Accounting for Nutrients in a Forest Environment
There are at least 17 nutrient elements, both macro and micro, important for tree and plant health, vigor, and growth. The nutrients important and most in demand by conifers- the macro nutrients include calcium (Ca), nitrogen (N), phosphorous, (P), and potassium (K), carbon (C), hydrogen (H) and oxygen (O). The micro-nutrients are required in smaller amounts and include boron (B), chlorine (Cl), molybdenum (Mo), nickel (Ni), copper (Cu), iron (Fe), magnesium (Mg), manganese (Mn), sodium (Na) and zinc (Zn).
All forests undergo a complex and extensive nutrient cycling system. Tropical forests tend to participate in a ‘direct’ system where the nutrient elements are stored in the foliage of the tree species and the many layers of epiphytes. Due to the heavy rainfall typical of these areas, most nutrients are leached out of tropical soils leaving a relatively inert soil environment. Hence it is critical that plants store the very nutrients essential for their survival, growth and reproduction. These nutrients are stored and recycled within the plant community. Temperate forests, including the cooler boreal forests employ an ‘indirect nutrient’ cycling system. In this system, nutrients are accumulated in especially the foliage and small branches (<3 inches diameter) as the tree grows and matures. When the trees die, these nutrients are ‘returned’ to the soil environment to be stored on exchange sites in the soil. These exchange sites are often referred to as the ‘cation exchange capacity’(CEC) of the soil. And typically, the higher the organic content of the soil, the higher the CEC. A cation is a positively charged ion. An anion is a negatively charged element. Plants can typically uptake only positively charged cations through their root structures. However, nutrients available in the soil are greatly influenced and dependent upon the soil pH, or hydrogen ion concentration in the soil.
Soil pH and the hydrogen (H) ion concentration in the soil environment is an inverse relationship. A pH of 4 has a much higher H concentration ( 1 in 10-4 sites= 1/10,000) versus a pH of 7 (1 in 10-7 =1/10,000,000 sites). The higher concentration of H ions (e.g., a lower pH) implies there are fewer available sites on the CEC for important nutrient elements, both micro and macro. Plants cannot take up these important nutrient elements until they are a positively charged ion on the exchange sites. A significant problem with the H ion is in the nature of the ion itself. Its chemical energy allows it to hold to the exchange sites with more energy than the nutrient element can exert to displace it. Hence, important nutrient elements are flushed from the root zone over time leaving a less fertile soil, simply because H ions occupy a greater number of the exchange sites. A periodic ‘flushing’ mechanism is essential to remove the H ions from the exchange sites and return important and critical nutrient elements back to the soil. Prescribed fire ( and historically wildland fire) in these forests provide this flushing mechanism. Fire oxidizes organic material, freeing up the nutrient elements stored in the foliage, small branches, and to a lesser extent the larger branches and woody stems. Thus, the ash that is left after a fire provides an overwhelming number of critical nutrient elements to remove or flush the H ions from the sites and attach themselves to these same exchange sites as they infiltrate and percolate into the soil environment. Hence, the notion that wood ash is a natural fertilizer.
The following Table 1 is a summary of nutrient flux in various forest cover types in a northwest United States temperate forest. Early seral forests (ponderosa pine, western larch, lodgepole pine and even Douglas-fir) tend to concentrate fewer nutrients in their smaller branches(<3 inches in diameter) and needles. However, Table 3 indicates a significant amount of critical nutrient elements and especially Ca (calcium) and K (potassium) are extracted from the soil environment and stored.
Table 1: nutrient concentrations by species in forest fuels < 3 inches diameter.
Table 2 indicates how hydrogen ion concentrations (pH) change as cation nutrient elements are concentrated and stored in the branches less than 3 inches in diameter as forests transition from early seral to climax.
Table 2: Soil pH by forest cover type.
Table 3 displays nutrient data from a forest inventory. This data analysis summarizes only 8 of the 17 critical nutrient elements. These data represent fuels 3 inches and smaller in diameter. It is measured in lbs/ acre and is relative to basal area of each tree species.
Table 3.
Table 4.
Replacement costs. Table 4 assigns a current dollar replacement value to the nutrients stripped from the forest environment and concentrated in large and small piles and generally lost to the over-all forest ecosystem. Chelated minerals were chosen to replace stripped forest nutrients because they are immediately available for plant uptake and use, much like the lost nutrients would be available promptly if managed appropriately.
This analysis assumes 40% of the fuels are left in the forest due to logging damage, breakage, and trampling and the remaining 60% are concentrated in small and large piles of logging slash. Given this assumption, replacement costs alone for the nutrients stripped from the site with a ‘whole tree’ logging system would total approximately $31,488/ acre. The calcium and potassium nutrients appear to suffer significant losses.
NOTE: This estimate does not include the application costs- whether it be a land-based spreader or aerial application.
We don’t know the effects of this magnitude of nutrient stripping and loss on the health, growth, productivity, and vigor of future forests, but it is reasonable to assume that if nothing else, we have reduced the forest’s ability and capacity to adapt to change ( e.g., climate change) and have negatively impacted its resiliency.
Also, much like underground carbon sequestration rates, we know little about weathering amounts from rocks and parent material. This natural recharge rate would offset a portion of these lost plant nutrients. Still, it is a reasonable assumption that weathering rates would offset only a small fraction of these lost critical nutrients.
NOTE: for these reasons, our Global Forest Resources Carbon Registry does not recognize ‘whole tree’ logging as a viable nor sustainable harvest system. Fuels <3 inches diameter are to be left on the forest floor and managed.
References:
Brown, James K., J. A. Kendall Snell, and David L. Bunnell 1977. Handbook for predicting slash weight of western conifers. USDA Forest Service Gen. Tech. Rep. INT-37, 35 p. Intermountain Forest & Range Experiment Station, Ogden, Utah 84401.
Brown, James K. 1978. Weight and density of crowns of Rocky Mountain conifers. USDA Forest Service Res. Pap. INT-197, 56 p. Intermountain Forest & Range Experiment Station, Ogden Utah 84401.
Cooper, Stephen V., Kenneth E. Niema, David W. Roberts 1991. Forest habitat types of northern Idaho- a second approximation. USDA Forest Service Res. Pap. INT-236, 152 p. Intermountain Forest & Range Experiment Station, Ogden Utah 84401.
Faurot, James L. 1977. Estimating merchantable volume and stem residue volume in four northwest timber species: ponderosa pine, lodgepole pine, western larch, Douglas-fir. USDA Forest Service Res. Pap. INT-196, 55 p. Intermountain Forest & Range Experiment Station, Ogden, Utah 84401.
Pfister, Robert D., Bernard Kovalchik, Stephen F. Arno, Richard C. Presby 1977. Forest habitat types of Montana. USDA Forest Service Gen. Tech. Rep. INT-34, 174 p. Intermountain Forest & Range Experiment Station, Ogden Utah 84401
Stark, N. 1979. Nutrient losses from harvest in a larch/ Douglas-fir forest. USDA Forest Service Res. Pap. INT-231. 11 p. Intermountain Forest & Range Experiment Station, Ogden, Utah 84401.
Stark, N. & H. Zuuring 1981. Predicting the nutrient retention capabilities of soils. Soil Science Vol. 131 No. 1. 10 p. School of Forestry, University of Montana, Missoula, Montana 59812.