The authors are US Forest Service forest researchers at the Northeast.Forest Experiment Station and the University of N.H.

This article, which appears in "Environmental Management" Vol. 13, No. 6, has been condensed by Don Gasper. The gist of the original article is retained.without changing the meaning. Don is a West Virginia trout biologist.

ABSTRACT: Both harvest removal and leaching losses can deplete nutrient capital in forests, but their combined long-term effects have not been assessed previously. We estimated changes in total soil and biomass Nitrogen (N), Calcium (Ca), Potassium (K), magnesium (Mg), and Phosphorous (P) over 120 years from published data for a spruce-fir site in Maine, two northern hardwood sites in New Hampshire, central hardwood sites in Connecticut and Tennessee, and a loblolly pine site in Tennessee. For N, atmospheric inputs counterbalance the outputs, and there is little long-term change on most sites. For K, Mg, and P, the total pool may decrease by 2%-10% in 120 years depending on site and harvest intensity. For Ca, net leaching loss is 4-16 kg/ha/yr in mature forests, and whole-tree harvest removes 200-1100 kg/ha. Such leaching loss and harvest removal could reduce total soil and biomass Ca by 20%-60% in only 120 years. We estimated unmeasured Ca inputs from rock breakdown, root-zone deepening, and dry deposition; these should not be expected to make up the Ca deficit. Acid precipitation may be the cause of current high leaching of Calcium.

QUESTION: The relationship of nutrient depletion to long-term productivity of forests depends on the total amount of each nutrient in the ecosystem as well as on the availability of that nutrient. The question addressed by this paper is: do the total amounts of nitrogen, calcium, potassium, magnesium, and phosphorus in a forest ecosystem change significantly over time? Only input and output rates and the total pool size of each nutrient are needed to answer this question. The complexities of various nutrient fluxes within the system and of the availability of nutrients to plants are avoided. Continual depletion of the total amount of any nutrient must sooner or later decrease its availability and, consequently, forest productivity Both timber harvest and leaching remove nutrients from forest ecosystems.

Most studies of nutrient removal by whole-tree harvest in the northeastern United States and eastern Canada have found that Ca was the element most likely to be depleted (Weetman and Weber 1972, Boyle and others 1973, Silkworth and Grigal 1982). Mann and others (1988) summarize a number of studies nationwide, all of which indicate a potential problem with Calcium.

In this paper we analyze the only studies from the eastern United States that we know of in which input, leaching, harvest removal, and total soil and biomass pools have all been measured. These studies cover six sites in Tennessee and New England, inclusive, including both residual and glaciated soils and both coniferous and hardwood forests (Table). At each site, the biomass and soil pools of nutrient elements, input from the atmosphere, output in leaching or streamflow, and harvest removal by clear-cutting have been previously reported. Starting with these published values, we use a linear extrapolation over time to show the expected changes in total pool sizes over 120 years. For Ca, which depletes rapidly, we estimate additional unmeasured inputs by dry deposition, rock breakdown (weathering) and root-zone deepening.

The total nutrient pool does not include the mass of [unweathered] nutrient elements in [rock] material [larger than] >2 mm. In our analysis, each of the six stands is a sample (but not random) of the population of all forests on acid soils in the eastern United States. There is general agreement in our results among sites; averaging over sites or statistical analysis for differences among sites is not warranted.

INPUT-OUTPUT: Annual inputs of nutrients in precipitation and outputs in streamflow or leachate for mature forests are generally consistent over the region (Table). For most elements at most sites, inputs are less than or equal to 2 kg/ha/yr. N input is 4-11 kg/ha/yr throughout the region, while Ca input in Tennessee is 6-7kg/ha/yr.¹ On the output side, Ca consistently has the highest mass losses, followed successively by Mg, K, N, and P. Negligible amounts of P move either in or out of all systems. Ca output exceeds input by 4-16 kg/ha/yr. On the other hand, N input exceeds output by 2-8 kg/ha/yr. Both K and Mg generally have net losses of several kilograms per hectare per year. (See Table)

The harvest removal ² plus harvest-induced leaching for a single harvest amounted to 3% or less of the total pool of K, Mg, and P at all sites. N loss from harvest was consistent at 4%-8% of the total pool. Ca loss for non-oak forests represented 2%-5% of the total pool, but for CT [Connecticut] and MO [Oak Ridge, TN.] forests, which contain oak and hickory, the loss was 13% and 19%, respectively, of the initial Ca pool.... (See Table )

DISCUSSION: Forest harvesting removes nutrients from the system in larger quantities but at infrequent intervals compared to the continual leaching loss. Over a 120-year period, leaching and harvest removal contribute roughly equally to nutrient depletion. Both must be considered in analyses of long-term productivity (D.W. Johnson and others 1988).

