James W. Hornbeck,USDA Forest Service,Durham, New Hampshire

(From "Proceedings of the Conference on the Impacts of Intensive Harvesting." Fredericton, N.B. January 22, 1990)

Compiled by M.K. Mahendrappa, D.M. Simpson, and G.D. van Raalte.

[ This now 10 year old paper is the first indication that forest hydrologists were concerned that forests on infertile geological strata may not have enough nutrients to regrow a stand of trees after logging. It presents valuable, amazing quantitative data, that seems to have been ignored. Streams draining such forests are often too pure (nutrient poor) for even trout. (There are 100 miles of such beautiful streams on the Monongahela Forest). The term kg/ha is about the same as pound per acre. Not a word is changed or added. -- Don Gasper]

During the past decade, the rapidly expanding use of biomass harvesting, increased product removals, and shorter rotations have raised legitimate concerns about depletion of nutrients from forests. The concerns are magnified when it is remembered that many forested sites in New England and eastern Canada had past uses, such as cropping, grazing, or lumber harvest, that involved substantial nutrient removals. Also, in more recent times, these forests have been subjected to atmospheric deposition which accelerates nutrient losses. Thus, it is not surprising that we are more frequently hearing questions about whether there will continue to be an adequate supply of nutrients for maintaining optimum productivity of our forests.

Forest managers must take these concerns and questions seriously, and begin to incorporate nutrient cycling into management activities and decisions. To do so it is necessary to learn about nutrient movement into, within, and out of forests, and about how to keep as much of the nutrient capital as possible within the forest and available for plant growth. This paper illustrates the kinds of information needed and gives some recommendations for protecting forest nutrient cycles.

Calcium is a logical element for illustrating nutrient considerations. Harvest removes a substantial amount of calcium in forest products and also triggers increased leaching of calcium from forest soils to streams and groundwater. A recent paper by Federer et al. (1989) has demonstrated the need to be concerned about depletion of calcium from forests.

A variety of pools and pathways are involved in supplying calcium for tree growth. Likens et al. (1977) show that the calcium cycle for northern hardwood forests can be divided into at least eight major pools and 13 interrelated pathways for transfer of calcium from one pool to another.

Though it is well to fully understand the calcium cycle, forest managers

can make decisions with only general indications of amounts of calcium stored in some of the pools.

This can be illustrated in the following table using data on calcium capitals available for three major forest types in New England where the USDA Forest Service and others have studied impacts of biomass harvesting. Two of the types, spruce fir and northern hardwoods, are found over sizable areas of eastern Canada as well. The calcium values for the mineral soil horizons and forest floor given in the table are for material that will pass through a 2 mm sieve. This size fraction is the calcium data for mature forests in New England. The data were derived from Hornbeck and Kropelin (1982), Trillon et al. (1982), and Smith et al. (1986).

Particles greater than 2mm, up to and including large boulders, probably contain several times the calcium found in the less than 2 mm fraction. However, this calcium in larger size fractions is usually tightly bound and is converted to plant available forms very slowly, sometimes at rates as low as a few kg/ha/century.

Calcium

Forest type and Mineral Forest Above-ground Merchantable Roots Input in Output in

location horizons floor whole trees boles Branches precipitation streamflow Net

_____________________________________________________________________________________________________

kg/ha kg/ha/yr ______________________________________________________________________________________________________________

Northern hardwoods 7,570 490 360 224 120 1 15 -14

Success, NH/

Spruce-fir 10,330 380 540 255 190 1 16 -15

Weymouth Point

(ME)

Central hardwoods, 3,320 100 590 451 240 2 10 - 8

Cockaponsell State

Forest, CT/

______________________________________________________________________________________________________________

The combined calcium found in mineral soil particles and forest floor can vary greatly among sites and forest types. For the three study sites, total calcium in these two important pools ranged from about 3,400 kg/ha for the central hardwoods in Connecticut to more than 10,700 kg/ha at the spruce-fir site in Maine (see table). Calcium in above ground whole trees ranged from 360 to 590 kg/ha, or 4 to 17% of the total found in the forest floor and mineral soil. A small amount of calcium, 1 - 2 kg/ha, is added to forest ecosystems in annual precipitation (see table). In turn, 8 - 15 kg/ha are lost each year as dissolved in streamflow.

Harvesting has two major impacts on calcium capitals. First, the calcium incorporated in the harvested products is lost from the site. Second, for the first few years after harvest, the cutover site is less efficient than mature forests at cycling calcium. As a result, more calcium leaches from the cutover site, increasing the amount of dissolved calcium lost in streamflow.

