| A unit of Lasting Forests
evolving since March 30, 1999 |
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A Total Forest Management Plan
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Soil sets the the productive potential of a site for myriad values. It is only one factor within the environment. (Jakinchuk (1982) observed that soils information can be over-empghasized in classifying habitat.) Soil is very complex and so the word itself may have many meanings and connotations. It may be support for trees, a sponge for water, a collection of minerals and nutrients, an organic "broth" of micro- and macro life forms. It may be the eroding material or the imperiled ancient forest floor of esthetic values. Among all of the factors of the system of trees and animals, it can be directly affected by mangement. Since National Forests (and hopefully other forests) must be managed in ways that will not impair their long-term productivity, then maintaining and improving soils may be critical under the umbrella of such a law.
There is now knowledge that this complex substance can be simplified. Soil porosity and site organic matter are the properties most influenced by management and most related to forest health and growth, of course within the limits imposed by climate and topography.
An amazing array of observations have been reported, some intuitively evident, some true after brief thoughtfulness:
Forest organic material and logging slash keep soils cool in the summer
Organic matter improves the amount ofwater that is available to plants at least by reducing evaporation (a function less important in cooler or wetter forests).
Organic matter reduces soil erosion.
Organic matter releases nitrogen, phosphorus, and other nutrients for potential plant uptake.
In sandy soils, organic matter as a source of nutrients is especially important.
Vegetation that competes with trees also dries out the soil and makes soil compaction (where it occurs) more harmful to trees than on sites without herbaceous vegetation.
Slight compaction may be good for coarse, sandy soils since it can improve available water-holding capacities of the soil.
The Rooting Volume
The below-ground system, the rooting volume, or rhizosphere is difficult to see and study and that accounts for how little is known about it. Much is known, however, and an effort is made here to summarize the key elements that may be of managerial significance or may open doors to increased profitability.
The hyporheic zone is that under streams and has relevance to the riparian volume.
Below the ground surface, key questions remain about root growth, the relations of above-surface plant growth to roots, sloughing of root cap cells, production of exudates, respiration, grazing by nematodes, growth of fungi and bacteria in the volume, immobilization of nutrients (nitrogen, phosphorus) by microbes, grazing by amoebae and nematodes on microbes.
Tree root diseases are usually interpreted by symptoms above ground. Heikkenen has shown that conifers may be long-dead before they show signs of stress or even death (for example, the still-green "Christmas tree"). It may be that root diseases, like many insects, follow moisture stress (and others) and do not "cause" tree death. Root pathogens (disease organisms) seem to affect groups of neighboring trees "in progressively expanding disease pockets or centers." Within these pockets there are trees of different ages. This is consistent with a stress-theory, one in which a group of trees grows to fill a rooting volume and then roots of an individual tree cannot compete with roots from surrounding trees or for the available moisture. Age, species, stem density, and the collective starting time rule the stand. Roots die and they are then the resource base for many fungi (once believed to be the cause of the tree death, no only the result. The meaning of tree death is clearly different from the meaning of conspicuous animal death, but the same problem remains with the debates over the meaning of human death.
In the West,laminated root rot associated with Phellinus weirii, Armillaria root disease (Armillaria ostoyae), and Anosus root disease (Heterobasidion annosum) are considered significant. These are associated with stands that are declining (stem density has declined from that of nearby stands), and broken or wind-thrown trees (now with functionless roots). Root-rotting Basidiomycetes, called root-disease fungi, are important drivers of ecosystems. They have co-evolved with the trees. Fire supression has been said to have increased mortality due to Annosus in Sequoia stands but it may have allowed encroachment of true firs, thus moisture stress. Increases in Anosus and Leptographium species in longleaf pine following prescribed burns have been noted (and mortality ascribed to the disease, not the fire-related stresses).
Nematodes feed on amoebae which seem to provide a better food source than bacteria.
These organism are essential for moving nutrients rapidly from nutrient-poor organic debris back into the ecosystem. Bacteria in general have a high demand for phosphorus and release little of it.
Carman (J. Appl. Ecol 1982 19:873-879) used red, blue, and yellow chlorotriazinyl dyes applied as a root drench on plants in a porous medium. The growth past the dyed portions was measured.
Vogt et al. (Ecology 1986 67(2):577-579) in a critique of net root growth studies, noted that root growth in terrestrial ecosystems does not occur randomly in time and that major pulses occur in response to specific climatic events. Knowing the events can help distinguish real from seemingly random growth (if such occurs.)
Perhaps we can master the growth of trees and grass adequately by measuring the above-surface materials. The costs of going below ground are excessive. We have to re-analyze the work already done on the rooting volume. Perhaps getting a relationship between the above and below ground materials, the root/shoot ratio, only tells us interesting things about roots, things that do not correlate well or any better than above-surface observations.
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Last revision January 17, 2000.