Consistency in above ground pool size of each element among sites and with other temperate forests of similar age (Morrison and Foster 1979) implies that harvest removals can be estimated easily. In the northeastern United states, a whole-tree harvest of a mature forest will remove roughly 250 kg/ha of N, 350 kg/ha of Ca (600 for oak-hickory forest), 150 kg/ha of K, 40 kg/ha of Mg, and 20 kg/ha of P. Stem-only harvest would remove about half of these amounts.

Input of nitrates from acid precipitation exceeds leaching losses.For P and K, harvest removals generally exceed the net leaching loss in the absence of harvest, whereas for Mg the reverse is true. For all three elements, projected long-term depletion of the total pools is cause for some concern, but the depletions are considerably smaller than the depletion expected for Ca.

Calcium depletion by leaching from mature northeastern forests in the absence of timber harvest is large with respect to the total nutrients excluding those in 2mm and larger rocks. Our projected rapid depletion from leaching and intensive harvest suggests that Ca deficiency could become a problem in the future.

Such a nutrient depletion scenario is usually countered by invoking various inputs not included in our analysis. These are dry deposition, weathering, and root-zone deepening. We now discuss each of these quantitatively.

For Ca at Oak Ridge, D.W. Johnson and others (1985) estimated ...dry deposition of Ca as 3.2 kg/ha/yr. For the New England sites, and for the Northeast as a whole (NAPAP 1987), Ca input is only 1-2 kg/ha/yr. Even doubling this annual input rate to account for additional dry deposition has a rather small effect on the rate of Ca depletion.

WEATHERING OF NEW NUTRIENTS: Weathering implies conversion of minerals from stable forms, as in rock or soil minerals, into forms that can participate in a variety of reactions involving exchange, leaching, and biological uptake.

This is related to the rate of formation of weathering rinds on rock (Clayton 1979). Birkeland (1984) summarized several studies of weathering rind formation giving average rates of 0.1-0.2 mm in 10,000 years and maximum rates of 3-6mm in 10,000 years for the most highly weatherable rock types. At Hubbard Brook (HB), bedrock is virtually unweathered where the till is more than 1.5m

thick (N.M. Johnson and others 1968). Glacial striations over 10,000 years old appear to have weathered only 1 or 2 mm.

Calcium content of local rocks at Hubbard Brook is roughly 1% (N.M. Johnson and others 1968, England 1976). The question becomes the rate at which breakdown occurs. Complete breakdown of a 1 mm weathering rind in 10,000 years provides only 0.026 kg/ha/yr of Ca from rock of 1% Ca. The total [weathering] rate depends on the number of rinds or the amount of stone surface per unit area of land.... For instance, 5% by volume of 5 mm gravel plus 15% by volume of 5 cm stones in a 70 cm root zone give 42 plus 13 = 55 effective weathering rinds. If each rind weathers at 1 mm in 10,000 years, then 55 X 0.026 = 1.5 kg/ha/yr of Ca is an estimate of [weathering]. There is obviously great uncertainty in this number, but better estimates are not yet available.

In residual soils, such as the Tennessee sites, weathering has been proceeding for millions of years. Bedrock normally is far below the root zone and coarse fragments often are small and composed primarily of chert, which has no calcium. The presence of any coarse fragments at all in ...old soils not affected by erosion implies that the rate of weathering is very slow, and cannot provide substantial Ca input at the Tennessee sites.

ROOT-ZONE DEEPENING: Gradual deepening of the lower boundary of the ecosystem (the root zone) would represent an input to the system and occurs when roots exploit new volumes of parent material not previously available to them. When the lower boundary is at the bedrock surface, weathering of the bedrock surface is effectively a deepening of the lower boundary, and is a system input. As was shown, the contribution of a bedrock weathering rind of 1 mm in 10,000 years is negligible with respect to measured inputs and outputs of Ca.

