” replies Miller, “it’s starting to look like it might be if allowed to accumulate to the extent that we now find in the upper watershed.”
Work on this interesting new source of nutrients, and what happens to them, is continuing. We’ll surely be hearing more about how nutrients are produced from forest litter, and how serious it might be.
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