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SummaryThe major ecological features of oldgrowth coniferous forests in the Douglas-fir region are reviewed. Special attention is given to characteristics that distinguish oldgrowth forests from managed and unmanaged (natural) young stands. The primary exemplary type is 350-to 750-year-old Douglas-fir-western hemlock forest typical of the western slopes of the Cascade Range, but other types and locales are discussed. Management techniques for maintenance of oldgrowth forests are also considered. Major conclusions are:1. Approximately 175 to 250 years are required to develop old-growth forests under natural conditions in both Coast and Cascade Ranges. Development of old growth is faster on good sites than on poor sites.2. Few plant or animal species are solely confined to old-growth forests, although many speciesincluding several vertebrates, saprophytic plants, and epiphytic lichens-find optimum habitats in such forests. Some organisms, however, may require old growth to maintain viable populations. Moreover, there are substantial differences in composition and relative abundance of species between young-and old-growth forests.3. Gross productivity is maintained at high levels in most old-growth stands, but mortality generally balances growth. Thus, the merchantable board-foot volume tends to remain constant for several centuries or gradually decreases because the amount of defect increases. Total organic matter keeps increasing because of accumulated masses of dead tree boles, mostly as down logs.4. Old-growth forests are highly retentive of nutrients; large amounts are incorporated into living and dead organic matter. Losses of limiting nutrients, such as nitrogen, are low.5. Nitrogen-fixing epiphytes are abundant in old-growth trees, and bacterial nitrogen fixation appears to be common in the large woody debris characteristic of old-growth forests.6. Small-to medium-size streams in old-growth forests depend mainly on forest litter for an energy base. These materials are invariably partially utilized before they are exported downstream.7. The structure of old-growth forest is more heterogenous than that of young forests; coefficients of variation in tree sizes are greater, and understory patchiness is much higher than in young-growth stands.6. Most of the distinctive features of old-growth forests can be related to four structural features: (1) large, live old-growth trees, (2) large snags, (3) large logs on land, and (4) large logs in streams. The structural features are related over time.9. A large, old-growth Douglas-fir is individualistic and commonly has an irregularly arranged, large, coarse branch system, and often, a long crown. It is ideal habitat for specialized vertebrates, such as the red tree vole, northern spotted owl, and northern flying squirrel, as well as nitrogen-fixing lichens.10. Large snags are valuable as habitat for a variety of vertebrates and invertebrates and as a future source of logs.11. Logs on the forest floor are important habitats for small mammals, including species that dispe...

Information on primary production, decomposition, hydrology, and element cycling was integrated in annual budgets of accumulation and flux among components of a mature Douglas—fir forest ecosystem. Annual N input in precipitation and dust was 2.0 kg/ha, and an estimated 2.8 kg/ha were fixed by cyanophycophilous lichens in the canopy. Annual N loss to groundwater was 1.5 kg/ha. N appeared to be accumulating in the ecosystem. An annual decrease of ~ 2.8 kg/ha in vegetation was offset by estimated increases of 5.0 kg/ha in fallen logs, and 2.8 kg/ha in soil organic matter. Microparticulate litterfall provided a large input of N to the forest floor (3.3 kg°ha—1°yr—1). Annual input of metallic cations in precipitation was only 545 eq/ha, whereas weathering input (net release of cations to solution from primary and secondary minerals) was estimated by difference at °9000 eq/ha. Total annual loss to groundwater was 9400 eq/ha and, because of little cation accumulation, loss almost exactly balanced input. Net transfers of P were small. Total annual input was 0.5 kg/ha, total loss was 0.7 kg/ha, and net accumulation was —0.2 kg/ha. Input of elements in precipitation and dryfall was small compared with that in the eastern United States. Water chemistry profiles showed that the biologically important elements N, P, and K increased in concentration as water passed through the canopy and litter layer but decreased as water passed through the rooted part of the mineral soil. In contrast, Na increased by a factor of 20 as water passed through the rooted soil. Concentrations of all elements except Mg were lower in the stream water than in solution at 2.0—m depth in the subsoil. At our site, unlike some eastern forests, Kjeldahl N (organic N plus NH4+) accounted for most of the measured N in solution. Nitrate levels were low, averaging @<20 mg/L NO3——N at all points in the profile. Titratable alkalinity dominated anion chemistry in the mineral soil, but in the upper parts of the water chemistry profile (precipitation, throughfall, and litter leachate) Cl— and SO4= together accounted for 30—40% of the negative charge. Total return to the forest floor in litterfall was greater than that reported for other Douglas—fir stands mainly because of plentiful microparticulate forms and coarse woody debris. Leaf fall accounted for less than half of the total litterfall input of N to the forest floor. Element accumulations in coarse woody debris almost cancelled the negative net annual increments in the living vegetation compartments. Overall cycling patterns show that only the biologically limiting element, N, was tightly conserved. For other elements, losses nearly equaled or even exceeded inputs. Redistribution from old to new foliage was also more important for N, P, and K than for Ca, Mg, and Na. Solution transport processes were important for all elements and dominated the cycling patterns of biologically less important elements such as Ca and Na. Vegetation absorbed metallic cations mainly from the mineral soil. However, much N and P w...

We investigated communities of denitrifying bacteria from adjacent meadow and forest soils. Our objectives were to explore spatial gradients in denitrifier communities from meadow to forest, examine whether community composition was related to ecological properties (such as vegetation type and process rates), and determine phylogenetic relationships among denitrifiers. nosZ, a key gene in the denitrification pathway for nitrous oxide reductase, served as a marker for denitrifying bacteria. Denitrifying enzyme activity (DEA) was measured as a proxy for function. Other variables, such as nitrification potential and soil C/N ratio, were also measured. Soil samples were taken along transects that spanned meadow-forest boundaries at two sites in the H. J. Andrews Experimental Forest in the Western Cascade Mountains of Oregon. Results indicated strong functional and structural community differences between the meadow and forest soils. Levels of DEA were an order of magnitude higher in the meadow soils. Denitrifying community composition was related to process rates and vegetation type as determined on the basis of multivariate analyses of nosZ terminal restriction fragment length polymorphism profiles. Denitrifier communities formed distinct groups according to vegetation type and site. Screening 225 nosZ clones yielded 47 unique denitrifying genotypes; the most dominant genotype occurred 31 times, and half the genotypes occurred once. Several dominant and less-dominant denitrifying genotypes were more characteristic of either meadow or forest soils. The majority of nosZ fragments sequenced from meadow or forest soils were most similar to nosZ from the Rhizobiaceae group in ␣-Proteobacteria species. Denitrifying community composition, as well as environmental factors, may contribute to the variability of denitrification rates in these systems.

Linear regression models were developed for Douglas fir needle, female cone, branch, and bark decomposition in seven stands representing four mature vegetation types in western Oregon. Rate constants (k) for annual weight loss of needles ranged from 0.22 to 0.31 year−1, from 0.047 to 0.083 year−1 for cones, from 0.059 to 0.089 year−1 for branches, and from 0.005 to 0.040 year−1 for bark. The decomposition constant (k) of needles had a negative linear correlation (P < 0.01) with maximum plant moisture stress and temperature growth index of the seven stands. In comparing substrate quality of needle and woody litter components, k was more closely correlated with lignin content than with C:N ratio.