Root-zone deepening in the absence of bedrock can occur as a long-term process only if the soil surface also is being lowered. On a time scale of thousands of years, the root-zone thickness of a forest must be virtually constant

In a 6 cm of rock thickness in 10,000 years with 265 kg/ha/mm of Ca, the Ca input from root-zone deepening is 1.3 kg/ha/yr. This is insufficient to alleviate the present input-output imbalance of Ca, but further refinement of the estimate should be attempted.

In the literature to date ³, dry deposition and weathering usually have been invoked to make up such deficits. Our analysis of Ca suggests very approximate values of 2.0, 1.5, and 1.3 kg/ha/yr of Ca for inputs by additional dry deposition,

weathering of new nutrients, and root-zone deepening, respectively. These add to about half of the reported input-output imbalance in Ca in the absence of harvest removal.

The values presented in the Table are measured or calculated with reasonable accuracy. General agreement among sites and among investigators provides some confidence in the values. By contrast, the additional inputs from dry deposition, weathering, and root-zone deepening are little better than educated guesses. In terms of ecosystem preservation and maintenance of long-term productivity, it seems prudent not to rely on such poorly evaluated inputs to offset the well-documented rates of nutrient depletion.

The current input-output imbalances cannot have been maintained for long or the basic elements already would be depleted. Current rates of cation depletion in the northeastern United States probably are the result of anthropogenic acid precipitation (D.W. Johnson and others 1988), and the net loss may only have existed over roughly 100 years of the Industrial Age. In much of the Northeast, precipitation pH averages as low as 4.0, whereas "clean" or preindustrial precipitation probably had a pH above 5.0 (Galloway and others 1984),.... As water passes through the forest ecosystem, the H+ in solution is replaced by the base cations Ca2+, Na+, Mg2+, and K+...(Reuss and Johnson 1986), Driscoll and others 1988). If preindustrial sulphuric and nitric acid inputs were one tenth of current rates, base-cation outputs also may have been on the order of one tenth of current rates. Assuming unchanged base-cation inputs, outputs would then have been equal to or less than inputs, and the system would have been in long-term balance with respect to the basic cations.

This excessive leaching rate cannot have continued for thousands of years and may have been induced by recent acidification of precipitation (D.W. Johnson and others 1988). Removal of Ca in above ground biomass by timber harvest can double this net loss of Ca. The combination of leaching loss and whole-tree harvest at short (40 year) rotations apparently could remove roughly 50% of biomass and soil Ca in only 120 years. [Italics and bold added by Editor]

The nutrients Mg and K are also subject to depletion by leaching and harvest removal, although not as severely as Ca. Phosphorus is tightly held and is not likely to be depleted. Nitrogen is replaced by anthropogenic atmospheric deposition and is only subject to depletion on some sites with intensive harvest.

Calcium deficiency is rarely mentioned as a limiting factor in growth of northeastern forests. If Ca depletion is a recent phenomenon, Ca deficiency may not yet have had time to develop. Available or exchangeable Ca could be depleted at a faster rate than total Ca, and cation imbalances also may be induced. Shortle and Smith (1988) have proposed aluminum-induced calcium deficiency as a cause of high mortality of red spruce (Picea rubens Sarg.)in the mountains of the northeastern United States. This could be the first indication of Ca deficiency.

Three measures are possible to mitigate Ca depletion: 1) reduction of acid deposition to preindustrial levels,

2) restrictions on short-rotation whole-tree harvesting, and 3) liming of vast forest areas on a scale similar to liming agricultural crops.

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¹ Elsewhere in the Eastern Forest annual calcium input in wet deposition is only 2 and dry (dust) about 3 kg/ha (kilograms/hectare). Kg/ha is about the same as pounds/acre.

² Harvest removal is the nutrients trucked away. The harvest induced leaching, is the added nutrient loss from clearcut sites when more nutrients are available and are washed away because there is more water without live tree roots taking up water and nutrients. This nutrient hemorrhaging or shock lasts 3 or more years. These values shown in the table are totals.

³ This article is nearly 10 years old, and no new studies of this nature have begun. _