For the three sites listed in the table, calcium in the biomass removed during a whole-tree harvest was 344 kg/ha for northern hardwoods, 494 kg/ha for spruce-fir, and 530 kg/ha for central hardwoods. Losses of calcium in stream-water approximately doubled in the first year after whole-tree harvesting, ranging from an increase of 10 kg/ha in central hardwoods to 23 kg/ha in spruce-fir. The increases nearly disappeared by the end of the third year after harvest, but totals of increased losses to leaching for the three years were 30 kg/ha for norther hardwoods, 43 kg/ha for spruce-fir, and 28 kg/ha for central hardwoods.

One way of evaluating these losses is in terms of how they drain the capitals of the forest floor and mineral soil. For example, combined leaching losses and removals of calcium for the spruce-fir site (537 kg/ha) represent a depletion of about 5% of the capital in the forest floor and mineral soil (10,770 kg/ha). This rather small depletion of a fairly large calcium capital would not seem to present a problem for future productivity. On the other hand, the leaching losses and removals for the central hardwood site (558 kg/ha) represent a depletion of over 16% of the capital in the forest floor and mineral soil (3,420 kg/ha) . The greater depletion of an already small capital raises a red flag regarding future productivity.

New England states are currently 60 to 90% forested. Nearly all of this forest is second or third growth arising after past harvests or clearing for crops or grazing. In many cases, the early harvests were intensive and were followed by fire in the logging slash. Thus, nearly every harvestable site in New England has a 100 to 200 year history of human activity that, in turn, has had impacts on nutrients.

Recent studies show that this history can often be reconstructed in considerable detail using court records, census data, tax records, and physical artifacts. For example, an investigation by Bormann (1982) showed that an area nearby and similar to the site of our whole-tree harvest study in New Hampshire was cleared for agriculture in the early 1800s. The clearing included burning, and removed an estimated 320 Ml/ha of forest biomass.

Cropping over the next several decades removed an estimated 2.3 - 3.9 Ml/ha/yr. Using nutrient contents from forest and agriculture literature, I calculated that biomass removed from the start of land clearing until

reversion to forest in the early 1900s included approximately 1,000 kg/ha of calcium.

The site for our whole-tree harvest study in Connecticut was cleared for grazing sometime prior to 1850, then allowed to revert to forest around the turn of the century. My rough calculations again suggest calcium removals of about 1,000 kg/ha from clearing and grazing, or about the same as estimated for early land uses on the New Hampshire site. The history of the spruce-fir study site in Maine has proven more difficult to reconstruct. There are no good cutting records, but the forest on the control catchment shows evidence of logging, most likely from one or more selection cuttings within the past century (Collidge 1963).

Thus, of the three study sites, the potential for cumulative effects of past land use and present day intensive harvests would be least for the spruce-fir forest in Maine. In the absence of complete clearing and heavy cutting, past nutrient removals were probably small and total nutrient capitals, as illustrated by calcium, are substantial (see table). The removals from northern hardwood sites in New Hampshire may be more of a consideration. Bormann (1982) found that forest recovery following agricultural disturbance was shower than on sites that had only been logged, possibly because of nutrient deficiencies.

Consideration of past land use is an absolute necessity for sites like the central hardwood forest in Connecticut. Present-day nutrient capitals are small, as evidenced by the total of just over 3,400 kg/ha of calcium in mineral soil and forest floor (table), partly because of removals during earlier land uses. Any additional removals and leaching losses could have serious consequences for site productivity.

Mobile anions, such as sulfate (SO4) and nitrate (NO3), associated with atmospheric deposition raise concerns over calcium depletion beyond those associated with nutrients removed or lost during harvesting. Input-output budgets for mature forests at all three study sites show a net annual loss of calcium (table). The mobile anions in acid precipitation are probably responsible (Federer et al., 1989). As the anions pass through the forest ecosystem, the hydrogen ions they were coupled with in precipitation are exchanged for other cations, such as calcium, potassium, and magnesium. The hydrogen ions stay behind to acidify the ecosystem while the cations are leached to streams and groundwater. The net losses appear small on an annual basis (table), but assume greater importance when extrapolated over longer times, such as a 100 year rotation. For example, net losses of calcium to leaching over such a rotation may total 800 to 1,500 kg/ha. Slight increases in leaching losses immediately after harvest coupled with removal of as much as another 500 to 800 kg.ha in harvested products (over a 100-year rotation) could mean a total depletion of from 20 to 40% of calcium capital for the rotation. Our studies indicate that additional inputs from rock breakdown, root-zone deepening, and dry deposition cannot begin to replace this lost calcium (Federer et al. 1989). _